The weather symbols and types of weather measurements described below are common parts of weather forecasts.
Warm and Cold Fronts
- Blue lines are cold fronts and red lines are warm fronts.
- High and Low Pressure Systems
- A blue "H" means a center of high pressure (usually calm, sunny weather).
- A red "L" means low pressure (which can mean storminess). Most fronts extend from low-pressure centers.
Each circle represents a weather station. The circle at each station is empty if skies are clear. The circle is white if it’s cloudy. A line within the circle or a half-filled circle means "partly cloudy."
Wind and Wind Direction
The little arrows attached to each weather station point in the direction the wind is blowing from. The more barbs at the end of each arrow, and the longer they are, the harder the wind is blowing. Each long barb is 10 knots (about 11.5 miles per hour or 18 kilometers per hour). Each short barb is half that amount. A barb that looks like a triangle is blowing at 50 knots (about 58 mph or 80 kph).
The number to the lower left of each station is the dew point temperature in degrees F (for U.S. maps) or degrees C (for other countries). The dew point is a measure of moisture; it shows how much you'd have to cool the air to get a relative humidity of 100 percent. The higher the dew point, the more water vapor there is for producing rain or snow.
The number to the upper right of each station is the barometric pressure. Since the pressure goes down with altitude, this reading has been adjusted to show the pressure as if the station were at sea level. The typical sea-level pressure is a little bit more than 1000 millibars. (The number is in kilopascals (kPa), which is the same as millibars).
The number has been compressed to fit the map by lopping off the first one or two digits (which are always a "10" or a "9") and omitting the decimal point before the last digit. For example, the code "085" would mean 1008.5 millibars, while 954 would be 995.4 millibars.
On a map of barametric pressure, you'll find lines, called isobars that go around centers of high and low pressure. They connect places with equal barometric pressure, so you can see where the highs and lows are. The wind usually follows the isobars, with a slight trend in the direction of the low pressure area.