12
"There are some things you learn best in calm, and some in storm."
-Willa Cather
The Skew-T/Log-P diagram is a type of thermodynamic chart that shows the interaction of heat and humidity in the atmosphere using a graphical presentation rather than long mathematical calculations. These diagrams can be used to predict cloud bases and the overall stability of the atmosphere and are based on daily radiosonde data received from the multitudes of weather balloons launched each day around the world. Although the name sounds complicated, it really isn’t. The "Skew-T" portion of the title simply refers to the fact that temperature lines (isotherms) on the diagram are tilted or "skewed" 45 degrees to the right. And, in another easy reference, the scale of the diagram that relates to pressure is logarithmic...hence the name Skew-T/Log-P. But what does it say? What does it show and how do you read it?
Lab Development Acknowledgement
For the beginning of this lab, you will use a tutorial produced by students of Penn State University’s meteorology program and recreated here for your use with Penn State's permission. Certain sections have been modified to fit particular examples used in this lab and other pieces have either been removed or added by R. Schmidt but the majority of the text is their original material. As part of PSU's Meteo 482: Weather Communications II course, undergraduate students produced this "how to" for Skew-T/Log-P diagrams with the intent it would be utilized by other students entering into the field of meteorology. Special thanks are in order to Beth Russell, Scott Dimmich, Shepard Stuck, Nicholas Sette, Lindsay Schwarzwaelder and Adam Marcal for their contributions to the text used here. Another special acknowledgement must be extended to Lee Grenci, (Emeritus) Instructor of Meteorology, Penn State University for his input in first bringing this lab investigation to Upper Dublin High School students in 2008.
Once you’ve gained some experience in the diagram’s basics, we’ll then move on to methods to draw out the diagram’s hidden "treasure trove" of information and how that data is then used in weather forecasting. Stick with it...there will be certain sections that may seem a bit technical (especially the sections on LCL, LFC, etc.) but if you refer to the text and diagrams often and give a wholehearted effort, you should be able to find your way to becoming a much better severe weather forecaster.
Introduction
Now that you have been introduced to the concept of stability, it is time to apply that understanding to real-world situations. In some regards, this "3D" look at the atmosphere will be very challenging since it uses a new tool you have never seen before, the Skew-T/Log-P diagram (Figure 1). Like meteograms and station models, Skew-Ts contain vast amounts of information about the atmosphere but a forecaster has to be able to interpret the data correctly in order to use it. Skew-Ts and the three dimensional sounding data they provide are essential for severe weather forecasters since atmospheric stability is intimately connected with the development of severe thunderstorms and tornadoes. In this exercise, you will be introduced to this tool, its structure and the ways in which its data can be integrated into a weather forecast.
The Skew-T/Log-P Diagram
Classic cumulus congestus clouds...an impressive but (often) ominous sign of instability in the atmosphere. Image: NOAA
The Skew-T Log-P Diagram
When severe weather strikes, forecasters turn to Skew-T/Log-P diagrams to illuminate their understanding of the atmosphere. This diagram is a fundamental meteorological chart of the atmosphere's vertical structure commonly used in weather analysis and forecasting in the United States. The Skew-T/Log-P Diagram, often referred to as just a "Skew-T", can seem intimidating at first. The intimidation may arise from the fact that there are actually five axes. The title, Skew-T/Log-P is based on the position of two of the axes. The temperature axis is at a 45° angle and therefore skewed. The atmospheric pressure is on the vertical axis (in millibars), but displayed on a log scale, hence the Log-P. The three other axes are mixing ratios, moist adiabats and dry adiabats, as labeled in Figure 1.
Variations of the Skew-T/Log-P have existed since the late 19th century. The differences arise from the placement of the axes. The first such diagram to be used was the emagram which was created in 1884 by Heinrich Hertz. The tephigram and stuve diagram are two other variations that appeared in the 1920s. Those remained the staples until the 1940s when newer techniques for analyzing the weather were introduced. The Skew-T/Log-P, which is most similar to the emagram,
Figure 1: An overview of Skew-T Log-P components typically used by meteorologists (Large size). Image: PSU
was created in 1947 by H. Herlofson for the United States Air Force. Skew-T/Log-P diagrams are most often used in the United States while most of Europe still uses the tephigram.
The information plotted on the Skew-T/Log-P diagram is gathered by a radiosonde, an instrument attached to a weather balloon that measures temperature, relative humidity and pressure throughout the atmosphere. As the weather balloon rises, the information recorded by the radiosonde is transmitted back to Earth by radio. Other variables such as dewpoint, wind speed, and wind direction are calculated from the recorded data. The recorded temperatures and derived dewpoints are then plotted on the Skew-T/Log-P diagram at their corresponding pressure levels. The points are then connected to create two profiles, also referred to as soundings, one of temperature and one of dewpoint. The whole process of launching a weather balloon (Figure 2) and plotting a Skew-T/Log-P diagram occurs regularly worldwide at 00Z and 12Z every day. During daylight savings time, 00Z is 8pm EDT, and 12Z is 8am EDT. When severe weather is likely, additional weather balloon launches take place to provide a more exact picture of the atmosphere.
To understand what is being measured by the radiosonde, it is convenient in meteorology to talk about a "parcel" of air. Think of the parcel of air as a self contained bubble which can contract and expand depending on changes in temperature and pressure as it moves. There is no defined mass or volume for a standard parcel of air. The only specification is that the temperature, pressure and other stated variables are constant throughout the parcel. For example, when the parcel is lifted, the pressure and all other variables change uniformly throughout the parcel.
It is these measured changes in the parcel as it rises or sinks that help forecasters learn about the weather. That is why the Skew-T/Log-P diagram is so important...it highlights the vertical structure of the atmosphere as recorded by the radiosonde in an easy to read format. One quick glance at the Skew-T/Log-P diagram provides a forecaster with a wealth of knowledge about the stability of the atmosphere. If a Skew-T/Log-P diagram is not used, many tedious calculations are needed to determine the same stability information that is provided in the diagram. The Skew-T/Log-P diagram is also used to quickly compute indices that are indicators of atmospheric stability. These indices provide insight into the current weather conditions and what is to come. For instance, certain indices are direct indicators of an unstable atmosphere, a necessary component for severe weather.
