Answer Meteorologists at the National Weather Service use information from ground stations and weather satellites to make these maps. Words like “rain” and “snow” are pretty obvious, but what exactly do the symbols on a weather map tell you about the weather? Use our handy dandy guide below to find out!
High and Low Pressure Areas
a blue H and a red L, which are symbols for high and low pressure systems
Earth’s atmosphere is a jacket of gases that surrounds the planet. Although it seems like these gases could easily float away into space, gravity is constantly pulling the atmosphere toward Earth’s surface. The force with which our atmosphere pushes down on a specific location on Earth is called atmospheric pressure.
Atmospheric pressure is mainly dependent on two things: the weight of the atmosphere in a specific location and the temperature of the air. If you’re at a low elevation—such as in a valley—there is a lot of atmosphere above you and the weight is very heavy. That means that you experience higher atmospheric pressure at lower elevations and lower atmospheric pressure in higher elevations.
A gif showing a short squat person at sea level and a tall thin person on a mountaintop to represent air pressure
When you're at a low elevation, you experience high atmospheric pressure because more of the atmosphere is pushing down on you.
Warm air can also cause the atmospheric pressure to rise. When the air is warm, gas molecules move around quickly in the air pushing out on the area around them. This causes high atmospheric pressure. In cold air the gas molecules slow down, causing low atmospheric pressure.
Water vapor in the atmosphere can also change the atmospheric pressure. Very moist air that has lots of water vapor is actually lighter and less dense than dry air. This is because water molecules are lighter than molecules of nitrogen or oxygen—the most abundant gases in our atmosphere. So, very moist air in the atmosphere can lead to low atmospheric pressure and very dry air can lead to high atmospheric pressure.
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.
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