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Central Illinois Skywarn: Brief Explainations
The following material is extremely short and extremely simple; it is intended only to give those unfamiliar with the types of weather data to which this site links some idea of what they are viewing and what the indications of severe weather are. Some of the items now also contain links to information at other web sites in addition to the general links under For More Info.
Explain:
Using various instruments and data from computer models, meteorologists can predict the chance of thunderstorms in a given area--and whether they will be severe. However, as we all know, weather science is far from exact. Therefore, storms may be more severe than they were predicted to be, pop up without being predicted, or may not develop though predicted.
The lower the lifted index number, the greater the chance of storms--and severe storms. The higher the lifted index number the less the chance of storms.
Dew Point is the amount of water vapor the air can hold at a given temperature. The higher the dew point the more likely there will be storms--and severe storms. Moist air is more unstable than dry air.
A thick blue line is a cold front. A thick red line is a warm front. A dashed yellow line is a trough. A trough is a wind shift line. That means on one side of the trough there may be a south wind, on the other a southwest wind. Severe weather is most likey to develop along a trough line or a cold front.
A solid purple line is an occulded front. That is a cold front that has overtaken a warm front. There will generally be heavy rain along an occulded front--and sometimes severe weather.
Thick alternating blue and red lines represent stationary fronts; as the term suggests, they are fronts that are not moving, or are hardly moving. For our purposes, stationary fronts need not concern us.
The thin blue lines are isobars; they show areas of equal atmospheric pressure. The closer the isobars are together, the higher the wind speed at the surface. Atmospheric pressure is measured in units called millibars. When the millibars increase in one direction that is a high. When the millibars decrease in one direction that is a surface low. There is a greater likelyhood of storms in the direction of a surface low.
The circles indicate the locations of reporting weather stations. The fraction of a circle that is filled indicates the amount of cloud cover. For example, a half filled circle means a half overcast sky.
The lines coming from the circles show the wind direction; the arrows perpendicular to those lines are called feathers and show the wind speed in miles per hour. A long feather represents ten miles per hour. A shorter feather represents 5 miles per hour. The more feathers the higher the wind speed. The higher the wind speed the greater the chance of severe storms and tornadoes.
The number above and to the left of the circle is the temperature at the reporting station. The number directly below that is the dew point. If you've already read about dew point, you know the higher the dew point, the greather the chance of storms--and severe storms.
Also, much of the information on the surface data plot can be found in text form by taking the Illinois Current Conditions (text) link on our Current Conditions Page.
See the surface data plot key.
Generally, the higher the temperature, the greater the potential for severe storms to develop if the right atmospheric conditions exist (very moist air).
Those who want to know about reading weather maps, may go to
Colored areas represent precipitation (eg. rain). The brighter the more intense, the lighter the less intense. Red generally indicates the most intense precipitation, light green the least intense. However, colors may vary on radar displays. In some cases, it is also possible to detect tornadoes and other severe weather phenomena with radar. See the
What you see is what you get. The white areas are clouds. The darker areas are land; that, of course, means there's little or no cloud cover over the area. The whiter the clouds, the higher their tops. High cloud tops often mean there's a thunderstorm because the tops of cumulonimbus clouds (thunderheads) usually rise to great heights.
Orange areas represent warmer areas, most often land. The ligher the orange, the less likely it is there are clouds over an area. Blue areas are colder areas--clouds. Since cloud tops are cooler at higher elevations, the darker the blue, the higher cloud tops. If you've read the previous item, you know high cloud tops may mean there are thunderstorms due to the fact thunderhead tops usually rise to great heights.
As most will know, infrared images can be seen at night; whereas, visible images can't be seen at night. Colors may vary on different satellite images.
Shows the water vapor that makes up the clouds. It can be seen both day and night. Like the visible image, the white areas are clouds; and the whiter they are the higher the cloud tops. You may already know, high cloud tops can indicate thunderstorms. The dark areas show clear skys. A black streak indicates the jet stream--actually a jet stream since there may be more than one. A swirl is an area of low pressure.
Other Satellite Information Resources:
Those who would like to delve further into the interpretation of weather satellite images may want to check out
San Fransico State University's
Also,
Before tornadoes you would most likely see:
However, this is only the classic example. Weather is
The Storm Prediction Center's Tornado FAQ is an awsome site for information on Tornadoes.
You may want follow the NOAA Storm Spotter's Guide link on our home page. You may also want to see the
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End of Skywarn: Brief Explainations.
Almost any typical introductory, college-level textbook on meteorology could provide the necessary background. Some examples with which I'm familiar include:
Indications of Tornado Risk:
For More Information:
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Last revised: March 26, 2000. © Mark Sefried