Figure 2: The launch of a cruise ship-based weather balloon with attached radiosonde in the Atlantic Ocean. Cruise ships are often the source of oceanic data due to their regularly scheduled routes. Image: U. of Miami
A superb example of unstable conditions producing severe weather was The Great Plains Tornado Outbreak of 1999. On May 3, 1999, 77 confirmed tornadoes touched down in Oklahoma, Kansas, and northern Texas. Many of these tornadoes were eventually classified as F4 or F5 events, the most powerful tornado classifications on the Fujita Scale. One storm in particular spawned the most violent tornado ever recorded just outside Oklahoma City when a nearby Doppler on Wheels (DoW) recorded a wind speed of 301 mph. In all, 46 people died and 825 were injured, with over one billion dollars of damage to local businesses and homes. During this time, forecasters turned to the Skew-T/Log-P as an indicator of the severe weather potential. All of the indices determined with a Skew-T pointed to severe weather for May 3, 1999 hours before it actually materialized and, because of this fact, an extra weather balloon launch was ordered for 18Z to create a better understanding of the situation's severity. This tornado outbreak will be explored further as the Skew-T/Log-P diagram is examined in closer detail, starting with temperature.
Temperature: The Thermal Trace
Being the most analyzed of the seven fundamental variables of meteorology, temperature gives meteorologists the greatest insight into the vertical structure and stability of the atmosphere. With data obtained from weather balloons, temperature values at different pressure levels in the atmosphere are plotted onto the Skew-T/Log-P diagram and connected to one another. This continuous profile of temperature with increasing height acts as a crucial tool for forecasting and during severe weather events.
The "Skew-T" part of the diagram's name comes about because isotherms, or lines of constant temperature, are oriented 45° to the right of the vertical. The orientation of the isotherms on the Skew-T/Log-P diagram allows meteorologists to easily calculate quantities related to stability and heating in the atmosphere. On a typical Skew-T/Log-P diagram, isotherms are labeled in degrees Celsius, as all radiosonde temperature data is recorded in Celsius.
Overlaying the vertical temperature profile in the atmosphere against these isotherms is the first step in creating and analyzing a Skew-/Log-P diagram. To determine the temperature of air at a given pressure level, it is necessary to locate the isotherm that intersects the vertical temperature profile in the atmosphere and then read the temperature value from the isotherm. When the vertical temperature profile at a given pressure level falls between isotherms, it is necessary to interpolate the value of the air temperature, as shown in Figure 3.
Like temperature, dewpoint is also plotted on the Skew-T/Log-P diagram (the green line in Figure 3), giving quantitative information on the humidity of air in the atmosphere. Surprisingly, the distance between the temperature and dewpoint at a given pressure level can give forecasters more valuable information than the dewpoint profile itself can. This is because the relationship between temperature and dewpoint requires an understanding of relative humidity. Qualitatively, when there is a large distance between the temperature and dewpoint profiles, the air in that region is dry and relative humidity values are low. Conversely, when there is a small difference between the temperature and dewpoint profiles, the air in that region is moist and relative humidity values are high. Where relative humidity values are high, especially near or up to 100 percent, clouds can easily form. In fact, relative humidity around 700 millibars (mb) is commonly plotted and forecasted in various meteorological models. In looking at a Skew-T/Log-P diagrams, however, temperature and dewpoint profiles can give forecasters an almost instantaneous look at where clouds and moist layers in the atmosphere can be found.
Figure 3: To determine the temperature at any point in the atmosphere, follow the temperature sounding (red) and line it up with the pressure level (blue) in the atmosphere you are curious about. Then using the isotherms (magenta) as a guide, read the temperature. For example, the temperature at the 700 mb height would be -1C while the temperature at 500 mb would be -13C. Image: PSU
The relationship between temperature and dewpoint also serves as the foundation for calculating many meteorological values and indices, especially those related to cloud processes and convective storms. Indices such as the lifting condensation level (LCL) are calculated by simulating the ascent of air parcels with a given temperature and dewpoint. Other indices related to the buoyancy of air parcels or atmospheric stability, such as the level of free convection (LFC), are located by finding where a parcel, while rising moist adiabatically, intersects the temperature profile. Don't worry, there will be much more on this soon.
Similar meteorological quantities can also be found by using the temperature of an air parcel at a given pressure level. Potential temperature, or the temperature of a parcel of air if it were compressed or expanded dry adiabatically to 1000 millibars, allows meteorologists to make comparisons between air parcels at different pressure levels and make a prognosis of where cold air and warm air is moving. Equivalent temperature, which results from releasing all heat within a parcel and then bringing it adiabatically back to a pressure level of interest, can be used to relate moisture content and temperature of the air.
Coupling these two ideas, equivalent potential temperature, or the equivalent temperature of a parcel where the parcel is brought down to 1000 millibars, can give insight about instabilities present in the atmosphere. Because heat is released in this process, the parcel can become highly buoyant so regions with high equivalent potential temperature (more commonly known as "theta-e"), are typically regions with the highest instability and therefore the greatest potential for thunderstorms or convective development. One of the many data sets available at the Storm Prediction Center under the Basic Surface tab is Theta-e (check out this SPC chart which shows the link between high Theta-e and convective thunderstorms.).
The temperature profile on a Skew-T/Log-P diagram aids forecasters in finding out how air parcels will move. In situations where temperature increases with height, more commonly called a temperature inversion in meteorology (Figure 4), rising air may encounter a virtual wall, inhibiting vertical motion. Such a scenario typically occurs in mountain valleys or against mountain ranges. As the Earth's surface cools after sunset, it cools the air above it and this cool, more dense air sinks to the bottom of the valley in a process known as cold air drainage. Temperature inversions can also occur during the late evening and overnight hours as the Earth's surface releases radiation to space and air aloft cools at a slower rate than air near the surface. Temperature inversions are typically present on most 12Z (morning) soundings in North America. Check out this obvious inversion on the 12Z Skew-T from Shreveport, LA on January 29, 2009.
Figure 4: The obvious effects of a temperature (thermal) inversion on a smoke plume in Lochcarron, Scotland on a cold January morning. Inversions are common on morning soundings as what heat existed in the ground is released into the air on a cold, clear, windless night. The heat warms the air just above the ground to the point that temperature rises with height instead of falling. This inversion creates an invisible lid or "cap" on the atmosphere that prevents air parcels from rising through it since the parcels will likely be cooler than the air in the cap. These very stable conditions, although occurring in "good" weather, are the source of many city's pollution problems. Image: PSU
Skew-T/Log-P diagrams also allow forecasters to determine the type of precipitation that will fall in advance of an approaching storm. As precipitation falls, it can travel through various layers of the atmosphere with greatly varying temperatures. If the temperature of the air through which a droplet falls remains below freezing, it is likely the droplet will crystallize and reach the ground as snow. Likewise, if the temperature of the air through which a droplet falls remains above freezing, it is likely that the droplet will reach the ground as rain. In certain situations, however, droplets can freeze in one layer and thaw in another as they fall yielding freezing rain or sleet by the time they encounter the surface (Figure 5)...a particularly dangerous situation on the ground and an equally difficult one to accurately predict without a timely sounding. One of the key applications of the Skew-T/Log-P diagram, therefore, is displaying information regarding the temperatures of the layers in which precipitation will fall in an easy-to-read format.
