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.
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.
Never miss a newpost!Sign up for our email list.
MEET THE AUTHOR
Duane Friendis an energy and environmentalstewardship educator with University of Illinois Extension, serving the organization in many roles since 1993. Duane provides information and educational programs to adult and youth audiences in the areas of soil quality, weather and climate, energy conservation, and disaster preparedness.These programs provide practical solutions for families, farms, and communities. He assists families in creating a household emergency plan, farmers with the implementation of soil management and conservation practices, and local government officials and business owners with energy conservation techniques.
ABOUT THE BLOG
All About Weatheris a blog that explores the environment, climate, and weather topics for Illinois. Get in-depth information about things your weather app doesn't cover from summer droughts to shifting weather patterns.
FAQs
These maps include symbols representing different weather phenomena like rain, snow, and fog. For instance, dots represent rain, while asterisks signify snow. Additionally, surface maps show front lines, indicating the boundaries of different air masses and often the zones of most significant weather changes.
What do the symbols on a weather map mean? ›
The central symbol tells you wind speed, direction, and cloud cover. The left symbol tells you the current weather. Cloud symbols above and below tell you what type of cloud can be seen. And the numbers tell you temperature (top-left), pressure (top-right), and precipitation (bottom right), among other things.
How to read a weather map like a professional meteorologist? ›
For example, high pressure (H) areas will have clear skies, while low pressure (L) areas can be stormy. Blue "cold front" lines bring rain and wind in the direction the triangular marks point. Red "warm front" lines bring brief rain followed by warming in the direction of the semi-circles.
How do you read a map? ›
North is directly up on standard maps; south is directly down; east is directly right; and west is directly left. If this is ever not the case, there will be something on the map that tells you this. This will usually be an arrow, a cross, or some kind of pointed star that shows which direction is north.
How do weather maps work? ›
A computer draws a station model for each observation location. The station model is primarily used on surface-weather maps, but can also be used to show the weather aloft. A completed station-model map allows users to analyze patterns in air pressure, temperature, wind, cloud cover, and precipitation.
How do I read the colors on a weather map? ›
The greens indicate light or moderate rainfall. Yellow, orange, and red indicate moderate to heavy rainfall and can also indicate hail. Whites or blues mean snowfall and pink indicates freezing rain, sleet, or a wintry mix.
What do the symbols H and L mean on a weather map? ›
“H” and “L” on weather maps, called high and low pressure centers, indicate the locations of areas of relative highest and lowest air pressure. These places are determined solely by air pressure, but it's valid to generalize about the kinds of weather associated with them.
What does the letter L mean on a weather map? ›
A region of low pressure, marked as "L" on a weather map. A low center is usually accompanied by precipitation, extensive cloudiness, and moderate winds.
What does h mean on a weather map? ›
Atmospheric pressure is measured with an instrument on the ground called a barometer, and these measurements are collected at many locations across the U.S. by the National Weather Service. On weather maps, these readings are represented as a blue “H” for high pressure or a red “L” for low pressure.
What are the 3 types of weather maps? ›
Tools such as surface maps and upper-level maps provide basic weather information, while satellite images and radar can track storms and systems. Thermodynamic images can also provide information about pressure and temperature changes throughout the atmosphere.
For example, if air pressure increases, the temperature must increase. If air pressure decreases, the temperature decreases. It also explains why air gets colder at higher altitudes, where pressure is lower.
How to tell which way a front is moving? ›
On weather maps, the surface location of a warm front is represented by a solid red line with red, filled-in semicircles along it, like in the map on the right (B). The semicircles indicate the direction that the front is moving. They are on the side of the line where the front is moving.
How do you know when a cold front is coming? ›
Clouds. Altocumulus clouds often are a sign of an entering cold front. If the cold front is highly unstable, cumulonimbus clouds producing thunderstorms commonly form along the front. Anvil cirrus clouds may spread a considerable distance downwind from the thunderstorms.
What does a 40% chance of rain mean? ›
(2) If the forecaster expected a widespread area of precipitation with 100% coverage to approach, but he/she was only 40% certain that it would reach the forecast area, this would, as well, result in a "40% chance of rain" at any given location in the forecast area.
What do the red and blue lines mean on a weather map? ›
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.
