So, when two different air masses meet, a boundary is formed. The boundary between two air masses is called a front. Weather at a front is usually cloudy and stormy. There are four different fronts- Cold, Warm, Stationary, and Occluded.

Four major types of air masses influence the weather in North America:

• Maritime Tropical (mT)
• Maritime Polar (mP)
• Continental Tropical (cT)
• Continental Polar (cP)

The air masses in and around North America include the continental arctic (cA), maritime polar (mP), maritime tropical (mT), continental tropical (cT), and continental polar (cP) air masses.

Terms in this set (9)

• CONTINENTAL POLAR. cold and dry air. forms on land.
• CONTINENTAL TROPICAL. hot and dry air. forms on land.
• MARITIME TROPICAL. forms over water. hot moist air.
• MARITIME POLAR. forms over water. cold moist air.
• maritime. = ocean (moist/humid air)
• continental. = land (dry air)
• polar. = cold.
• tropical. = warm/hot.

An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time sufficiently long to acquire the temperature and moisture properties of that surface. The Earth’s major air masses originate in polar or subtropical latitudes.

A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.

stationary front

Warm air rises, creating a low pressure zone; cool air sinks, creating a high pressure zone. Air that moves horizontally between high and low pressure zones makes wind. When cool air descends, it warms. Since it can then hold more moisture, the descending air will evaporate water on the ground.

Cold fronts form when a cooler air mass moves into an area of warmer air in the wake of a developing extratropical cyclone. The warmer air interacts with the cooler air mass along the boundary, and usually produces precipitation. Cold fronts often follow a warm front or squall line.

Moist air

Tropical air masses form in low-latitude areas and are moderately warm. Polar air masses take shape in high-latitude regions and are cold. Equatorial air masses develop near the Equator, and are warm. Air masses are also identified based on whether they form over land or over water.

Winds and air currents cause air masses to move. Moving air masses cause changes in the weather. A front forms at the boundary between two air masses. Types of fronts include cold, warm, occluded, and stationary fronts.

When cool air descends, it warms. Since it can then hold more moisture, the descending air will evaporate water on the ground. Gases at sea level are also compressed by the weight of the atmosphere above them. At higher altitudes the atmospheric pressure is lower and the air is less dense than at lower altitudes.

Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms.

Rising Air: a key process in the production of clouds and precipitation. Imagine a block of air, or air parcel, rising upward through the atmosphere. The air parcel expands as it rises and this expansion, or work, causes the temperature of the air parcel to decrease.

What are characteristics of an unstable air mass?

What are the characteristics of stable air? A—Good visibility; steady precipitation; stratus clouds. B—Poor visibility; steady precipitation; stratus clouds. C—Poor visibility; intermittent precipitation; cumulus clouds.

Weather is strongly affected by how stable or unstable the atmosphere is. Stable air means that the weather is likely to be calm. It may rain or snow slowly and steadily, it may be sunny, but the weather will not change quickly. Unstable air means that the weather might change quickly with very little warning.

Temperature and humidity are two characteristics used to classify air masses.

To be “unstable”, the lowest layers of an air mass must be so warm and/or humid that, if some of the air rises, then that air parcel is warmer than its environment, and so it continues to rise. This condensation releases heat, which warms the air parcel, which can cause the parcel to rise higher still.

If the environmental lapse rate is like the blue one on the graph, a bubble of air a degree or two warmer than the surrounding ground will not rise very far before cooling to the same rate as the surrounding air and stopping. This is therefore a stable air mass and will experience no convectional rain.

Indicates stability: stability of the atmosphere is determined by vertical movements of air. Warm air rises when the air above is cooler. Cooling from below: surrounding air is warmer, which would increase the stability of an air mass.

The difference between the cold parcel air and the warmer surroudings gets larger and larger with increasing altitude. Sunlight warms the ground and the air next to it during the day. This steepens the environmental lapse rate and makes the atmosphere more unstable. Cooling air above the ground has the same effect.

The coldest air masses are Arctic air masses. These air masses originate at the poles of the Earth in Greenland and Antarctica.