Temperature and dewpoint profiles, when plotted on the Skew-T/log-P diagram, act as the keystones to understanding weather conditions. The diagram gives forecasters an almost instantaneous view of the air above us all the way to the top of the troposphere, providing information regarding the stability of parcels as they move about, the likelihood of cloud formation or storm development, the type of precipitation likely to occur at a given location and various other meteorological quantities that give further insight into the structure of the skies.
Figure 5: The 12Z sounding from Wichita, KS on December 5, 2002. Note the shallow pocket of warm air (blue) to the right of the 0oC isotherm. This kind of situation is a typical formula for a "wintry mix" instead of just snow. Full Skew-T. Image: NC State Univ.
Perhaps the most important indices emphasizing the severity of this particular weather system are related to the energy of the storm. Convective available potential energy, or CAPE, serves as an index that describes the maximum energy available to an ascending parcel of air. CAPE is defined as the area within which the theoretical parcel temperature is greater than the measured air temperature at each pressure level in the atmosphere. The vertical temperature profile in the atmosphere, therefore, has a strong influence on CAPE values. To many, CAPE value is one of the easiest variables to use in predicting the potential for severe weather. For example, on a typical summer morning in the Great Plains, a Skew-T/Log-P diagram might have CAPE values near 0 Joules per kilogram (J/kg). In the 3 May 1999 12Z sounding from Norman, Oklahoma, CAPE values were well over a 1000 J/kg, a strong indicator that storms later that day had the potential to develop on the order of minutes. Take a look at the reference to CAPE in this SPC mesoscale discussion excerpt issued during the 2008 Super Tuesday Tornado Outbreak on February 5, the deadliest one day tornado event since 1950 (Full Report).
Temperature alone, however, is only one part of the Skew-T/Log-P diagram. What relates the skewed temperatures to the logarithmic scale of pressure in the atmosphere? Let's take a look.
Pressure: The Force of the Atmosphere
Pressure is a vital component on Skew-T Log-P diagrams and standard meteorological observations are generally based upon specific mandatory pressure levels in the atmosphere (Figure 1). As a review, pressure is the amount of force per unit area being applied to a surface. In meteorology, pressure is usually measured in units called millibars (as opposed to inches) from the Greek word "baros" which means weight, and "milli" for one-thousandth of a bar. For the atmosphere, pressure typically ranges from 0 millibars to 1050 millibars and on the Skew-T, pressure is plotted as horizontal lines with the associated pressure values displayed to the left side of the chart. Where the temperature and dew point plots start at the bottom of the Skew-T Log-P diagram is the pressure at the surface when the radiosonde is launched. Note this is not necessarily at the bottom of the chart. Meteorologists mostly use the Skew-T Log-P for pressure levels between approximately 250 millibars and 1050 millibars because that is where the troposphere is located and is where most of the weather affecting the surface of the earth is occurring.
For a creative way of thinking about pressure, imagine taking three 1' x 1' x 1' slabs of cement and stacking them on top of each other one by one and that the pressure levels are where the cuts are between each slab. First, take one slab and lay it flat on the ground. The weight of the whole slab rests on the side of the slab that is in contact with the ground. Due to pressure being a force per unit area, that then means there is a pressure exerted on the side of the slab that is in contact with the ground. Take another cement slab and lay it on top of the first slab so that the edges match. Now, the weight of both of the slabs is resting on the side of the first slab in contact with the ground. The pressure exerted at this point has now doubled due to the increase in weight. Where both slabs are in contact, there is also pressure exerted on the first slab by the second
Figure 6: This map shows the mandatory pressure levels used by meteorologists to describe weather phenomena. In almost all cases, meteorological observations are represented by these pressure levels rather than altitude. Image: NOAA
slab. By adding a third slab, the pressure exerted on the side of the first slab in contact with the ground now triples to accommodate the other two slabs. The pressure exerted between the first two slabs doubles. The pressure exerted between the second and third slabs will be the same as the first and second slabs before adding the third slab. In essence, this is how pressure in the atmosphere works although it is air molecules that are causing the force on the surface of the Earth.
From the cement slab example, take a column of air that goes from the ground up to the tropopause and slice it into slabs as well. The first slab in contact with the ground is the total weight of all the molecules exerting the pressure force at the surface. By adding a second slab of air to the first, the pressure will now increase as well because there will be more molecules that add to the total weight of the column, so there will be a greater pressure at the surface. Looking at the Skew-T, it is noticeable that pressure doesn't exactly double. As pressure decreases the farther away from the surface of the earth the sounding gets, the greater the distance between the conventional pressure levels becomes. This result is due to gravity which pulls the air molecules to the earth's surface causing the density of the atmosphere to be greater at the surface than at the top. This implies that pressure is also greater at the surface than at the top (Figure 7).
Figure 7: This image illustrates the effects of gravity on air pressure. As gravity pulls down on the air molecules, density naturally increases and surface pressure rises. Image: NOAA
Pressure in the atmosphere plays an important role in what happens to air parcels as they rise and sink. As an air parcel rises in the atmosphere, the force exerted on the sides of the parcel weakens, causing the parcel to expand and cool on ascent which helps in forming clouds. When a parcel is sinking, pressure increases with decreasing height so the force exerted on the sides of the parcel from the outside atmosphere pushes the sides of the parcel closer together. This causes compression which in turn causes the air parcel to warm on descent which helps to make clouds disappear. These pressure related changes in temperature occur regardless of the addition or subtraction of actual heat energy and are known as adiabatic changes.
There are also differences in pressure due to elevation. Think about Denver, Colorado, and Miami, Florida. Miami is at sea level so the weight of a column of air at this location is physically heavier than a column in Denver which is one mile higher in elevation. Consequently, even though the general weather conditions may be almost identical in both cities, the physical atmospheric pressure may be quite different. For example, the air pressure in Denver rarely ever reaches 900 mb due to its elevation yet 900 mb in Miami would likely mean a hurricane was bearing down on the city!
