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"Don't knock the weather; nine-tenths of the people couldn't start a conversation if it didn't change once in a while."
-Frank M. Hubbard
Introduction
There is probably no natural phenomena that affects our daily lives more than the weather. Each day, atmospheric conditions play a major role in what we do, what we wear and even how we feel. It should therefore not be surprising that our fascination with observing exactly what is happening in the atmosphere is also high. In the United States alone, there are now more than 11,000 cooperative observers that volunteer to observe and report daily weather conditions around the nation in addition to other unmanned stations like the one shown at right. From National Weather Service professionals to multitudes of amateur weather enthusiasts, tens of thousands of observations are collected and evaluated each day in order to "build a better forecast." Weather can be like history; the past can foretell things about the future. During the early spring, for instance, fierce Pacific storms that bring rain and snow to the Pacific Northwest can scream into the Midwest spawning severe
A NWS ASOS (Automated Surface Observing System) station. These unmanned stations relay data from hundreds of sites around the US 24 hours a day. Image: NOAA
thunderstorms and tornadoes. Midwest forecasters prudently watch conditions to their west in order to make predictions about what weather they could receive in the near future. By the same token, tropical forecasters keep a wary eye to the east during the summer months since many Atlantic hurricanes are the result of atmospheric conditions that first show themselves off the coast of Africa.
Even during more benign times in the atmosphere, skilled US forecasters routinely consider weather conditions to their west in the hopes of more successfully predicting the weather over their own location. Frankly, there is always a big payoff for forecasters that focus on the minute details revealed in hourly observations made in hundreds of locations across the US each day since the atmosphere often "tips its hand." This investigation is designed to introduce you to some of the data sources used by professional meteorologists, how they use this data to create forecasts and finally, how to interpret the data they collect.
Telling Time
Although you have already been introduced to the 24 hour time system, meteorology is where you need to perfect its use. Note, for instance, the time stamp on the satellite image at right (indicated by the arrow). The "Z" is short for "Zulu" time, the military version of Greenwich Mean Time (GMT). Since meteorological observations are synchronized the world over, it makes sense that a universal time system is used across the planet. Zulu time effectively accomplishes this. Also known as the Universal Time Code (UTC), this hypothetical clock sets the entire planet on the same time system so if, for instance, global observations are to be taken at 1200Z and 0000Z, the observer’s local time is immaterial. Using UTC, all observers would go outside and take their measurements at the same time. Don’t get confused though...its easier than you think. "Z-Time", GMT and UTC are all the same thing and you do not have to memorize three different systems. Depending upon the actual source of weather data, any of these systems may be used for the time
Note this GOES satellite image's time stamp. In this case, the image was taken on October 7, 2010 at 0115Z hours or 9:15 PM (2115 hrs) EDT. Image: NESDIS
stamp. Notice the lack of a colon between the hour and minutes in the image above as well...its generally considered bad form so meteorologists usually do not use them.
To convert to local times, realize that each time zone accounts for about 15o of longitude on the Earth. As such, the East Coast of the United States, for example, lies five time zones behind Greenwich, England (the planet rotates counterclockwise). So, if it is 1200Z, the equivalent time in Philadelphia would be 0700 EST. Finally, do not forget that there is an additional one hour time offset that needs to be taken into account during the Summer months when "Daylight Time" is used. On the East Coast, the conversion is only 4 hours as opposed to the regular 5. Click here for a full scale world time zone map
Station Models
Since immense amounts of weather data are collected every day, meteorologists need systems of shorthand to read it all quickly. If this data was relayed in text form, it would take volumes of pages just to report it all. For this reason, the NWS organizes observations into coded symbols called station models. Station models depict the current weather conditions and for certain characteristics the recent past with numbers and simple symbols in a compact manner that allows the meteorologists to quickly figure out exactly what is happening at any location where data has been collected. On a weather map, these models are placed directly on top of where the observation was made. The figure at right illustrates how to interpret them. Now, don’t panic! First, station models NEVER have all of these symbols in them at the same time. More often than not, they usually only contain about half of
these symbols without much variety so once you learn the basics, the rest is pretty simple.
