Station model
A station model is a symbolic illustration used on weather maps that represents and reflects the weather occurring at a given reporting station. When analyzing weather maps, a station model is plotted at each point using either its surface weather observation or its observation aloft based upon its radiosonde or pilot report. The convention of coding station models has changed little since August 1, 1941. Within the station model, the temperature, dewpoint, wind, sea level pressure or height of the respective pressure surface above sea level, pressure tendency, and ongoing weather are plotted.[1] The circle in the middle represents cloud cover.[2] Outside the United States, and for station models seen on upper level charts, temperature and dewpoint are plotted in degrees Celsius. In the United States, the quantities plotted within the station model can be in Imperial units, such inches or degrees Fahrenheit.
Once a map has a field of station models plotted, isobars (lines of equal pressure), isallobars (lines of equal pressure change), isotherms (lines of equal temperature), and isotachs (lines of equal wind speed) can be analyzed.[3] The abstract present weather symbols used on surface weather analyses for obstructions to visibility, precipitation, and thunderstorms were devised to take up the least room possible on weather maps.
Plotted winds
More than a century ago, winds were initially plotted as arrows facing downwind, with feathers on both sides of the staff to indicate wind direction.[4] In the United States, the change to the modern convention of flags shown on one side of the staff to indicate wind speed took effect on August 1, 1941.[5][6] Half of a flag depicted five knots, a full flag depicted 10 knots, while a pennant (filled triangle) on the staff stood for 50 knots of wind.[7]
Winds have a standard notation when plotted on weather maps. Winds are depicted as blowing from the direction the flags are facing. Therefore, a northeast wind will be depicted with a line extending from the cloud circle to the northeast, with flags indicating wind speed on the northeast end of this line.[8] Once plotted on a map, an analysis of isotachs (lines of equal wind speeds) can be accomplished. Isotachs are particularly useful in diagnosing the location of the jet stream on upper level constant pressure charts, usually at or above the 300 hPa level.[9]
Cloud cover
Along with wind direction, cloud cover is one of the oldest atmospheric conditions to be coded on a station model.[4][10] The circle in the middle of the station model represents cloud cover. In the United Kingdom, when the observation is taken from an automated weather observation site, the shape is a triangle.[11] If the shape is completely filled in, it is overcast. If conditions are completely clear, the circle or triangle is empty. If conditions are partly cloudy, the circle or triangle is partially filled in.[2] The cloud cover shape has different looks depending upon how many oktas (eighths of the sky) and covered by cloud. A sky half full of clouds would have a circle that was half white and half black. Below the shape indicating sky cover, the station model can indicate the coverage of low clouds, in oktas, and the ceiling height in hundreds of feet. The ceiling height is the height at which more than half the sky is covered by clouds.
Knowing the degree of cloud cover can help determine whether or not various weather fronts, such as cold fronts or warm fronts, have passed by a location. A nephanalysis, contouring areas that are cloudy with scalloped lines, can be performed to indicate a system's cloud and precipitation pattern.[12] This technique is rarely performed nowadays, due to the prevalence of satellite imagery worldwide.[13]
Cloud types
Above or below the shape indicating sky cover can lie a symbol indicating cloud type. One cloud type is depicted for each of three possible cloud layers, if known. The top two cloud layer types are depicted above the sky cover circle in the station model, while the lowest cloud type is indicated below the sky cover circle.[8] The symbols used for clouds emulate the cloud shape. Cirrus is indicated by a couple hooks, cumulus are indicated by a mound shape, with cumulonimbus indicated with an upside down trapezoid on top of the cumulus symbol to indicate its anvil. Since the station model has limited room, when there is more than one cloud type present per level, the cloud type with the highest priority is included.[14] Knowing the cloud type at various locations can help determine whether or not a weather front has passed by a particular location. A low deck of stratus could indicate a station is still north of a warm front, while thunderstorms can indicate the approach of a squall line or cold front.
| Low Cloud Types | Mid Cloud Types | High Cloud Types |
|---|---|---|
 |
 |
 |
Present weather and visibility
To the left of the cloud shape in the center of the station model is the symbol depicting present weather. The present weather symbol depicts the current weather which normally is obstructing the visibility at the time of observation. The visibility itself is shown as a number, in statute miles in the United States and meters elsewhere, describing how far the observer can see at that time. This number is located to the left of the present weather symbol.[8] Precipitation, fog, and thunderstorms are the predominant present weather symbols shown on the station model. Dust, sand, and haze are also depicted using their own present weather symbols.
