Last June Seattle Backpackers Magazine posted a short article on tracking barometric pressure with a GPS.  Recently my son reminded me of a little known theorem that helps the hiker’s situational awareness.  This theorem is called Buys-Ballot’s Law.

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In 1857 Dutch professor Christopher Buys-Ballot postulated that there was a relationship between wind direction and air pressure. Buys-Ballot’s law provides a rough approximation of the location and direction of the low pressure system as it tracks through a region.

Simply put, in the northern hemisphere, if one faces the wind the center of a low pressure system will be to the right and slightly behind the observer.  High pressure will be to the left and slightly ahead of the observer. Further, weather systems in the northern hemisphere track from west to east.

Important for the hiker, a low pressure system is associated with rain, snow and bad weather in general.  A high pressure system is associated with improving weather conditions.

So, if the hiker determines that high pressure is to the west of the present location the weather may be improving because the system will move from west to east.

The YouTube video by meteorologist Vince Condella presents this nicely.

Buys-Ballots Law and a GPS are both useful tools to improve the hiker’s ability to monitor and anticipate the weather in the backcountry.

buys-ballot's law2

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