There are a couple of other things you should know about the models to get started. First, with the exception of the wind direction flag, station model data is always shown in the same location on the model. For instance, air temperature is always at the "10 o’clock" position with regard to the model’s central circle. If information is unavailable for any data type, that area is simply left blank. Second, this is the United States so most meteorological measurements still use the old Imperial measurement system and not the metric system so temperature readings, for example, will always be in degrees F. However, especially if the data is from other nations, the measurements will likely be in degrees C (like Canada) so be careful. In most cases, a bit of thought to consider what temperature scale just "makes sense" is the best tool to figure out whether or not it was measured in degrees F or C.
This weather map (with analyst notations) indicates how a typical set of station models might appear. In this case, a strong influx of warm, moist Gulf air is meeting cooler, drier air from the north behind a low pressure system centered in western Kansas. This formula is a classic for severe weather and several tornados were reported in Kansas that afternoon. Click here for a stormchaser video shot at the outset of the event.
Dew Point Temperature
Although this is a rather generic measurement, its importance is often undercut by the general public’s lack of understanding. For the record, although relative humidity is the most often reported water indicator on TV weather, its complete absence from station models should hint at its lack of true scientific value. It has its uses but generally the dew point temperature is a much more effective measurement when speaking about atmospheric moisture. The dew point temperature is the temperature to which a parcel of air would have to be cooled in order to make the water vapor present in it condense into liquid water droplets. In other words, the point at which the atmosphere would reach 100% relative humidity or become saturated. Life experiences should tell you that temperature should play some role in this process. You do not have to look any further to see my point than dew on
a field of grass on a summer morning, beads of water on the side of a glass filled with a cold drink or frost on a window pane in the winter. For a forecaster, the dew point temperature hints at the amount of moisture hidden in the air. Although it may be invisible (in vapor form) when an observation is made, its presence can be a time bomb for severe weather later in the day so whether or not the observer can see it or not is immaterial...the fact that it is there is what is important.
To understand how dew point can tip off a forecaster, hypothetically imagine the air temperature is 72F in two towns in Montgomery County. Town A’s dew point is 62F and Town B’s dew point is 42F. Which town has more water vapor in the air? At first, you may think this is impossible to predict because you are used to using "relative humidity" as a function of water content. But think of it this way...100% relative humidity is reached when the air temperature and dew point are the same. That means that Town A’s temperature would have to drop 10F in order to reach the dew point while Town B would have to drop 30F. Therefore, Town A has more water vapor in it and a higher humidity.
Wind Speed and Direction
Like dew point, wind is not what would typically be considered a special measurement in weather forecasting. However, reporting wind direction often causes problems for the novice observer since we tend to think about wind in terms of the direction it is blowing. However, the direction a wind is blowing is not nearly as important as where the wind came from. Whether or not a wind is cold, hot, wet or dry lies in its origin, not its destination. For this reason, wind direction is always reported (and drawn on a station model) according to the direction it came from and not where it is blowing. For instance, look at the Wind Speed and Direction chart (below right) and the example wind barb. Note the wind staff is sticking out of the central circle at the 10 o’clock position. This means the wind is a northwesterly wind (although it is blowing southeast). For the record, northwesterly winds in the Mid-Atlantic states are generally cool and dry since they originate in Canada. In the Great Lakes region however, northwesterly winds are a prime component of the Lake Effect snows that hit the leeward (eastern) sides of the Lakes each winter (at right). Nor’easters, coastal winter storms common to the Eastern Seaboard of the United States, got their name from fishermen who always noticed the onshore winds originated from the north-northeast as the storm approached. It is a common misconception that Nor’easters actually got their name due to the fact they generally move in a northeasterly direction up the coast…not true!
