From these categories come the five combinations of air mass types that influence our U.S. and North American weather.

The source region of an air mass defines its main characteristics. The main source regions are the high pressure belts in the subtropics (giving rise to tropical air masses) and around the poles (the source for polar air masses). …

Five air masses affect the United States during the course of a typical year: continental polar, continental arctic, continental tropical, maritime polar, and maritime tropical.

• Continental Polar (cP) Air. John E Marriott/All Canada Photos/Getty Images.
• Continental Arctic (cA) Air.
• Maritime Polar (mP) Air.
• Maritime Tropical (mT) Air.
• Continental Tropical (cT) Air.

When a front passes over an area, it means a change in the weather. Many fronts cause weather events such as rain, thunderstorms, gusty winds, and tornadoes. At a cold front, there may be dramatic thunderstorms. At a warm front, there may be low stratus clouds.

Pressure is a force exerted on or against an object by being in contact with a substance. Thus, air pressure is due to the bombardment of the air (gas) particles with a surface. Density is defined as the mass per volume of a substance. Thus, air density is defined as the mass of air per unit volume.

The greater the pressure difference between the pressure zones, the stronger the wind blows. Warm air can hold more moisture than cool air. When warm air rises and cools in a low pressure zone, it may not be able to hold all the water it contains as vapor. Some water vapor may condense to form clouds and precipitation.

Wind results from a horizontal difference in air pressure and since the sun heats different parts of the Earth differently, causing pressure differences, the Sun is the driving force for most winds.

Commonly, when the cold front is passing, winds become gusty; there is a sudden drop in temperature, and heavy rain, sometimes with hail, thunder, and lightning. Lifted warm air ahead of the front produces cumulus or cumulonimbus clouds and thunderstorms.

There are four basic types of fronts, and the weather associated with them varies.

• Cold Front. A cold front is the leading edge of a colder air mass.
• Warm Front. Warm fronts tend to move slower than cold fronts and are the leading edge of warm air moving northward.
• Stationary Front.
• Occluded Front.