Air Masses
When a large body of air has fairly uniform characteristics it is referred to as an air mass. An air mass may be polar or tropical, which refers to its temperature. An air mass is also described as maritime or continental, in reference to its humidity.
The characteristics of an air mass are set when that air mass remains relatively still for a period of time, perhaps several days. For example, if an air mass originated from Canada, it would be cold and dry; a polar continental air mass. If an air mass became warm and moist as it moved over the Pacific Ocean, it would be described as a tropical maritime. The areas from which the characteristics of an air mass are usually established are known as the source regions.
A moist, unstable air mass is characterized by cumuliform clouds and showery precipitation, good surface visibility, and turbulence. Stable air masses are characterized by stratiform clouds with steady precipitation, poor surface visibility, and smooth air.
Once set in the source region, the characteristics of an air mass remain relatively set, but may be modified as the air mass passes over varying land and water features.
The place where two air masses meet each other is a front. There are four types of fronts: warm, cold, stationary, and occluded. Temperature is one of the most easily recognized changes that occurs with the passage of a front. However, a change in wind direction is something that always occurs with frontal passage.
Warm Front
A warm front exists where a warmer air mass overtakes a cooler one. The less dense warmer air slides over and above the cooler air ahead, gradually pushing it out of the way. A warm front will usually progress at a speed of about 10 to 25 miles per hour. Weather is produced as the warm air is lifted.
Weather associated with a warm front is stratiform clouds and poor visibility. Cumulonimbus may exist during the summer, and haze may persist after the front passes.
Cold Front
A cold front occurs when cold air overtakes warmer air. The colder air is more dense, and so plows underneath the warmer air ahead, lifting the warmer air as it goes. Cold fronts typically move faster than warm fronts, at speeds of around 25 or 30 miles per hour. A cold front may be fast moving, however, at speeds of up to 60 miles per hour.
The type of clouds associated with a cold front will vary depending on the stability of the warmer air mass ahead. Typically, cold front weather consists of cumuliform cloud types, poor visibility, gusty winds, and rapid temperature changes.
A fast moving cold front may bring a narrow band of intense weather along preceding the front, known as a squall line.
If the warmer air ahead is stable, overcast skies and rain will precede the front for some distance. However, if the warmer air is unstable, a line of thunderstorms or squall line may form ahead of the cold front.
After cold front passage, skies clear rapidly.
Stationary Front
A stationary front is one in which neither air mass is advancing. Colder air will exist on one side, while warmer air will exist on the other. Stationary fronts bring poor weather that often persists in an area for several days.
Occluded Front
An occluded front exists when a cold front overtakes a warm front. This means an occluded front is colder air overtaking warmer air which is, in turn, overtaking colder air. This results in two colder air masses pinching the warmer air mass in between them, forcing it aloft.
The two colder air masses will also have a temperature difference relative to each other. A cold front occlusion is one in which the air behind the occluded front is colder than the air ahead. A warm front occlusion is an occluded front in which the air behind the occluded front is warmer than the air ahead.
Occluded front weather has warm and cold front characteristics. Nimbostratus and cumulonimbus clouds are both common with an occluded front, along with poor visibility. Precipitation may be light or heavy. Occluded fronts normally become stationary fronts, which may remain in place for days.