Classic Lake Effect snows as seen by a POES satellite. Notice how the snow is concentrated on the eastern sides of the Great Lakes as cold, northwesterly winds howl down out of Canada into the US. Image: NOAA
Reporting wind speed is a bit more straight forward but it is always best to consider the wind in terms of knots instead of miles per hour when building station models (1 kt = 1.15 mph). Again, this is a leftover from transportation since both air and sea traffic refer to winds in terms of knots (not miles per hour). For station models, wind speed is indicated by adding a "half barb" (also called a hachure) for every 5 kts of wind (right). At 50 kts, a pennant is used but then the barbs continue until two pennants are used to indicate 100 kts, etc. For musicians in particular, this pattern should not be very difficult to figure out since eighth notes, sixteenth notes, etc. follow the same basic pattern. In all honesty, converting between knots and miles per hour is no big deal and can be achieved by simply taking the base number and adding or subtracting "a little" depending on which conversion is being made. Oddly, the use of knots does not carry over into the Saffir-Simpson Scale (hurricanes) or the Enhanced Fujita Scale (tornado) scales. These standards are divided according to easy to remember miles per hour increments. For instance, the minimum sustained wind speed for a Category 1 hurricane is 75 mph (as opposed to 64 kts).
Barometric Pressure
Atmospheric pressure is one of the largest determining factors in global weather patterns and also one of the easiest facets of weather to track. Because of the indicative spins of low pressure systems and the clear skies of high pressure systems, pressure centers and their impact on local barometer readings are among the most important indicators of both future weather trends and passing storm centers. In the case of station models, barometer readings are always placed to the upper right of the center circle.
However, barometer readings do have a problem. Because a typical barometer reading will have 5 digits and would be annoyingly long to write out (xxxx.x mb: station models always use millibars), barometer readings are coded into 3 digit abbreviations. For this reason, it is vital that you understand what a "reasonable" pressure reading is on a given day. First, you must understand how a barometer reading is actually interpreted.
Take a look at the figure at right. The pressure reading is shown at the upper right of the station circle and is listed as "089". So, that means the barometric pressure is 89 mb, right? Remember, that a "normal" pressure reading near the surface should hover around 1000-1020 mb so a reading of 089 mb is only about 1/10 the normal pressure and is obviously either wrong or encoded. In this case, it’s encoded. When writing pressure readings, station models leave out either the first digit of the reading if it was a "9" or the "10" if the reading was over 1000 mb. Also missing is a decimal point between the last two digits.
So what is the pressure in the figure? This is where the forecaster must understand pressure. If a "9" was to be inserted in front of the coded numbers and the decimal re-added, the pressure would be 908.9 mb, an extremely low reading normally only found in the most severe of storms. The lowest reading ever recorded was in Typhoon Tip in 1979 at 870 mb. In the Atlantic, Hurricane Wilma of the record breaking 2006 hurricane season shattered the former lowest barometer record of 888 mb (H. Gilbert in 1988) when it reached 882 mb. So, unless this station model was produced near an intense hurricane, the pressure can not be 908.9 mb...it just doesn’t make sense. However, placing a "10" in front the 089 would make a pressure of 1008.9 mb, a perfectly normal reading. So, as indicated earlier, it is very important a forecaster "self checks" his/her interpretation of the data so it makes sense.
Finally, the last pressure reading shown on station models is known as the "barometric trend." Located directly beneath the pressure, a pressure trend is also coded in the same way as the pressure itself. Using the same figure, the pressure trend is shown as "-26". This actually translates into a change of -2.6 mb since the last time the conditions were checked (usually 3-6 hours). If the number is negative, it means the pressure has dropped by the amount indicated. Again, think about what makes sense considering the pressure discussion we already had. A drop of 26 mb in pressure is an immense drop and, for a period of only 3-6 hours, practically unheard of. There is a phenomenon known as "bombogenesis" when a storm intensifies so rapidly that its pressure "bombs out" but even this type of
event is usually characterized by pressure drops of about 18-24 mb in a 24 hour period, not 3-6 hours. Some of the Mid-Atlantic's most notorious winter Nor'easter events have been bomb cyclones.
