Weather & Weather Systems

Cloud cover and the rainfall likely to fall from this nimbostratus cloud are characteristics of weather.

What is Weather?

Weather may be defined as the short-term state of the atmosphere over a particular region of the Earth’s surface. It encompasses a number of specific descriptions of atmospheric conditions such as temperature, humidity (moisture content), precipitation, cloud cover, wind speed and direction, and air pressure. With the movement of weather systems, atmospheric conditions can change from hour to hour and day to day. Weather is therefore a short-lived phenomenon, unlike climate which refers to atmospheric conditions averaged over a long period of time (usually 30+ years).

What are weather systems?

Weather systems are areas of relatively high or low air pressure that form and dissipate while moving from one area to another. Weather systems can bring about clear, calm weather, or violent, stormy weather. They may cover thousands of square kilometers, like a mid-latitude cyclone or a large hurricane, or the may cover only a few hundred meters, such as a tornado or small thunderstorm. Some weather systems form and travel halfway around the world, while others may only cover a kilometer or less before dissipating. Common types of weather systems include high-pressure anti-cyclonic systems that generally bring about sunny, pleasant weather, and low-pressure cyclonic systems that are associated with cloudy skies, precipitation, and strong storms.

Weather systems are strongly affected by the surface of the Earth and the other atmospheric conditions and weather systems they encounter as they move. Hurricanes, for example, form over the tropics where abundant heat and moisture fuels their development. As they move northward, the water begins to cool and they start losing energy. Once they reach land, hurricanes tend to dissipate quickly because they no longer have a source of water from which to draw upon. Mountain ranges also significantly affect weather systems by trapping cool air and moisture on one side, creating what is known as a rain shadow.

Air Masses

Changes in weather are often driven by the movement of large, relatively uniform bodies of air known as air masses. Air masses can cover several thousand square kilometers and extend over 15 kilometers into the atmosphere. Air masses are classified based on where they originate. The surface conditions air masses develop over give them their unique properties, most notably temperature and humidity. Air masses that originate over water are called maritime air masses, while those that develop over large landmasses are called continental air masses. The classification becomes more specific when one factors in at what latitude the air mass developed.

The primary air masses that affect North America. cA = continental arctic, cP = continental polar, cT = continental tropical, mP = maritime polar, mT = maritime tropical.

Although air masses can develop at any latitude they are generally classified as tropical, polar, and arctic. Tropical air masses develop in the mid-to-low latitudes and have a relatively warm temperature throughout. Polar air masses originate over the northern latitudes (around 50 – 70 degrees), and have a cool to cold temperature, while arctic air masses develop over the very northern latitudes and are the coldest of the three. Any air mass can thus be described in terms of latitude and whether it developed over land or water.

An air mass that originates over water in the tropics is considered a maritime-tropical air mass. Because warm air can hold more moisture than cool air, maritime air masses tend to have more moisture than continental air masses. Maritime-tropical air masses are often the most moist, as they are composed of warm air and develop over an abundant supply of water. In the U.S., maritime tropical air masses that originate over the Caribbean Sea and the Gulf of Mexico bring warm air and and rainfall to the eastern half of the country, particularly the southeast.

Maritime-polar air masses, by contrast, are relatively cool, but still moist. Continental air masses are drier than maritime air masses. Continental-polar and continental arctic air masses bring cool-to-cold, dry air to the mid-latitudes in winter. In the northern hemisphere, the two largest source regions for continental-polar air masses include Canada and Siberia.

Like weather systems, the properties of air masses change as they move from location to location. A continental air mass moving south from Canada, for example, will warm as it moves across the U.S. and down into the subtropics. If a continental air mass reaches a large body of water, its moisture content may increase significantly. Likewise, a maritime air mass will usually lose moisture through precipitation as it moves over land.

Fronts

It’s likely you’ve heard the terms “warm front” and “cold front” many times on your local weather channel. Fronts are simply the boundaries between different air masses. Fronts form whenever two air masses of significantly different densities (related to their temperatures and humidity) either collide or move past one another. Fronts can be hundreds of kilometers long or just a few hundred meters. A front does not rise at a perfect 90-degree angle from the ground, but rather tilts at an angle. The angle of the tilt depends on the types of air masses present and the speed at which they are moving.

Severe weather, such as this tornado, may accompany cold fronts. Credit: NOAA.

A front is considered a warm front when a warm air mass catches up with a slower-moving cooler air mass. Warm fronts are presenting by lines with orange bubbles on weather maps. Cooler air is more dense than warmer air, and therefore tends to settle toward the ground. When a warm air mass encounters a cooler air mass, the warm air is pushed upward into the atmosphere, even as the cooler air at the surface is slowly replaced by warmer air. A warm front therefore tends to have a relatively low-angle tilt.

Because the warm air is pushed up into the atmosphere at a relatively slow rate, warm fronts are not generally associated with strong, sudden storms. If the air has enough moisture, however, light precipitation may accompany a warm front, often lasting a day or more. Because the angle of the warm front is so low, the boundary of the front might pass over you, high up in the atmosphere, several hours before the front reaches you at ground level and you actually feel a change in temperature.

Cold fronts occur when a cooler air mass overtakes a warmer air mass. Cold fronts are represented on maps as blue lines with triangles. The denser cold air mass quickly pushes the warmer air mass high into the atmosphere, creating a front with a relatively high (almost vertical) angle.

The rapidity at which the cool air thrusts the warmer air into the atmosphere causes the warmer air to cool quickly and release precipitation. Strong thunderstorms, tornadoes, wind and heavy precipitation are all usually associated with the arrival of cold fronts due to the rapid rate at which they cause instability in the atmosphere along the frontal boundary. Unlike with warm fronts, the precipitation events associated with cold fronts are usually short lived, and are often followed by a day or more of cool, calm weather.

Warm fronts are typically slow moving and often result in light or no precipitation. A cold front, however, results in the rapid uplift of warm air, often causing heavy precipitation and strong storms. Credit: NOAA.
An occluded front. Credit: NOAA.

Occasionally, a cold air mass will displace a warm air mass and catch up with another cold mass, or visa versa. This situation, which often happens at some point in the life-cycle of a mid-latitude cyclone, results in what is known as an occluded front. An occluded front is represented by a purple line with alternating purple bumps and triangles, representing the mix of multiple air masses. Fronts, and the movement of air masses, have the most direct and dramatic control of our everyday weather. The atmospheric conditions that result when two or more air masses meet is critical to many meteorological models.

On a weather map, L indicates low pressure; H high pressure, blue triangles a cold front, red bumps a warm front, red bumps and blue triangles on opposite sides a stationary front, and purple symbols indicate an occluded front. Credit: NOAA.
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