Atmospheric Pressure: Highs, Lows, Troughs, and Ridges

Most flight schools are quick to point out that aviation weather is one of the most difficult things for student pilot to learn. By the time, you know enough to pass your private pilot written exam, you’ll most likely know more than 99% of your friends, and probably enough to host the evening weather on TV.

The greatest effect on the weather you’re going to experience as a pilot is caused by atmospheric pressures consisting of highs and lows, as well as troughs and ridges. Variations in atmospheric pressure are caused by variations in surface temperatures. As surface temperatures rise or decrease not only will air circulation patterns change, but air density as well as air pressure will change.

Meteorologist at the National Weather Service plot air pressure across the country. They connect similar air pressure a line. These lines of the same air pressure indicate air pressure gradients. Sometimes those gradients are very steep and you’ll notice a dramatic difference in air pressure just a few miles apart. Connecting the lines of like atmospheric pressures also allow us to identify areas of highs and lows.

  • A high is an area of higher pressure surrounded by lower pressure;
  • A low is an area of low pressure surrounded by higher pressure.

Sometimes these high and low pressure areas will not form very symmetrical circles and you will see an elongated area of low or high pressure that sticks out into the higher or lower pressured areas. These elongated areas are called troughs, if they’re low pressure or ridges, if they’re high pressure. As a pilot, you need to watch out for troughs in particular. These are the elongated areas of low pressure. Because low pressure air cannot move into the higher pressure areas and can’t move into the ground, it’s going to result in rising air. As you’ll learn, rising air results in cloudiness & precipitation. That’s why we generally associate low pressure with bad weather.

The areas around a trough or ridge can sometimes have very steep pressure gradients. The steeper the pressure gradient, typically, the stronger the wind going through that area. As a pilot, you need to watch for these areas and strong wind conditions.

As you prepare for your oral examination, remember a trough is an elongated area of low pressure and a ridge is an elongated area of high pressure.

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