You may have heard the weather station talking about the barometric pressure of an area. High levels of barometric pressure can lead to cooler temperatures and cloudless skies, whereas low levels of barometric pressure often lead to warmer temperatures and clouds, possibly accompanied by rain. But what exactly is barometric pressure and what causes it to change? The causes of barometric pressure—density, temperature and altitude—are closely interconnected.

Barometric pressure is another term for air pressure. We think of air as weightless, but in truth air does have weight.The air molecules above a specific point on Earth weigh down (or exert pressure) on that point. This pressure is called barometric pressure. Barometric pressure is measured with a barometer.

Like all molecules, air molecules are pulled to the ground by gravity. The pressure the molecules exert on the ground is dependent on the force of gravity. For example, the barometric pressure on the moon would be less than the barometric pressure on Earth because there is less gravity on the moon.

The density of a mass of air affects the barometric pressure. If the mass of air over a specific point on Earth is more dense, there are more air molecules exerting pressure on that point. Therefore, the barometric pressure is higher. If the same mass of air is less dense, there were fewer air molecules exerting pressure on that same point, which means the barometric pressure is lower.

Hot air is less dense than cool air, which is why hot air rises and cool air falls. This can be explained by thinking about how molecules move in hot air and in cool air. Molecules in hot air are moving quickly, so they tend to bounce off of each other and move apart, creating a less dense mass of air. Molecules in cold air move more slowly, so they tend to stay together, creating a denser mass of air.

The altitude of a location affects the barometric pressure indirectly, because altitude affects temperature. For example, the temperatures in the mountains are colder, so the mountains have a higher average barometric pressure than the temperatures at the beach. In addition, flying in an airplane may cause your ears to pop because of a sharp increase in barometric pressure. This increase occurs because the plane is passing through cool air at high altitudes.