Weather reports often mention high or low pressure systems headed toward a city or town. If you're in the path of one of these systems, expect a change in weather conditions. Pressure refers to the force the atmosphere exerts on everything below it. High and low pressure systems operate using similar principles, but the effects they produce differ significantly.
Air Likes to Congregate
View a weather map, and you may see distinct regions that represent different air masses. An air mass is a region of air that has about the same humidity and temperature throughout. When an air mass remains over a location for a while, it assumes the characteristics of the region. Air masses can be cold or warm and they may contain moist or dry air. All air masses have the ability to move away from their source region into other locations. For instance, a Canadian air mass may move down toward the United States.
Meteorologists call the boundary between two air masses a front. A cold front's motion through a warm front is a low-pressure system. If a cold air mass replaces a warm air mass, you have a cold front. Air in the cold air mass is often drier than air in front of it. As the air masses collide, the wind usually changes direction and rain or thunderstorms often occur.
A warm front is associated with a high-pressure system and occurs when a warm air mass replaces a colder air mass. Because warm air moves closer to the ground, a high-pressure system often brings clear, calm weather. Warm fronts travel slowly and are weaker than cold fronts. You may often see a warm front after fog or precipitation occurs.
Cold fronts move faster than warm fronts because a cold front contains denser air. Cold fronts also often travel from west to east while warm fronts move from north to south. As a cold front approaches, temperatures may rise as warmer air flows in from a warm air mass. However, when the cold front hits, temperatures can drop dramatically within a short time.