University of Illinois Extension
How to read a weather map
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[June 22, 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.
![](http://archives.lincolndailynews.com/2021/Jun/17/images/ads/current/LPL_small%201.gif)
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. [to top of second
column] |
![](http://archives.lincolndailynews.com/2021/Jun/17/images/ads/current/graue_21ENCLAVE_060721.png)
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] |