Temperature and dewpoint
To left of center in the station model, the temperature and dewpoint are plotted. Within the United States on surface weather maps, they are still plotted in degrees Fahrenheit.[8] Otherwise, they will be in units of degrees Celsius. When plotted on a map, isotherms and isodrosotherms (lines of equal dewpoint) can be analyzed.
Sea level pressure and height of pressure surface
On the top right corner of the model for a surface weather map is the pressure, showing the last two integer digits of the pressure in millibars, or hectopascals, along with the first decimal. For instance, if the pressure at a certain location is 999.7 hPa, the pressure portion of the station model will read 997. Although the first digit or two of the pressure is left off, other nearby stations give away whether or not the first digits is a 10 or a 9. Most of the time, choosing first digits that would lead to a value nearest to 1000 works out best.[8] The plotting of this value within the station model allows for the analysis of isobars on weather maps. Within maps which plot data on constant pressure surfaces, the pressure is replaced with the height of the pressure surface.[15]
Pressure tendency
Below the pressure will lie the pressure tendency figure, which indicates the pressure change over the past three hours. The number depicting the pressure change will usually have two digits and indicate the pressure change in 0.1 millibar increments. There are nine different figures which represent the pressure change. A slant angled up and right indicates a steady rise, while a downward slant to the lower right indicates a steady fall. Steady rises can indicate improving conditions and approaching high pressure, and normally occur in the wake of a cold front. Steady falls can indicate deteriorating conditions and an approaching low pressure area, with the largest falls occurring in advance of a surface cyclone and its attendant warm front.[16]
The time of the day must be considered, as there are two natural rises per day (locally around 10 am and 10 pm) and two natural falls per day (locally around 4 am and 4 pm). These daily changes in pressure can mask the movement of pressure systems and fronts past a location. The lowest natural dip in the pressure, in a stagnant weather pattern, occurs around 4 pm while the highest natural peak in pressure occurs around 10 am.[17] Once plotted on a map, the analysis of isallobars (lines of equal pressure change) can be plotted on a map, which can indicate the direction of motion of high and low pressures systems across the map domain.[18]
Past weather
Station models can have past weather plotted within them, which would be located just under the pressure tendency. They indicate the type of weather experienced during the past six hours. The weather types are limited to obstructions of visibility and precipitation.[8]
See also
References
- ^ National Weather Service. Station Model Example. Retrieved on 2007-04-29.
- ^ a b Dr Elizabeth R. Tuttle. Weather Maps. Retrieved on 2007-05-10.
- ^ CoCoRAHS. INTRODUCTION TO DRAWING ISOPLETHS. Retrieved on 2007-04-29.
- ^ a b United States Weather Bureau. Daily Weather Map: January 1, 1871. Retrieved on 2008-01-22.
- ^ United States Weather Bureau. Daily Weather Map: July 31, 1941. Retrieved on 2008-01-22.
- ^ United States Weather Bureau. Daily Weather Map: August 1, 1941. Retrieved on 2008-01-22.
- ^ Hydrometeorological Prediction Center. Decoding the station model. Retrieved on 2007-05-16.
- ^ a b c d e f JetStream. How to read weather maps. Retrieved on 2007-05-16.
- ^ Terry T. Lankford. Aviation Weather Handbook. Retrieved on 2008-01-22.
- ^ United States Weather Bureau. Daily Weather Map: July 31, 1941. Retrieved on 2008-01-22.
- ^ MetOffice. Interpreting weather charts. Retrieved on 2007-05-17.
- ^ Glossary of Meteorology. Nephanalysis. Retrieved on 2008-01-22.
- ^ AEROGRAPHER'S MATE 1&C. SHORT-RANGE EXTRAPOLATION. Retrieved on 2008-01-22.
- ^ JetStream. Cloud type priority. Retrieved on 2007-05-17.
- ^ American Meteorological Society. Map Symbols. Retrieved on 2008-01-22.
- ^ University of Wisconsin-Madison Department of Atmospheric and Ocean Sciences. Air Masses and Fronts. Retrieved on 2008-01-22.
- ^ Frank Singleton. Tides in the Atmosphere. Retrieved on 2007-05-16.
- ^ Dr. Giles Harrison. Isobaric analysis and pressure tendency. Retrieved on 2008-01-22.
