U S Weather on October 21, 2006: Oklahoma City is about to get hit by a cold front.
Reading a Weather Map
Weather is whatever happens to the air around us each day: things like rising and falling temperatures, winds blowing, rain or snow falling, and cloudy gray or clear blue skies. Meteorologists are the scientists who use what's happening in the weather today to predict what will happen tomorrow. They pore over complex and detailed weather maps that display surface conditions over a wide area, maps that provide a variety of information they need to make their predictions. For we ordinary people, however, televisions, websites and newspapers publish generalized maps. These maps summarize the details and show us part of the data, which helps keep ordinary people aware of what's to come with the weather.
Detailed weather maps display a wealth of information, but with just a generalized map or set of maps you can still make your own predictions or, at the least, understand what's happening in the sky above you. So, what kind of information can you get from a weather map?
Hot or cold? Your thermometer knows!
The weather maps the pros use show air temperature at hundreds of official stations, but for a layperson the data is usually summarized. Depending on the purpose of the map, it might display current temperatures, high or low temperature for the previous day, or tomorrow's predicted temperature. Generalized maps don't show all the posted temperatures: instead, they use contours called isotherms, lines connecting points of the same temperature, to show areas of similar temperature. Using color makes the differences in of temperature stand out even better: hot areas are usually in shades of yellow to red and cold areas are blue to violet.
A detailed weather map also shows a feature called the dew point temperature, which is the temperature at which the air's relative humidity would be 100%. This information helps forecasters predict rain or snow. For ordinary folks, a low dew point, say 40Â°F, means the air is dry and comfortable. A high dew point, such as 75°F, indicates moist and sticky air.
Surface weather maps let you visualize how atmospheric pressure, or barometric pressure, varies from place to place. This is important because masses of air with low or high atmospheric pressure move around on the surface, changing the weather as they pass. A high pressure system is usually accompanied by warm, moist air; while cool, dry air makes up low pressure systems. Low pressure cells are often accompanied by storms, with the lowest pressures ever recorded at the center of hurricanes.
Official weather stations report barometric pressure along with the other data. The pressure readings are printed at each station on detailed maps, while on generalized maps barometric pressure is represented by a contour map. Isobars, or lines connecting points of equal pressure, curve across this map. These lines are labeled to allow readers to identify regional trends in pressure and locate zones of high and low pressure. By convention, the centers of high- and low-pressure cells are marked by large letters H and L, respectively.
A front is the leading edge of a moving mass of warm or cold air. Mapmakers use a heavy line to show the approximate location of a front, and add symbols to show the direction the air mass is moving. The lines marking cold fronts have triangles added that point in the direction the cold air mass is moving. Warm fronts are marked by half-circles on the side toward which the warmer air is moving. Sometimes an air mass stops moving, becoming what meteorologists call a stationary front. This front is marked by triangles on the cool side and half circles on the warm side.
Generalized weather maps show predicted rain or snow. Areas where precipitation is likely to occur are identified by patterns on the map; with special symbols for thunderstorms, hail, sleet and ice.
Detailed maps show the degree of cloud cover at official stations by using coded circles. An empty circle means the station reports clear skies, while a filled circle means conditions are overcast.
Wind is plotted on detailed maps using special symbols that show both the wind's speed and direction. The symbols have small flags showing the average wind speed in knots (1.151 MPH or 1.852 km per hour). Wind information is not displayed on general maps, although an experienced reader can interpret wind direction and estimate speed from other information.
Could You Predict an Approaching Storm?
As this cold front advances through a Dutch town, a roll front cloud marks the leading edge of a sharp line of thunderstorms.
So: Making your Prediction
Start by looking at the fronts and which directions they're moving. If there are no fronts approaching your area, then chances are good that tomorrow's weather will be a lot like today's. If, however, there's a front bearing down on you, expect a change. What kind of front is headed your way controls the kind of weather you can expect.The movement of fronts is important, especially cold fronts:
- A passing cold front usually brings storms, sometimes even violent thunderstorms containing tornadoes. Once the cold front has moved past, the air will be cooler and drier than before.
- Warm fronts aren't usually accompanied by storms, but they may bring large areas of overcast skies and steady rain. The air will be warmer and more humid behind a warm front.
- A stationary front can wreak havoc over a small area, as storms follow the front like trains on a track. This "training" effect can caused localized flooding as large amounts of rain fall on a narrow band.
- Cold fronts usually move faster than warm fronts, so they approach more rapidly, but pass by more quickly.
You can make some guesses about tomorrow's wind by studying the maps, especially the isobars and locations of high- and low-pressure cells. Things to keep in mind are:
- The winds circle low-pressure cells counterclockwise and high-pressure cells clockwise.
- The wind usually blows more or less parallel to the isobars in a region.
- The isobars also allow you to estimate how hard the wind is blowing: it will be windy where isobars are close together and almost parallel, and more calm where the isobars are far apart and wander about the map randomly.
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Anyone can be an amateur meteorologist, so take a look at today's weather map and make your own prediction about tomorrow's weather!