Weather 101: Air Pressure

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Air pressure plays a big part in weather forecasting. The juniors and seniors at Bellefonte Area High School are learning about weather, and had a few questions about how it works. It can be a bit confusing, so here is a quick explainer on how air pressure works!

Air pressure begins because the sun heats the earth unevenly, creating different areas of pressure across the globe. Air pushes on us at all times! From the top, bottom, and sides. At sea level,  air pressure is 14.7 pounds per square inch! Our bodies are used to it so we don’t feel it surrounding us. To get an idea of how this feels, think about when you dive to the bottom of the pool. You feel the pressure of the water surrounding you. In meteorology, we like to think of the atmosphere as the ocean and we live on the bottom of that ocean.

We measure air pressure with a barometer and our unit of air pressure on a weather map is called a millibar. Average sea level air pressure is 1013 millibars. We use pressure in weather to tell if it is going to be a nice day or a rainy day. High pressure (above 1013 millibars) means we will have dry weather and sunshine. If it is below or the pressure is dropping, we know that some type of clouds or precipitation is on the way!


Air pressure depends on the temperature of the air and the density of the air molecules. We use the ideal gas law to find this.

Ideal Gas Law: PV = nRT

The P = pressure, the V= Volume, the n= number of molecules R=gas constant and the T= temperature

Using this equation, you can find if pressure increases, so does the temperature. If pressure decreases the temperature will lower.

In the video, we show you that if you fill up a balloon, that would represent an area of high pressure because it is more dense with the amount of air molecules in it. With the bottle as a closed off environment, it keeps the balloon inflated without tying it off. Remember, air does take up space! When we open the environment the air flows out of the balloon where there was high pressure into low pressure to try and fill it up. The atmosphere is always trying to come into balance.


To understand high and low pressure, we need to understand the coriolis effect a little bit. The Coriolis Effect makes the air on earth blow in a curved motion instead of a straight line. So on earth, we see air moving out of a high pressure system and into a low pressure system.


High pressure is when we have 1013 mb or above. Winds due to the coriolis effect will blow clockwise around an area of high pressure in the northern hemisphere. When the air pressure is high, winds will blow away from the area of high pressure. So air in the sky will sink down, and try to fill in the space. With the air sinking, it will make a “clear” or nice sunny weather.


Low pressure is when we have 1013 mb or below. Winds due to the coriolis effect will blow counterclockwise in the northern hemisphere. When pressure is low, the winds blow towards the low to try and fill it up. This air then rises into the sky and condenses water vapor. With the condensing of water vapor, it creates clouds which then creates more “stormy” weather.


We have sites across the country and globe that collect weather data, including air pressure. These sites are typically at airports. We then plot the pressure on a map and play connect the dots for the areas of similar pressure. These are called contours and can show us where there are areas of high and low pressure so we can know where there will be nice weather or where it will be stormy! The high pressure is designated as a blue H on a weather map. The low pressure is designated as a red L on a weather map.

Air pressure can be confusing to understand. If you have more questions email


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