The line symbol next to the pressure trend shows how the pressure has been changing over the same period as the pressure trend number. In our example, the symbol can be interpreted as "...the pressure has been consistently dropping since the last time it was observed..." The figure at right shows the other symbols that are used in this part of the station model.
Before I finish with pressure, let's remember one more important thing. All of the measurements that have been discussed here are based on typical sea level pressure. If you want to examine pressure readings in a place like Denver, CO (elev. 5,280 ft.), you must remember that what would be considered "normal" pressure there is noticeably less than cities along the coast.
Sky Cover
For now, we will not worry about telling the difference between low, middle and high clouds but will instead concentrate only on how to correctly fill in the center circle of a station model with regard to sky cover. In general, the sky cover indicator shows the relative amount of clouds in the sky and is one of the few areas of a station model that is open to some interpretation...it’s not meant to be an exact science. There are two general systems that can be used for sky cover...one that divides the sky into 8ths and the other that divides the sky into 10ths. Personally, it does not matter (to me, anyway) which system you use when working on station models as long as you follow the basic rules that are common to either system.
However, sky cover is never filled in like pizza pieces with little wedges representing the percentage of sky covered by clouds. This is one of the most obvious rookie forecaster mistakes. Be sure to refer to the station model symbol chart before you make this mistake! For additional help, consider the photo of a U.S. Army facility in Iraq taken in 2002. Assuming a sky observation could be taken in this dust storm, the sky cover would be reported as "obscured" since sky cover only refers to actual atmospheric clouds and not other phenomena like smoke, dust and haze that would block a forecaster's view of the sky.
The table below uses the 8ths system and also shows you how the television news system ties into the meteorological version. Ever wonder what the difference was between partly cloudy and partly sunny? Now you know.
Visibility and Weather
It is important to keep in mind that one of the primary uses for station models is for aircraft pilots. For that reason, all U.S. airports are required to track and report meteorological data as long as they are open. For obvious reasons, visibility is one of the chief factors that pilots are concerned with so it is frequently encountered on station models and is guaranteed if the observation was made at an airport. In the example on the last page, "3/4" was indicated to indicate the horizontal visibility was three-quarters of a mile (not very far if you’re in a plane). But, how is that number determined?
Since visibility can vary considerably as you turn around in a circle and look towards the horizon, there a few basic rules to follow when reporting this measurement. Weather observers report the distance that objects can be seen over more than 180 degrees of the 360 degree view around them. Believe it or not, this takes quite a lot of training to get it right.
When precipitation or some other special type of event is occurring in the atmosphere near a station, it is also always reported since this can have an impact on any number of human activities especially flight. Remember, that airports are required to gather and disseminate weather information since any adverse conditions can have a massive impact not only on the behavior of the airplane but also conditions on the runway. It can obviously affect visibility as well so airports are required to report any visibility inhibitors within a seven mile radius of the airport. Why seven miles? The reason for this lies in the fact that an airport’s "outer marker" (a radio beacon that aircraft use to guide them to the runway) lies 7 miles away from the runway (shown at right). Visibility will always be shown at the 9 o'clock position of the station model with a "weather" symbol (WX) right inside it.
Scientifically speaking, visibility decreases because light waves scatter off of the water droplets in the atmosphere instead of hitting your eyes. For more practical purposes, the amount of scattering can be deduced by looking at the WX symbol on the station model. The higher the precipitation rate (deduced by the symbol), generally the lower the visibility. Here’s another little tip...anytime the visibility is less than 7 miles, there will always be a reason why somewhere on the station model. Also, even if visibility is not reported (i.e. visibility is farther than 7 miles), it is still customary to report any WX information.
Links and Downloads
Introduction to Atmospheric Variables lab
International Station Model Symbols (Web based)
International Station Model Symbols (Chart form)
Basic Station Model Symbols and Coding (Penn State) - Part 1, Part 2
Links
National Center for Atmospheric Research (METARs)
Aviation Weather Center (METARs)
Cooperative Institute for Meteorological Satellite Studies (Station Model practice)