Pressure also plays an important role in aiding meteorologists to forecast temperatures and determine where the jet stream (Figure 8), which is associated with the top of the troposphere, is situated. The Skew-T Log-P indicates wind speeds on the right side of the chart as the radiosonde continues to climb in the atmosphere. The jet stream is typically located between 300 and 250 millibars but can occasionally occur at much lower altitudes. While it is often little more than a guide for synoptic weather patterns, its influences can sometimes be much greater and either make or break a major surface storm system. Using information plotted on Skew-T Log-P diagrams, meteorologists can make predictions about wind speeds at different heights, the impact of temperature advection along with many other aspects of the upper atmosphere that can greatly aid in accurate forecasting.
Figure 8: A 300 mb map from 21Z, 25Mar09. The colored shapes represent concentrations of high wind speeds embedded in the jet. Note the 150 kt "jet streak" over Washington. A close examination will also reveal two different jets, the sub-polar jet (the more northerly of the two) and the sub-tropical jet, the southerly of the two...can you find them both? Click for the answer. Image: PSU
the beginning of where the dry adiabats of Skew-T Log-P diagrams come into the picture.
As discussed previously, as pressure changes in the atmosphere, parcels are forced to expand and contract. According to the ideal gas law (some physics here), decreases in pressure cause parcels to expand and therefore cool. Conversely, as a parcel's overall height decreases, it is forced to contract and warm. An adiabat is a line on a Skew-T Log-P diagram that diagnoses how atmospheric temperature changes with height as it's pressure changes. These lines (trends) are possible to chart because not only do air parcels warm up or cool off with changes in pressure, they do so in predictable ways since they obey the ideal gas law where pressure and temperature are mathematically related.
Adiabatic processes do not include the affects of heating and cooling on a parcel by actual heat energy. Heating and cooling are often a result of the phase change of water in the atmosphere (remember earlier discussions about latent heat). Evaporation, for instance, leads to parcel cooling while condensation leads to parcel warming. These are known as adiabatic effects which are simply processes that force a heating or cooling in a parcel. These moist processes are not considered when looking at dry adiabats.
There are two types of adiabats on a Skew-T Log-P, moist and dry. Dry adiabats show how the temperature of the atmosphere will change with height as long as the air remains unsaturated (regardless of the actual water vapor content). Moist adiabats show how temperature changes when the atmosphere is saturated (relative humidity is 100%). These rates of change in temperature with height are called lapse rates (Figure 10). Adiabats on a Skew-T Log-P diagram are used most frequently to determine how stable the atmosphere is. They are also used to determine how a hypothetical parcel of air will behave at various altitudes in the atmosphere. The fundamental difference between moist and dry adiabats on a Skew-T Log-P is the difference in the rate of cooling (the lapse rates). The dry adiabatic lapse rate is approximately 9.8 °C/km while the moist adiabatic lapse rate is considerably smaller (~6°C/km). Additionally, dry adiabats are lines of constant potential temperature. If the atmospheric lapse rate is dry adiabatic, potential temperature does not change with height.
Figure 9: A classic late afternoon temperature profile for the boundary level after it has been heated by the sun. The planetary boundary level can be identified on a Skew-T by the total distance the temperature parallels the dry adiabats. This layer usually extends no further than ~850 mb. Full Image. Image: PSU
Day Adiabats: The Key to Atmospheric Stability
In many parts of the United States, thunderstorms occur most often in the late afternoon and early evening hours since these are the hours of the day when the ground and the air near the surface are heated to a maximum. This layer of heated air near the surface of the Earth is often called the (planetary) boundary layer. As it is warmed, it becomes less dense than the free atmosphere above it. This warming allows the layer to move upward in the atmosphere (Figure 9). Upward motion is one of the triggers that allow these afternoon thunderstorms to form.
This layer of air near the surface rises due to simple buoyancy. Think of what happens when ice cubes are placed in a cup of water. Frozen water is less dense than liquid water so the ice cubes float at the top of a body of water. In the atmosphere, buoyancy is the property that allows less dense air to rise through layers of air with higher density. These upward motions in the atmosphere are critical in creating the afternoon severe weather described above. Analyzing these vertical motions is
Figure 10: On a Skew-T diagram, the dry adiabatic lapse rate is indicated by parallel skewed lines similar to temperature but in the opposite direction. Wet adiabatic lapse rates are represented by a series of curved lines that will not be parallel to each other (in this case, green dashes). In Figure 10a (left), the blue line illustrates the rate at which a hypothetical parcel of air would cool if raised dry adiabatically from its current position P to a height of 700 mb. Figure 10b (right) illustrates the same concept but raises the parcel wet adiabatically. Which lines to use depends upon whether or not the air parcel is saturated. If it is not, it would cool according to the dry adiabatic lapse rate. If it is saturated, it would cool according the slower rate found along the wet adiabats. Images: PSU
So how do adiabats factor into atmospheric stability? Here are a few guidelines...
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When the lapse rate of a parcel of air is greater than 9.8°C/km, the atmosphere is absolutely unstable.
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If the lapse rate of a parcel of air is between 9.8 °C/km and the moist adiabatic lapse rate (~6°C /km), the atmosphere is called conditionally unstable.
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If the lapse rate is less than the moist adiabatic lapse rate, the atmosphere is called absolutely stable.
One of the most common uses of the dry adiabatic reference lines on a Skew-T Log-P is to find the Lifting Condensation Level (LCL). To determine the LCL (Figure 11), the surface temperature and dewpoint must be located. The dewpoint determines the mixing ratio of the parcel at the surface. The mixing ratio at the surface is then extended upwards into the atmosphere. Then, the temperature of the parcel must be found and extrapolated upwards along a dry adiabat so that potential temperature remains constant. The LCL is the level at which the mixing ratio and dry adiabat lines intersect. This is the point at which a parcel of air raised dry adiabatically from the surface would become saturated. Why might someone care about this? Because if the air was actually to reach this point, it could systematically create an unstable situation. Since this level is where the air becomes saturated, it is also the height in the atmosphere where the bases of clouds are often found.
The Convective Condensation Level (CCL) is another useful level on a Skew-T log-P diagram used in forecasting convection (Figure 12). The CCL is the level at which the sounding temperature curve intersects with the saturation mixing ratio line. Once found, follow the dry adiabat that goes through the intersection point down to the surface. The temperature measured at the surface is the temperature which a parcel must obtain before it can rise dry adiabatically to the LCL. So, if your current air temperature is less than this number, it will not likely rise (and therefore remain stable) without the addition of more heat (like that encountered during a hot, sunny day for instance).
