University of Illinois Extension
How to read a weather map
 

[June 17, 2021]    We watch the news, search the web, or go to the weather app to see what the great outdoors is like at the moment and what it will be for the next few days. You may go ahead and look at the weather map. It typically has many lines, symbols and colors. Any idea what it all means?

A few years ago I provided content for a series of short weather videos on 17 weather topics. The one that by far has received the most views: How to read a weather map! I was really surprised by that, but sometimes, we take for granted that people fully understand what is being shown. So, let’s talk about that.

Air pressure

First let’s talk about the H’s and L’s. These are symbols for air pressure, essentially the amount of force exerted by air. At sea level, almost 15 pounds of force is exerted on every square inch of earth’s surface. Pressure can vary slightly from place to place. H’s stand for higher pressure, L’s for lower pressure. A general rule is that high pressure means fair weather, while low pressure brings cloudiness or storms. Another rule is that air will flow from high to low. Winds blow in a clockwise motion away from high pressure and spiral in a counter clockwise fashion into low pressure. That’s in the northern hemisphere. We won’t talk about the southern hemisphere pressure systems, but they spin differently.

Fronts

The blue lines with triangles are called cold fronts. They represent the leading edge of colder air. The triangles are pointed in the direction the cold air is going. Red lines with semi-circles are warm fronts, the leading edge of warmer air. The semi-circles also point to where the warm air is going.

An alternating red and blue line means there is warmer air and colder air next to each other but neither is moving, hence the name stationary front.

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Rarely discussed, a purple line or red and blue line with both triangles and semi-circles on the same side of the line is called an occluded front. Sometimes, the cold front catches up to a warm front, lifting the warm air completely off the ground. Main thing to remember about this line is that it means the low pressure it’s associated with is about to die out.

Fronts are only associated with low pressure. You’ll never see any frontal line coming out of an H. If you do see a front coming out of an H, you’ll know the person making the map doesn’t have a clue about the weather. I did see that once a few years ago on an insurance commercial. I’m sure they got a lot of letters from weather geeks. And no, while I am one of those, I didn’t send one.

Troughs

Depending on the map you may also see dashed lines. This is called an upper air trough. It basically means that area may be having clouds with some precipitation, but typically it’s not too extreme.

White lines show changes in air pressure. Numbers on these lines are called millibars, a measure of force. The numbers usually range from about 1000 to 1020 millibars. If you see something lower than 1000 it will be an intense area of low pressure, like with a hurricane. Above 1020 means strong high pressure, probably associated with cold, clear air. The closer the lines are together, the greater in the wind in that area.

That’s the basics of a weather map. Now go forth and impress your friends and neighbors.

[Written by Duane Friend, University of Illinois Extension, Energy and Environmental Stewardship Educator]