Under Pressure

2012-08-21 18:22:35.000 – Cyrena Briede,  Director of Summit Operations

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If you read the forecast (available HERE) you will see the term “high pressure” mentioned when talking about the weather we are experiencing right now and will see for a few days to come. But what does exactly does “high pressure” mean?

High pressure means that relative to the surrounding air in the region, the pressure of the air at that location is higher, it weighs more. Areas of high pressure are often associated with clearer sunnier skies, and there are reasons why this is generally true. High pressure areas have downward vertical motion. This falling air warms at it gets closer and closer to the ground (for those that are familiar with the Ideal Gas Law), at a rate of about +3.5 degrees F per 1000 feet. As the air warms, the relative humidity falls, making it harder for clouds to form. In turn, low pressure has rising motion where the temperature decreases as it rises, relative humidity increases, making it easier for clouds to form.

Air pressure also has a direct effect on winds, something that is very important up here at the summit of Mount Washington. Wind blows from areas of high pressure to low pressure. Want to see it for yourself? Before you head up Mount Washington, grab an empty water bottle and put the cap on tight. As you drive or hike up, notice how the pressure in the bottle increases. Once you get to the summit, open the bottle slowly and you will hear the wind, the high pressure air escaping the bottle and move to the surrounding areas of lower pressure. Put the cap back on tight and head down again. When you get to the base of the mountain, the bottle may be deformed or caved in due to the low air pressure inside. Again, remove the cap and listen to the wind as the surrounding high pressure air moves into the bottle where the pressure is lower.

So be sure to get out and enjoy the high pressure forecast for the region over the next few days. You never know when the next low pressure area is just around the corner!

Cyrena Briede,  Director of Summit Operations