Another application of the dry adiabatic processes is analyzing how a parcel behaves when it encounters changes in terrain. When a flow encounters changes in topography, it is likely to be forced upwards in the atmosphere. In the western United States, a saturated parcel of air from the Pacific Ocean will often encounter a mountainous region. As the parcel is forced upwards, it cools at a moist adiabatic lapse rate (less rapidly with height than dry adiabatically) and causes precipitation on the windward side of the mountains. Consequently, these are some of the wettest places in the United States. As moisture is condensed out of the parcel of air, it is warmed due to condensational heating. This is simply the reverse of evaporational cooling, which is the property that allows sweat to make you feel cooler.
As the flow reaches the lee side of the mountains, it will encounter much colder air. As it traverses down the slopes, it is compressed to the
Figure 11: Determining the LCL allows a forecaster to figure out how high an air parcel would have to rise into the atmosphere reach its lowest level of condensation given its current temperature and dewpoint. To find this altitude, cool the parcel of air dry adiabatically until it intersects with a line drawn from the dewpoint up along the mixing ratio lines. Image: PSU
Figure 12: To find the CCL, draw a slanted line upward from the surface dew point parallel to local mixing-ratio lines until it intersects the temperature sounding. Now trace downward parallel to local dry adiabats to the surface pressure. The temperature of this point is the Convective Temperature. Image: PSU
surface by colder air aloft and is further warmed. Thus, when the parcel reaches the altitude it was at near the base of the mountains, it will be much warmer. These warm winds on the lee side of mountains are often called "down sloping winds". The most analyzed down sloping wind in the United States is the "Chinook". The Chinook is today used to describe a seasonal wind that originates in the eastern Rocky Mountains and often causes abrupt heating to take place in the western Great Plains during the Winter and early Spring seasons. The name originally came from Pacific Northwest Indian tribes' word for "snow eater" and was used to mark the coming of Spring.
During the warmest months of the year, when the sun shines for the longest duration, the surface of the earth warms the most. However, this extended period of heating at the surface does little to affect the temperature of the atmosphere aloft. This, in turn, sets up a particularly volatile situation where temperature drop off most rapidly with height especially during the summer months. If a hot parcel of air heated at the surface were to rise into these rapidly cooling heights, it could rise explosively since it will be significantly warmer than the air around it creating extremely unstable conditions. Meteorologists will often keep a watchful eye skyward during the summer months when remarks like "steep mid level lapse rates" are found in a SPC convective outlook since it means that, with perhaps nothing more than a little nudge, intense convective storms could break out as heated parcels of air rise up into the upper atmosphere.
Moist Adiabats: Explosive Convection Starts Here
The moist adiabat is a useful tool that allows forecasters to investigate the trajectories of saturated parcels of air in the free atmosphere. It illustrates whether parcels will sink, rise or remain neutrally buoyant and further allows meteorologists to determine if the atmosphere has strong convective (severe weather) potential.
Once saturated, a parcel of air ascends or descends along the moist adiabat, a graphical representation of the scientifically proven tendencies of moist air (Figure 10 of Dry Adiabats). Recall that a rising parcel cools due to adiabatic expansion whether or not it is saturated or not...saturation simply slows down the overall cooling rate. Conversely, a sinking parcel warms due to adiabatic contraction. Saturated parcels have an average lapse rate of 6°C/km but, unlike, the dry adiabat lapse rate, the moist adiabat is a curved line on a Skew T diagram since it is not a constant.
A moist adiabatic process occurs when the relative humidity of air is 100%. This occurs within the cloud at the LCL where condensation causes the release of latent heat, which in turn warms the parcel (since this temperature change is due to heat energy entering the air, it is not directly considered adiabatic). Buoyancy and gravitational forces are in a constant tug of war with air molecules so this is also where stability comes into play. Since condensation releases heat into the atmosphere, it can have the overall effect of raising the temperature of an air parcel above that of the air around it causing the parcel to rise and increase instability. On s Skew-T diagram, when the environmental sounding (from the weather balloon) has a greater slope than the moist adiabat, the parcel will undergo a net acceleration upward and therefore become unstable.
Typically in an unstable atmosphere, CAPE is present. This common acronym stands for Convective Available Potential Energy. For a rising parcel of air, CAPE is a measurement of how much available energy the parcel would have available to it during its ascent; hence, the greater the CAPE, the better chance the atmosphere will produce convection and potentially thunderstorms. CAPE is measured in Joules per kilogram of air (J/kg). Although there are several varieties of CAPE, amounts ranging between 1500 and 2500 J/kg is considered large, while anything above 2500J/kg is extreme. This is not to say CAPE below 1500J/kg cannot produce severe weather, but this gives a ballpark range. CAPE is denoted by the positive area between the environmental sounding and the moist adiabat above the LFC (Level of Free Convection) on a Skew-T diagram (Figure 1). An exact value for CAPE can be determined by finding the area between these two curves (moist adiabat and environmental sounding) or computing the integral (calculus is required).
In spite of CAPE's potential, it does not mean the atmosphere will automatically take advantage of it and produce convective storms. Occasionally, there are also forces acting directly against CAPE. These forces are cumulatively known as Convective Inhibition or (CIN or CINH) and throws a monkey-wrench into an air parcel's attempt to take full advantage of high CAPE. As CAPE is the measure of positive area, CIN is the amount of negative area on the sounding. When negative area
Figure 13: The Level of Free Convection (LFC) represents the altitude to which an air parcel must be lifted (either dry or moist adiabatically) in order for it to become warmer (less dense) than the air around it (in other words, unstable). To find the LFC, find the LCL and then lift the air parcel moist adiabatically until it crosses the environmental temperature sounding and becomes warmer than the environment. If the air was lifted to this pressure, the parcel would be "free" to bubble upward (by convection). Image: PSU
computed on a Skew-T diagram exists, a convective lid or cap helps to prevent the onset of convective development (think back to earlier class discussions about temperature inversions). Mathematically, CIN is the energy needed to lift a parcel from its originating level to the LFC (level of free convection). This amount of energy is needed to get the parcel to where buoyancy forces (the CAPE) take over and accelerate the parcel upward. This same concept can be applied to a rocket traveling through the earth's atmosphere. CIN would measure the thrust/work the rocket would have to expend in order to reach a height free of the earth's gravitational field. Just as the buoyancy and gravitational forces are in constant tug of war, CAPE and CIN suffer from the same dilemma. The stronger the CIN lid on the atmosphere, the more energy would be required to get the parcel to the LFC, which lessens the CAPE and the potential for severe weather. On the Skew-T diagram, CIN is the area bounded between the temperature line and the dry adiabat if below the LCL and the moist adiabat if above. CIN is denoted as the negative area or the integral between these two curves. Don't let the small amount of area occupied by CIN on a Skew-T fool you...it cannot be compared directly to the amount of area CAPE takes up on a diagram. A little CIN goes a long way in preventing instability in the atmosphere! Take a look at this SPC convective outlook to see what the presence of CIN can have on the potential for thunderstorms.
