Air Mass

Like an unseen global ocean, the Earth's atmosphere flows over every point on the planet's surface, and showing many features that are similar to the oceans of water that lie beneath it. The winds are its currents, hurricanes are its enormous whirlpools, and storm fronts ripple through it like tsunami. And just as the watery ocean can be divided into 'seas' -- such as the Sargasso Sea or the Arctic Ocean -- by geographical boundaries and water temperature, so the atmosphere is divided into air masses.

An air mass is a region of air that's different from the air surrounding it -- it's warmer, cooler, wetter, drier, or has a different atmospheric pressure. These air masses tend to act as units, and generally don't mix together very rapidly. Instead, they push against each other, flowing across the planet in succession and bringing us our daily weather. Most of our familiar weather -- from rainstorms to thunder, from blizzards to sudden shifts in the wind on sunny days, come from the pushing and jostling of different air masses.

Where do air masses come from?

An air mass is a huge swath of air that has basically the same temperature and humidity through its whole length, width, and depth. Air masses form over areas of the Earth's surface where conditions remain the same for long periods of time, which are called source regions. For example, the Arctic is frigid and parched -- so the air which remains over it becomes cold and dry as well. The tropical oceans are warm and wet, steaming beneath the fierce equatorial sun -- so hot, soggy air masses form over the seas near the equator. The arctic and the tropics, in fact, are two of the major places where air masses are created.

An air mass doesn't remain permanently in its area of origin, fortunately. If it did, most of the Earth's surface would be without rain and would be a lifeless desert. The atmosphere is not a stable, immobile place. Instead, there is a constant rolling and stirring caused by temperature differences, the action of the sun, and the Earth's rotation. It is this pulsing of the atmosphere that makes it act like a huge, somewhat chaotic pump, moving air masses north and south from their point of origin.

So, the combination of cooling air sinking, hot air rising, and the winds and pressures generated by the different temperatures of land and sea all combine to get air masses moving from their point of origin. It takes some time for an air mass to mix with other air masses and dissipate, and during that time it moves into the middle latitudes and pushes against other air masses from elsewhere on the globe. It is this invisible shoving match that generates most of our weather.

Weather -- the visible form of air masses

Almost all of the weather we experience is a result of air masses. When an air mass meets another in our vicinity, we see various effects such as rain, thunderstorms, fog, and similar phenomena. These effects occur at the edge of air masses, but can be many hundreds of miles deep -- the stronger the impact between the air masses, the more lasting and impressive the effects. The boundary between two air masses is known as a front.

Cold air masses often arrive spectacularly, wedging their way under the prevailing warm air and throwing its moisture heavenwards in towers of cumulonimbus, filled with fierce strokes of lightning, torrential rain, and occasionally, hail or tornadoes. Warm air masses are usually more mild in their approach, since they push in from above into a cooler air mass that has less water to turn into violent storms. A period of mild, rainy weather, often without thunderstorms, is typical of a warm air mass' gentler arrival.

The weather at a front isn't the only way an air mass manifests itself, however. Once the front is past and the shift from one air mass to another is complete, the weather we experience until the next front arrives is the air mass itself. Arctic air masses tend to be cooler and drier, producing pleasant, mild days with plenty of sun and crisp white cumulus clouds. Tropical air masses tend to be hot and muggy, with haze and sticky, humid conditions prevailing.

Types of air masses

Although we've been talking mainly about arctic and tropical air masses, there are several different kinds, and with a little practice, you will be able to form a good guess about the type of air mass that's causing your current weather and the region it came from. Each type of air mass comes from a certain kind of source region, and tends to move either north (if it is a tropical air mass) or south (if it is a polar air mass).

The six main types of air masses that you might experience include:

• Maritime polar air masses are those which begin their life over cold polar seas, or perhaps over cold continental regions before passing over ocean waters and picking up moisture. They tend to be cold, although not as cold as continental polar air masses, damp, and unstable, meaning that precipitation such as rain, sleet, and snow can occur within the air mass as well as along the frontal boundary.
• Maritime arctic air masses are similar to maritime polar except for their point of origin.
• Continental polar air masses start over large expanses of northern land such as the wintery regions of northern Canada or Russia. These air masses are cold and dry, with high pressure and stable air. The fact they're usually dry means that there are few clouds to hold in the heat, and they can be especially bitter in winter. Those searing winter nights when the stars seem more vivid than at any other time are continental polar or continental arctic nights. In the summer, these masses produce dry, sunny, mild weather -- often the best outdoor days of summer.
• Continental arctic air masses are very similar to continental polar air masses, except that they originate over the permanent ice cap near the north or south poles.
• Continental tropical air masses start over land in the tropical regions, where solar heat is intense and there is little water to draw from to make the air mass more humid. Hot, dry weather is the rule in an area 'controlled' by such an air mass. Deserts and high plains are the usual breeding grounds for these air masses. These air masses enter the temperate regions mostly in the summer months. If a continental tropical air mass moves in and stays for a while, a drought may result.
• • Maritime tropical air masses have the equatorial oceans as their source region, and tend to be warm, wet, and unstable. Summer weather may turn hot and muggy, with hazy sunlight and scattered thunderstorms in the afternoon. Winter weather turns warmer and damper -- perhaps resulting in a mild thaw if the air mass is strong enough.

The fate of air masses

Air masses that move outside their source regions tend to lead eventful lives. They jostle against other air masses, sometimes wrestling with them for control of a region for days. They may be deflected by another, more massive air mass, halted in their tracks by one of their peers -- leading to the development of a stationary front where the two masses meet, and leading to days of rain for the inhabitants of the boundary, or they settle in for a prolonged stay, bringing in long periods of similar weather. That weather may be pleasant and sunny, hot and dry, damp and drizzly, or whatever else characterizes the air mass.

Air masses are changed by the terrain they pass over, although since they are usually in motion, this change is not as thorough as their initial 'programming' by their source region. Hot air moving over cooler terrain -- such as a large body of water, a winter landscape, and so on -- will become cooled, while cool air can be heated by 'hot' terrain like a desert or open plains, or simply by solar heating as it moves southwards.

Another way an air mass can be changed is by a change in elevation. Air cools as it rises because it expands, and warms as it sinks because the temperature is raised by the air's greater density. So, if an air mass flows down from the mountains to the plains, it will warm, while if it ascends from the plains to the hills, it will cool. This temperature change can cause it to become very different if it is sufficiently strong. The temperature changes about 30 degrees Fahrenheit per mile that air rises or sinks. So, if an air mass climbs onto a thousand-foot-high plateau, it will cool by around 6 degrees. Large mountain ranges like the Rocky Mountains can transform a moist maritime-type air mass into a dry continental-type one simply by lifting it and causing it to cool and dry. The excess moisture will fall as snow, and the air mass will emerge on the far side of the mountain range 'wrung dry.'

Although an air mass can last a long time -- especially considering that it is made up of something as literally light and insubstantial as air -- it will eventually break apart and mix into the general atmosphere. By that time, however, one of its brethren will likely be moving out of the source region to keep the weather cycle going, and bringing a changing panoply of sun and rain, storm and starlight to our days and nights.