What is defined as air pressure is the result of molecules of air weighing down on objects in our environment. As each molecule strikes an object, it exerts a small amount of pressure. These small amounts can add up to be fairly significant, however. As the density of air increases, the number of molecules striking each object in its environment increases, causing an increase in air pressure. Therefore, the causes of atmospheric pressure changes are actually the causes of changes in the density of the ambient air.
The most commonly known cause of changes in atmospheric pressure is a change in elevation. As elevation increases, the number of air molecules in the surrounding atmosphere decreases. This, in turn, decreases the density of the ambient air, causing a decrease in the ambient air pressure. While this change is not as noticeable within the first mile of atmosphere above sea level, it is very noticeable at extreme elevations. On Earth, the atmosphere extends more than fifteen miles from sea level. Of all of the atmospheric air within this fifteen miles, however, about half of the air molecules are contained within the first 18,000 feet. This means that air pressure drops logarithmically as measurements are taken further from sea level.
Weather patterns can also have significant effects on air pressure. In general, hot air is less dense than cold air. This is because the molecules that make up hot air have greater velocity and are farther apart from each other than they are in cold air. This means that ambient air pressure is dependent on changes in temperature. In fact, the most noticeable change in air pressure occurs twice a day with the rise and fall of the sun. The middle of the night is usually the time the lowest air pressure is observed, while the middle of the day is the time at which the highest daily air pressure is observed. This difference is most pronounced in areas near the equator, and the effect is less pronounced in areas around the poles.
In addition to these daily fluctuations, however, are the even larger pressure changes resulting from migrating weather systems. As precipitation develops, air molecules cool, causing them to release their moisture content. The cooling of the air will lower the ambient air pressure as the fall in temperature decreases air density. On the other hand, as a front of warm air passes through a region, the air pressure will increase as the air becomes warm and more dense.
Although these changes are usually too slow to observe without equipment such as a barometer, ambient air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature and altitude.