Another way of looking at convective potential is to use the Showalter Index (SSI or SWI). There are, in fact, many other types of indices which indicate the potential for convective results but the Showalter Index utilizes the moist adiabat in finding a numeric value so its discussion here fits nicely. The Showalter Index is found by locating the LCL at 850 mb and extending a line moist adiabatically to 500 mb. The temperature previously found is then subtracted from the actual 500 mb temperature. Showalter Indices below zero constitute an unstable situation that is capable of producing severe weather. The more negative the SSI, the more likely a severe weather outbreak will occur. It should be noted though that indices are merely guides and represent the potential for severe weather...not a certainty. A Showalter Index of -3 does not guarantee that a strong thunderstorm will occur at the given location...it states that conditions are favorable for a thunderstorm outbreak. If a Skew-T shows high CAPE, low measurable CIN and indices favoring an unstable atmosphere, meteorologists would say its time to focus on the LFC (level of free convection) and EL (equilibrium) levels.
Both represent exact pressure levels in the atmosphere and were indicated in the earlier CAPE diagram. Since the LFC was already outlined in Figure 13, its time to look at the other end of the atmosphere, the Equilibrium Level (EL). In essence, if the LFC marks the bottom of where positive (CAPE) is measured, the EL marks the top. The equilibrium level is the point near the tropopause where the parcel temperature is the same as the ambient temperature thus ceasing upward momentum. The equilibrium level is found by extending a line moist-adiabatically from the LFC until it intersects the temperature sounding profile. In some instances (especially within powerful thunderstorms), a parcel of rising air has so much upward speed that it overshoots the EL before settling back down to it and creates an impressive feature known as an "overshooting top" (Figure 14) at the top of the thunderstorm. When this occurs, it is a clear indicator of air rocketing up within the storm. Since air parcels stop rising once they reach the EL, this is also part of the reason the tops of thunderstorms are typically flat creating the classic "anvil" appearance. momentum to continue its upward momentum after the equilibrium level was reached. This is called the maximum parcel level. The greater the upward momentum the further above the EL level the parcel will travel. This is more likely for a very unstable atmosphere, supporting a high CAPE. Imagine the rocket, needing high engine power to get through the earth's atmosphere, but shutting its engines off once the gravitational pull is not as strong, while still maintaining an upward acceleration.
Figure 14: This photo (which eventually won several awards) was taken by a college student in a commercial airliner as it passed the thunderstorm which would spawn the F4 La Plata Tornado, the deadliest tornado ever to strike Maryland. Note in particular the clear overshooting top in the center of the supercell's updraft. In a strange coincidence, UDHS AES students on a paleontology field trip in St. Leonard, MD would stand in the exact path of the tornado only 24 hours after it occurred. Full size image. Image: S. Maciejewski
Mixing Ratios: Hinting at Cloud Formation
Have you ever heard of a day described as "muggy?" My favorite meteorological expression, a muggy day is usually described as a hot day with high humidity but, in reality, it can be just as humid in the winter yet since that total amount of water available in the atmosphere is significantly less, it does not create the same discomfort so goes happily unnoticed. Among all of the other atmospheric characteristics a Skew-T can figure out, humidity (relative humidity in this case) is also on the list. In order to do this, the last two sets of lines on the Skew T need to be explained.
Figure 15: Another version of the standard Skew-T diagram showing the mixing ratio lines as the grey, right-skewed lines. Be careful...they can easily be confused with the temperature lines (red)! Image: NOAA
The mixing ratio is the ratio of the mass of water vapor (in grams) to the mass of dry air (in kg). Lines of constant mixing ratio are used as a vital step on the path to understanding the concepts of many components of a Skew-T (Figure 1). Specifically, they can lead one to finding various indices emphasizing stability. The mixing ratio at a certain level of the atmosphere can be computed as well, along with the saturation mixing ratio at any level of the atmosphere. In order to find the mixing ratio for a given level, the dewpoint sounding on a Skew-T is used. The value of the mixing ratio line which intersects the dewpoint plot at this level is the mixing ratio value. The process of finding the mixing ratio at a given level is revealed in Figure 1. In this specific figure the mixing ratio found at 815mb is 8.5g kg-1, while the mixing ratio found at the surface (970mb) is 14.Og kg-1.
The saturation mixing ratio is not identical to the mixing ratio. The saturation mixing ratio is defined as the maximum mass of water vapor (in grams) which can exist in a mass of dry air (in kg). The saturation mixing ratio has the same standard units as the mixing ratio (g/kg) and is determined on a Skew-T plot by reading the value of the mixing ratio line which intersects the temperature plot on the graph (Figure 16).
Figure 2: On a Skew-T diagram, the saturation mixing ration can be quickly found by following the saturation mixing ratio lines up into the Skew-T until it intersects the temperature curve. Image: PSU
OK...so why bother doing this? As it turns out, relative humidity (RH) can be directly calculated once the mixing ration and the saturation mixing ratio are known. Think this one through...RH is defined as the amount of water in the air compared to the maximum amount of water the air can hold at a given temperature. Well, mixing ratio is calculated by using the dewpoint sounding which is a measure of the amount of water in the water and the saturation mixing ratio is calculated by following the current temperature sounding. So, in essence, the ratio of the mixing ratio to the saturation mixing ratio (x100) is the relative humidity at a given level in the atmosphere. As long as the temperature and dewpoint do not change, neither will the RH. The best part about this way of calculating RH is that it can be done at any level in the atmosphere, not just at the ground. For instance, if a forecaster wanted to know the RH at 700 mb, all he/she would have to do is pinpoint the temperature and dewpoint at that level and follow it down to the mixing and saturation mixing ratio numbers and calculate the ratio. Easy!
