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Air Pressure

24-hour Air Pressure

This graph shows the recent history of atmospheric pressure. A falling trend can mean that stormy weather is approaching, while a rising trend usually heralds fair weather is on the way.

Air Pressure High and Low

The color spectrum shows the current, high, and low values for air pressure since midnight. The trends at the time of each measurement is shown by the arrow.

Air Pressure Dominance

This pie chart shows the percentage of all the measurements taken in the last 24 hours that fall within the ranges shown.

Atmospheric pressure is caused by the weight of the air above. At sea level on Earth, on average, atmospheric pressure is about 15 pounds of weight distributed over every square inch of surface area. This means that a column of air an inch across and stretching upward to the "top" of the atmosphere only weighs 15 pounds.

Differences in surface air pressure from one region to another are caused by solar heating. As the Sun's energy heats up Earth's surface, the atmosphere above also heats up, causing the air to expand and become less dense. Less dense air weighs less than a similar volume of denser air, and so atmospheric pressure is less. It is differences in atmospheric pressure between one region and a neighboring region that drive winds.

Air at lower pressure is less capable of containing water vapor (water in its gaseous state), and so low pressure regions are more prone to the formation of clouds by condensation, especially if the air is cooled and becomes even less capable of holding water vapor. Storm systems are typically associated with low pressure zones.