What is air pressure, and why does it change?

Some of the more common questions that we receive at Cloudy Headquarters are related to atmospheric pressure: What is it, and how is it related to the weather? Cloudy's resident meteorologist Prof. David Schultz answers your questions below.

Atmospheric pressure is the weight of the atmosphere above us. Think of living on the surface of the Earth as living at the bottom of a big swimming pool. If you dive to the bottom of the deep end of a pool, you'll feel the pressure of the water on your body. The same is true living at the bottom of the Earth's atmosphere. On average, at sea level the pressure on the surface of the Earth is 14.7 pounds per square inch, which can also be expressed in other units such as 29.92 inches of mercury, 1013.25 hectoPascals (hPa), or 1013.25 millibars (mb).

Ordinarily, we may not feel the pressure because we are used to living down here at the bottom of this pool of air and our bodies have become accustomed to it. But, perhaps some of us are more sensitive and feel those pressure changes in our bodies. That is one of the hypotheses that we will be exploring with the data that Cloudy participants are providing to us.

Why does pressure change? Where we live, in Earth's midlatitudes (where the UK is), weather systems generally move from west to east. These weather systems are associated with changes in the surface pressure, essentially how much air is above us. When the pressure is decreasing and you hear people say that low pressure approaches, the air in the atmosphere tends to ascend, which is favourable for the production of clouds and precipitation. In contrast, when the pressure is increasing and you hear people say that high pressure approaches, the air in the atmosphere tends to descend, which suppresses clouds and precipitation.

Because the pressure, cloudiness, and precipitation (as well as wind, temperature and humidity) are linked in low- and high-pressure systems, one of the challenges in Cloudy is to separate out the pressure effects that people may be feeling from those that may be due to temperature or humidity instead. That's why we need as much data from as many different participants as possible.