If the relative humidity is 100%, the water in the air will undergo condensation and form water droplets and, in turn, develop clouds or even precipitation. If we see a relative humidity of at least 100% at 850mb, then there are likely clouds at 850mb. It's easy to tell whether or not a level is at 100% relative humidity - simply by evaluating if the dewpoint plot and temperature plot meet at the given level. If they meet, then the mixing ratio and the saturation mixing ratio are equal and therefore the relative humidity is 100% (figure it out mathematically if you want). It's also important to note that there can be rain in an area with a surface relative humidity under 100%. A RH of 100% is only necessary where the clouds and moisture are present, not necessarily where the precipitation is falling. So, if precipitation is falling from great heights but enters a pocket of dry air before reaching the ground, much of it may evaporate in the dry pocket...a Skew-T can easily tell a forecaster of the presence of any dry pockets. Also, remember that a Skew-T is a point measurement...it is only a measurement of one point in time. Therefore, if a Skew-T shows no area of 100% relative humidity, clouds cannot form at that moment in time but they may be able to form at an alternative time.
The lifting condensation level (LCL), or the level in the atmosphere where clouds will form if air is lifted dry adiabatically, is obtained, in part (if you recall), by using the mixing ratio line. Also, the convective condensation level (CCL), or the level at which condensation will occur if lifted parcels of air reach saturation, is attained by using the saturation mixing ratio so these figures are vitally important for a variety of atmospheric measurements.
Stability Calculations
This section is intended to assist you with determining the whole spectrum of Skew-T variables discussed earlier. For each one (listed alphabetically) formulas are listed along with any appropriate charts. Remember that, when considering the potential for severe weather, a forecaster must consider a wide range of atmospheric characteristics. From heating properties to dew point to cloud cover, all can have a significant impact on the likelihood of a severe weather outbreak so no stone goes unturned when a prediction is made. You should have noticed along the right side of a Skew-T that there are several sets of numbers that usually follow an acronym. A few of these (like CAPE and CINH) have already been introduced to you and although we will not go over all of these acronyms, I have included four more severe weather indicators on this list that are frequently used by forecasters but not mentioned in the text. Together, most of these numerical values are the result of various calculations that are know as Convective Stability Indices. As the name implies, each
index uses different atmospheric measurements in an effort to determine overall atmospheric stability (or lack thereof) in order to determine the potential for severe weather development. Try your hand at a few of the ones listed below.
Convective Condensation Level (CCL)
The CCL is an important forecasting tool for estimating the chances of severe weather. Keep in mind that, in the morning hours, thunderstorms are not all that common since the amount of CINH is usually more potent than the level of CAPE in the atmosphere. In short, cooler and more stable air found during the morning hours generally limits the desire for air to rise in the atmosphere. However, if a hot and sunny day is on tap, the CAPE in the atmosphere can rise to dangerous levels that can erode and then overwhelm the CINH’s ability to prevent the air from rising and "presto", thunderstorms break out. So, an air parcel’s convective temperature is the temperature to which the air would have to be heated at the surface to make it rise. The CCL is therefore, the altitude (pressure level) to which an air parcel, if sufficiently heated at the surface, would rise adiabatically until it becomes saturated. The CCL therefore represents the level of the atmosphere where the base of any convective (cumulus) clouds would lie. If this level is near the ground, that means any thunderstorms that form could grow to a massive scale. Thunderstorms where the CCL is a very high altitude can create "elevated convection" (elevated thunderstorms) which are usually not as dangerous as storms that exhibit "surface convection" (sometimes referred to as deep convection). To determine the CCL:
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Find the surface Td
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Draw a parallel line up along the mixing ratios until it intersects the T sounding.
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The resultant pressure level represents the CCL
Convective Temperature (Tc)
If CAPE represents convective potential in the atmosphere and CINH represents an opposing force that blocks convection, how do we know which value has the "advantage"? Enter convective temperature. The convective temperature of an air parcel is an important temperature reading to know since it gives a forecaster a good idea of exactly how much heating will have to occur in the atmosphere in order for the air to overcome the negative convective value of any CINH that may be present. To find the convective temperature:
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Determine the CCL
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From the CCL, draw a line dry adiabatically down to the ground
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This temperature is the convective temperature for the air
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Now look at a weather forecast. If the forecast indicates that high temperatures for the day will exceed the convective temperature, watch out!
Equilibrium Level (EL)
The equilibrium level is essentially the exact opposite of the LFC. It represents the level at which the temperature of an unstable (positively buoyant) air parcel drops back below the surrounding air and becomes stable again. This level is almost always very high in the atmosphere and is also the altitude where the unstable air reaches some of its highest vertical speeds. Occasionally, rising air parcels are traveling so fast that they overshoot the EL before falling back into the top of the thunderstorm. Visually, this creates an "overshooting top" at the top of the thunderstorm and is an indicator of the storm’s intensity. To find the EL, follow the following instructions.
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Starting at the LFC, follow the moist adiabatic lapse rate lines upwards until it intersects the temperature sounding.
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This intersection point is the EL.
Lifting Condensation Level (LCL)
Determining the LCL for the atmosphere is a straight forward calculation made almost every day on atmospheric soundings. Recall that the Lifting Condensation Level is the lowest altitude the air will reach saturation (100% RH). Therefore, this altitude is the lowest level of the atmosphere where condensation will occur and cloud formation will begin. To find the LCL:
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Note the temperature and dew point at approx. 50mb off the surface. This 50mb "fudge factor" is sometimes used in order to get away from the inconsistent temperature mixing that takes place near the surface.
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Next, (from the dew point temperature) draw a line parallel to the mixing ratio lines up into the Skew T diagram
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Next, identify the temperature at the same pressure as the dew point you just used
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Draw a line parallel to the dry adiabats up into the Skew T diagram (this simulates an unsaturated, rising parcel of air)
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The intersection point of the two lines will be the LCL and therefore the lowest level at which the air will become saturated and form clouds
Level of Free Convection (LFC)
The LFC is a vital measurement in severe storm prediction since it represents the level at which a hypothetical parcel of air forced to rise in the atmosphere would become warmer than the surrounding air and continue to rise all by itself. This is a "dangerous" number in the sense that, if air in the atmosphere
reaches this condition (especially at low levels), it has the potential to explosively rise in the atmosphere and create severe weather. Conversely, if an air parcel never intersects the temperature sounding, absolute stability is almost assured! Here’s how to determine the LFC...
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Locate the LCL as previously discussed.
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From the LCL, raise the air parcel moist adiabatically until that curve intersects with the air temperature.
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This intersection point is the LFC. By consider the pressure level the LFC is at and a forecaster can begin to prepare for the possibility of severe weather.
