Beyond The Forecast - High and Low Pressure

Centers of High and Low pressure are useful in determining what weather is heading our way

Happy Monday, and welcome to another edition of Beyond The Forecast!

One of the most helpful pieces of data we have in forecasting is atmospheric pressure. It can be hard to tell on a generic day, but air is made up of molecules that exert forces on other molecules. When you feel the wind blowing at you, that wind is literally the impact of those molecules against your skin. Even under calm conditions, there is still air pressing down on us from higher in the atmosphere.

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Though each of the air molecules are very small the immense number of them in the atmosphere creates a lot of pressure

The presence or absence of those molecules defines the air pressure in the atmosphere. The scientific unit for pressure is a bar. Bars measure the amount of force done by an object (in our case, the force of gravity pulling air molecules towards Earth) over a particular area.

High pressure collects molecules together and makes it hard for air to rise while low pressure has much more space

At sea level, the average pressure is 1.013 bars. We use the sea level pressure standard to determine if air pressure will be relatively high or low as weather systems move over us.

A change of even one one-thousandth of a bar can have a big impact on weather

High pressure means there is more air stacking up in a single spot raising the number of molecules we feel making contact with our skin. The weight of the air makes it difficult for moisture to rise from the surface; high pressure generally means we have clear skies.

Low pressure on the other hand means there is less air in a particular spot than the global average. Low pressure is often the result of warm air rising and leaving fewer molecules at the surface to press down on us. That lack of pressure makes it easier for warm air to rise and cause rain or storms.

Low pressure systems often bring a warm front with showers and a cold front with storms in the summer

The atmosphere is in a constant state of change between high and low pressures. When air builds up in a particular location and causes high pressure the laws of physics want that pressure to even out. Air rushes away from the center of the high to try to achieve that balance. On the other hand, a low-pressure system means there is a relative lack of air molecules. Air from the surrounding environment swoops in to fill the gaps.

High pressure at the end of the week brings a lot of cold air our way while keeping us clear. Know what to expect in the sky each day; you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.

You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!

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-- Marshall Downing


About the Author

Marshall Downing presents the weather Saturday and Sunday evenings at 6:00 PM and 11:00 PM, and you can see him during the week at 12:00 PM and 5:30 PM.

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