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Air Masses
Air Masses
An air mass is a large body of air—often thousands of miles across—that has similar temperature and humidity throughout. Air masses form when air stays over one area for a long time and takes on the characteristics of the surface below it.
Source Region
Source Region
The area where an air mass forms is called its source region. Source regions are usually large, flat areas with fairly uniform conditions, such as oceans, deserts, plains, or ice-covered land. Because conditions don’t change much across these regions, the air can become evenly warm or cold, moist or dry.
Common source regions include:
Tropical oceans (like the Gulf of America), which create warm, moist air
Cold land areas (such as Canada), which produce cold, dry air
Warm deserts (like northern Mexico or the southwestern U.S.), which form hot, dry air
Polar ice and snow-covered regions, which generate very cold, dry air
Types of Air Masses
Types of Air Masses
Air masses are classified by:
Temperature
Tropical (warm)
Polar (cold)
Moisture
Maritime (moist)
Continental (dry)
These combine to form common air masses found almost everywhere on Earth such as:
Maritime Tropical (mT): warm and humid
Continental Polar (cP): cold and dry
Continental Tropical (cT): hot and dry
Maritime Polar (mP): cold and humid
Some air masses only found in certain parts of the world include:
Continental Arctic (cA): extremely cold and dry
Continental Antarctic (cAA): extremely cold and dry
Maritime Equatorial (mE): extremely warm and humid
Air Mass Weather
Air Mass Weather
Air mass weather occurs when a single air mass stays over an area for several days and dominates the weather conditions. Because air masses have similar temperature and humidity throughout, the weather during this time tends to be fairly consistent from day to day.
When air mass weather is present, skies may stay clear or cloudy for long periods, and temperatures change slowly. For example:
A continental polar (cP) air mass brings cold, dry weather.
A maritime tropical (mT) air mass produces warm, humid conditions.
A continental tropical (cT) air mass causes hot, dry weather.
A maritime polar (mP) air mass produces cold, humid conditions.
Air mass weather often develops when winds are weak and large high-pressure systems are in place. This allows the air mass to remain over one region instead of being pushed away by moving weather systems.
Lake-Effect Snow
Lake-Effect Snow
Lake-effect snow is an example of air mass weather that occurs near large lakes, such as the Great Lakes. It forms when cold, dry continental polar air moves over the warmer lake water. The air warms from below, picks up moisture, and becomes unstable. As the air rises and moves downwind of the lake, it cools and drops the moisture as heavy snow.
Lake-effect snow can produce:
Very localized snowfall
Rapidly changing conditions
Snowfall rates much higher than surrounding areas
Places downwind of the Great Lakes can receive several feet of snow while nearby areas get little or none. We often see lake-effect snow form near Lake Michigan in the Chicagoland area.
Fun Fact:
Fun Fact:
In the United States, one of the cities that gets some of the most snow each year is Buffalo, New York. On average, it receives around 90 – 95 inches of snow annually.
The main reason Buffalo gets so much snow is lake-effect snow — the city is located near both Lake Erie and Lake Ontario.
Buffalo is the home of the NFL's Buffalo Bills football team. Because of all the snow they get, the team often hires Bills' fans to shovel out the stadium before home games!
Click on the image above to get a more in depth look.
Weather Fronts
Weather Fronts
A weather front is a boundary between two air masses that have different temperatures and moisture levels. Air masses are large bodies of air that form over specific regions, like oceans or land, and they often have similar weather throughout. When two different air masses move toward each other, a front forms—and that’s where changes in weather usually happen.
There are four main types of weather fronts:
- Warm Front
A warm front occurs when a warm air mass moves toward a colder one. The warm air gently rises over the cooler air. This slow lifting usually produces clouds and steady precipitation like light rain or snow over a wide area. This precipitation takes a long time to pass. After a warm front passes, temperatures often increase.
Cirrus clouds often signal the approach of a warm front. As the front moves closer, the cirrus clouds thicken and lower, turning into stratus or nimbostratus clouds.
Warm Front Passage Timelapse Video
2. Cold Front
2. Cold Front
A cold front forms when a cold, dense air mass pushes into a warmer air mass. The cold air slides underneath the warm air, forcing the warm air to rise quickly. This rising air can create thunderstorms, heavy rain, or snow, for short periods of time. Cold fronts are usually followed by cooler, drier air. Cold fronts often move fast and can cause sudden weather changes.
Cumulus clouds are the most common type of cloud produced in the early stages of the formation of a cold front. These cumulus clouds often grow into cumulonimbus clouds, which often produce thunderstorms.
Cold Front Passage Timelapse Video
3. Stationary Front
3. Stationary Front
A stationary front forms when two air masses meet but neither one is strong enough to move the other. Because the front stays in one place, it can bring several days of cloudy skies and ongoing rain or snow.
The clouds associated with stationary fronts are usually stratus or nimbostratus clouds.
4. Occluded Front
4. Occluded Front
An occluded front forms when a cold front catches up to a warm front. Because cold air moves faster, it eventually overtakes the warm front and lifts the warm air completely off the ground. This type of front is common in mature storm systems and can bring cloudy weather and widespread precipitation. Because the warm air is lifted more gently and is more spread out, thunderstorms are less common, but can occur. Either way, long-lasting precipitation is typical.
Clouds associated with occluded fronts are usually nimbostratus or cumulonimbus.
Weather Front Symbols
Weather Front Symbols
Meteorologists use colored lines and symbols on weather maps to show where weather fronts are located and which direction they are moving. Each type of front has a unique symbol so it can be identified quickly.
Warm Front Symbol
Warm Front Symbol
Symbol: A red line with semicircles
Semicircle direction: Points in the direction the warm air is moving
What it means: Warmer air is advancing and rising over cooler air
Weather clue: Widespread clouds and steady precipitation ahead of the front
Cold Front Symbol
Cold Front Symbol
Symbol: A blue line with triangles
Triangle direction: Points in the direction the cold air is moving
What it means: Colder air is advancing and pushing under warmer air
Weather clue: Thunderstorms or heavy precipitation ahead of the front
Stationary Front Symbol
Stationary Front Symbol
Symbol: A line with alternating blue triangles and red semicircles
Symbol direction: Triangles and semicircles point in opposite directions
What it means: Neither air mass is moving much
Weather clue: Long periods of clouds and precipitation
Occluded Front Symbol
Occluded Front Symbol
Symbol: A purple line with alternating triangles and semicircles
Symbol direction: Both symbols point in the same direction and show the direction the front is moving
What it means: A cold front has overtaken a warm front
Weather clue: Cloudy skies and widespread precipitation near the storm center; possible thunderstorms
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