Atmospheric pressure is really the ‘weight’ of the air above a location and as air moves over and past, the quality of that air (temperature, humidity and thus density) is weighed as the air’s pressure.

Trio of Reasons

On average, air pressure exerts roughly 14.75 pounds per square inch at sea surface. Meteorologists prefer to talk about standard sea-level pressure in the SI units as 1013.2 mb. Variations in air pressure are responsible for three important consequences in the atmosphere. They include:

Creating winds on the large scale that travel the oceans and continents to localized breezes felt in our own back yard

Causing the upward and downward motions of air within areas of high and low pressure systems.

Changes in air pressure readings are warning indicators of what the weather will bring in the hours to follow.

Measurements

The most common measurements of air pressure in the United States include "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from weather reports on news. At sea level, the standard air pressure is 29.92 inches of mercury.

A millibar is 1/1000th of a bar and is the amount of force it takes to move an object weighing a gram, one centimeter, in one second. Millibar values used in meteorology range from about 100 at the top of the tropopause to 1050 at stations below sea level. At sea level, the standard (average) air pressure is 1013.2 millibars. Weather maps showing the pressure at the surface are drawn using millibar contours. Under most 'normal' weather events pressures vary between 980 and 1025 mb.

High and Low Pressures

Low Pressure systems are inward spiraling, and ascending, circulation systems. With ascent, cooling and condensation occurs and the change from vapor to liquid (called a phase change) releases energy. This 'heat' energy converts into kinetic energy (motion or wind) – and is essentially the makings of ‘weather’. 

High Pressure systems are descending, outward spiraling systems. Here, as the air sinks, it warms (as pressure increases) and the air's capacity to hold moisture (vapor) increases. Thus evaporation processes occur, clouds diminish – and the occurrence of ‘weather’ lessens. 

Simplistically, Lows are associated with clouds and rain and weather, and Highs with clearing skies, sunny conditions and generally nice weather.

Author's Bio: 

Athena is not just a High Priestess of Wicca and a spiritual adviser, she is also an avid weather geek and is a storm spotter and storm chaser.