An air mass is a large body of air that exhibits uniform temperature and moisture characteristics and helps to determine the weather within an area. Warm air masses tend to form over oceans in warm-temperature regions of the tropics and sub-tropics, while cold air masses tend to form over land in cold-weather regions nearer the poles. The cold air masses that form at the poles move toward the equator, while the warm air masses that form at the equator move toward the poles. These air masses can undergo gradual, but significant, changes as they pass over warmer or colder land surfaces encountering different types of air. These boundary lines are called fronts.
Cold fronts occur when a cold air mass moves into warmer air. Since cold air is heavier than warm air, the incoming colder air forms a wedge that displaces the warm, and often moist, air at the surface. As the warm moist air rises, towering clouds often form releasing energy in the form of brief, intense rain storms with high winds and lightning. On the other hand, warm fronts will ride up and over cold air near the surface. These fronts can also cause clouds and rain, but they tend to be less violent and longer lasting.
Air pressure is determined by how much the air weighs in a given space. Pressure varies with altitude, with denser air residing in lower altitudes and lighter air in higher altitudes. This is caused by the Earth's gravity ? its strength decreases the farther away from the center of the planet, therefore it holds fewer molecules together in the same amount of space. Air pressure is also a function of temperature and density; differences in pressure can be an important weather maker. As air descends, it warms, inhibiting cloud formation which is often associated with good weather. Air rises in areas where the pressure at the surface is low. As it rises, the air cools and the humidity condenses to form clouds and precipitation. The centers of all storms are areas of low pressure; therefore, low pressure is often associated with bad weather.
WW2010: Air Masses and Fronts The World Weather 2010 (WW2010) project at the University of Illinois has a webpage on air masses, or "uniform bodies of air," that illustrates the formation and movement of air masses through diagrams, pressure maps, and other graphics.
WW2010: Forces and Winds The University of Illinois' WW2010 website has an excellent collection of pages relating to wind and atmospheric pressure, defining key concepts and using graphic illustrations.
Air Masses and Frontal Transition Zones PhysicalGeography.net is an educational web portal that focuses on explaining the spatial characteristics associated with the Earth's atmosphere. It provides colorful graphics and tables that explain the movement of various air masses over North America, in addition to showing how to read a weather map and interpret weather patterns.
Atmospheric Pressure NASA's kid-friendly website explains atmospheric pressure; it also provides experiments, discussion questions, and links to additional sites with related information.
FOR THE CLASSROOM
Pressure and Winds Learning Module The Department of Geography at the College of Alameda provides learning materials on air pressure and winds, including study guides, tutorials, and practice quizzes for students. [Grades 9-12]
WW2010: Air Masses Activity The University of Illinois created this activity for students learning the basic characteristics of air masses and how to identify them. [Grades 9-12]
WW2010: Pressure Activity The University of Illinois created this activity to introduce students to pressure characteristics by having them interpret a diagram of a pressure field. [Grades 9-12]