Wet Bulb Temperature (Tw)
Will you get a snow day? It’s the question on every student’s mind if some type of frozen precipitation is in the forecast. Let’s talk about condensation and evaporation again for a moment. Quite often, precipitation can begin at great heights but never reach the ground because the generally warmer air near the surface remains unsaturated. The descending precipitation evaporates into the dry air near the surface and people on the ground are never the wiser. However, evaporation also has a cooling effect so if precipitation falls into a dry pocket of air, it will both raise the humidity levels of that air but also drop its temperature as heat energy is pulled from the atmosphere in order to make the evaporation take place. If the air temperature is hovering around the freezing mark and is also somewhat below its saturation point, you might ask how much the evaporating water vapor will lower the temperature since, if the water vapor can lower the temperature below freezing (as it gets closer and closer to its saturation point), eventually precipitation should also arrive in a frozen form. If you want the answer to this question, the wet bulb temperature is the number you want to know. Here’s how you find it...
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Identify the LCL height for the pressure level you want by using the methods above
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From the LCL, draw a line along the moist adiabats down to the pressure level you want. This simulates the idea of descending, saturated air
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Read the temperature off the temperature axis. This is the Tw or, the temperature to which the air will drop if/when it becomes saturated. If that number is less than the freezing point, snow will likely occur.
SELECT STABILITY INDICES
K Index (KI)
Sometimes, especially in the Summer on hot and humid days, thunderstorms can break out without the presence of fronts, orographic lifting or any of the other mechanical factors that can force rising air. The atmosphere’s inherent instability is enough to produce thunderstorms. Known as "air mass thunderstorms," these pop-up storms are not usually severe but, if conditions are just right, still have the potential to pack a punch. The K-Index is a measurement that is used to predict the potential of air mass thunderstorms by looking at mid-level lapse rates combined with low level moisture. Keep in mind, though, that the K-Index is like LI in that it represents a potential for the appearance of thunderstorms but is in no way an absolute predictor. In fact, simple variables like elevation can make the atmosphere appear more unstable than it is if the K Index value is taken at face value. The chart at right shows what various K-Index values may mean for thunderstorm development. Here's how to determine the K-Index:
K = T 850mb - T 500mb + Td 850mb - (T 700mb - Td 700mb)
Lifted Index (LI)
LI is a crucial piece of information for severe weather forecasters. Essentially, the LI is a product of an air parcel’s buoyancy at 500mb. This is a bit different than CAPE which represents potential buoyancy over a much larger cross section of the atmosphere. In general, if LI values are negative, this translates into an air parcel that is warmer than the air around it which increases the chances the atmosphere will undergo deep convection and produce severe weather. However, it is also important to
keep in mind that a strongly engative LI value means that the potential for severe weather is enhanced if thunderstorms actually break out...it is not a direct indicator of whether or not they will occur. In fact, experienced forecasters often report strongly negative LI values where thunderstorms fail to materialize at all. LI is calculated through the following method:
LI = T 500mb - T parcel 500mb
Showalter Index (SWI or SI)
The Showalter Index (SI) is a Skew T indicator that assigns a number to the environment to reflect its overall stability and likelihood for convection. For SI values, 850mb is used as a start point instead of the surface since 850mb is generally near the top of the well mixed boundary level (here again, this is partly why 850mb is a mandatory pressure level). A thick (deep) boundary level can sometimes mask an unstable environment above it so by taking measurements starting where its effects wear off, a better feel for the overall atmosphere can be gained. To determine the Showalter Index:
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Find the T and Td at 850mb.
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From the 850mb T, draw a line parallel to the dry adiabat up on to the Skew T
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From the 850mb Td , draw a line parallel to the mixing ratio lines.
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The intersection point of these two lines will be the 850mb LCL. From the LCL, draw a line parallel to the wet adiabats up to a level of 500mb.
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Compare the temperature you just intersected with the actual temperature at 500mb.
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Subtract the 500mb parcel temperature from the actual 500 mb temperature. The resultant number will be the SI number.
Total Totals Index (TT)
Perhaps it's a goofy name but this index still does the job when predicting convective weather! The Total Totals Index is calculated by paying particular attention to conditions at 850 mb and 500 mb, both key mandatory pressure levels when considering severe weather. However, just like all the other indices, TT represents potential for convective weather, not certainty so it's always good to see if other indices are showing similar characteristics. One case in point would be when the atmosphere is exceptionally cold at 500 mb, a condition that can result when a feature called a "closed low" exists in the upper atmosphere (especially in winter). Although the 500 mb may be extremely low and give a high TT value, an intense cold pocket of air in winter is hardly the recipe for convective thunderstorms!
TT = (T 850 mb - T 500 mb) + (Td 850mb - T 500 mb)
Links
College of DuPage (IL) Sounding Data - The College of DuPage is the third largest community college in the US and has an excellent meteorology website. This page is an excellent page for Skew-T soundings with a special emphasis on the Midwest. CoD also has links to off hour soundings (15Z, 18Z, 21Z) if they were taken.
National Center for Atmospheric Research Upper Air Page - A federal government site for Skew-T diagrams. These Skew-Ts also include hodographs, important severe weather forecasting tools not covered in this lab. Mandatory pressure level maps also occupy a prominent place on the main Upper Air page.
NOAA Technical Acronym List - This list published by NOAA provides explanations to most acronyms used in their technical forecast products. This link will be especially useful when working with mesoscale discussions, convective outlooks and area forecast discussions which are generally not intended for the general public.
Penn State University E-Wall - An excellent one stop shopping webpage for all things meteorological including computer models, forecast soundings, satellite imagery, etc.
Storm Prediction Center - The federal government's primary office for severe weather and mesoscale meteorology. The site includes up to the minute maps/ charts showing a wide variety of atmospheric conditions. Includes current watches/warnings, mesoscale discussions, convective outlooks, storm reports, etc.
University of Wyoming Upper Air Maps - This site will produce Skew-T diagrams for the current time or from archived charts. Multiple Skew-T diagrams can be selected in order to produce Skew-T animations. Some archived Skew-T diagrams go all the way back to the 1980s.
Files and Downloads
Atmos. Stability Lab Part 1: Three Dimensional Atmosphere (Completed as class exercise)
Atmos. Stability Lab Part 2: The Skew-T Log-P Diagram
Atmos. Stability Lab Part 2: Take Home Quiz
Files and Downloads
Part 3 Data Sets: Skew-T Diagrams
Skew-T "A" Skew-T "B" Skew-T "C" Skew-T "D" Skew-T "E"
Part 3 Data Sets: Area Forecast Discussions