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Characteristics of Weather Fronts

Meteorologists classify air masses by humidity and temperature. Dramatic or unstable weather happens when air masses move around the Earth. Fronts mark the boundary at the Earth's surface where two large air masses meet. Besides the Earth's surface, fronts occur vertically; vertical movement of fronts causes clouds and precipitation. Strong fronts indicate severe weather, and great temperature changes take place in cold and warm fronts.

Air Masses
- An air mass is a huge amount of air that takes on the characteristics of its source region. Humid air masses are classified as maritime, or marine, because they originate over an ocean or other major body of water. Dry air masses are continental because they originate over land. Warm air masses are tropical, and cool air masses are polar. Extremely cold air masses are arctic. Air masses can control weather for less than a week or for a whole season.
Cold Fronts
- Cold fronts form when a denser cold air mass wedges beneath a thinner warm air mass, and vertically lifts the warm air to create thicker and lower surface clouds that can produce gusty winds, thunderstorms and short-lived heavy precipitation that can include rain, snow, sleet or hail. Traveling at 30 miles per hour, cold fronts are fast moving and can sweep into a region with amazing speed.
Warm Fronts
- Warm fronts occur when a warm air mass displaces a cool or cold one. Warm air is less dense than cold air so it advances by gently sliding over the top of a colder air mass. Warm fronts in the summer produce steady, light, and widespread rain; however, in the winter, rainfall can turn into snow, freezing rain or sleet.
Stationary Fronts
- A clash of equal cold and warm air masses creates a stationary front. Stationary fronts occur when cold and warm air masses meet at the Earth's surface but both are about the same strength; the two fronts may seem to hover over one location for hours or even days. Stationary fronts are more likely to occur during the summer rather than winter.
Occluded Fronts
- Occluded fronts involve three air masses: cool polar air, colder polar air and warm air. When the two polar air masses converge, the colder polar air slides beneath the cool polar air to block, or occlude, the warm air from the Earth's surface. Warm air must stay above both polar air masses so the surface effects of the warm front evaporate. Occluded fronts take one to two days to form and are more likely to occur in the northern hemisphere than in the southern hemisphere because well-developed warm fronts are not common south.

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