Week of August 24, 2025 – August 30, 2025
by Lizzy Suzedell, Environmental Educator
When we think about the weather, measurements like the temperature, clouds, rain, and wind always come to mind. However, to make a forecast, one of the most important considerations is the air pressure. We hear meteorologists talk about it all the time on TV as high and low pressure systems approach, but what does that actually mean for the forecast?
Air pressure, also known as atmospheric or barometric pressure, is the force of the air exerted on earth’s surfaces. Air molecules don’t weigh very much, but the earth’s atmosphere has a LOT of air, so the weight adds up. At standard atmospheric pressure, the air exerts a force of 15 pounds per square inch. This makes it sound like the air should feel like a bunch of bowling balls sitting on top of us, but because the air pressure acts in all directions and our bodies exert equal pressure back onto the air, it feels relatively weightless to us.
The air pressure around earth is constantly changing due to two main factors: temperature and altitude. The molecules in warmer air move faster, so they spread out and exert greater pressure than that of cold air, which is slower and denser. Also, as you go higher up in the atmosphere, the amount of air above you (and the air pressure) drastically decreases. At sea level, the average pressure is around 1013 mb (millibars), while Denver, CO, located one mile above sealevel, has an average pressure of about 820 mb. Differing temperatures and elevations around the globe make the air pressure change constantly.
Air tends to move from areas of high pressure to areas of low pressure. In a low pressure system, air converges and rises. When air rises, it cools, and water vapor condenses to form clouds and rain. In a high pressure system, air diverges and sinks. Because the air sinks, clouds have difficulty forming and conditions will be sunny and stable. This is why low pressure systems are associated with bad weather (rain storms or hurricanes) and high pressure systems mean good weather (clear and sunny days).
As we head into the fall, low pressure systems will be more common, giving us regular gloomy weather. However, the atmosphere is dynamic, and high pressure will still come around to bring blue skies! What’s your favorite type of autumn weather?