Weather comes in all shapes and sizes. As you know, it changes constantly. There's an old saying in Maine that if you don't like the weather, just wait a minute! However, certain patterns of weather seems to form time and time again. These somewhat more predictable features of the weather make the job of weather prediction somewhat easier.
Air masses are simply big mounds of air that sit over an area. The air in an air mass tends to have the same temperature and humidity throughout the whole mass. The way that this makes our job of weather prediction easier is that certain air masses tend to bring us certain kinds of weather. If the weather in your area is being partially influenced by an air mass, it makes your job of weather prediction somewhat easier, since certain air masses tend to bring about certain kinds of weather.
There are four basic types places that air masses can form:
Over the ocean: These are called MARITIME AIR MASSES, they tend to contain moist air. (Because they formed over water, and water is moist--get it??)
Over land: These are called CONTINENTAL AIR MASSES, they tend to contain dry air.
Over warm areas: These are called TROPICAL AIR MASSES, they tend to contain warmer air.
Over cooler areas: These are called POLAR AIR MASSES, they tend to contain cooler air.
The four types of air masses that can form, are every combination of the above masses possible;
MARITIME TROPICAL AIR MASSES: These contain moist, warm air. They can bring hot and humid conditions in the summer, as well as precipitation. In the winter, they tend to bring precipitation of some type.
MARITIME POLAR AIR MASSES: These contain moist, cool air. In the summer they can bring cool rains and precipitation to an area. In the winter, they are usually responsible for heavy snow.
CONTINENTAL TROPICAL AIR MASSES: These contain dry, warm air. They tend to bring dry, warm weather conditions, and no precipitation.
CONTINENTAL POLAR AIR MASSES: These contain dry, cool air. They bring cooler temperatures in the summer, and in winter are often responsible for bitterly cold temperatures.
When two air masses meet, a FRONT forms. A front is simply the boundary between two moving air masses containing different temperatures, humidity or winds. The weather is usually unsettled and stormy along a front. Four different types of fronts are possible.
COLD FRONTS: A cold front is the movement of a cold, or cool, air mass. The cool air mass meets and replaces warmer air. The cold air forces its way underneath a warm air mass and pushes it upwards. For the United States, Canada is like a giant icebox that holds cold air which is ready to move into our country when winds begin. The air slides from north to south as if on a ramp. It is in the shape of a wedge with a sharp edge. Cold fronts move fairly rapidly, certainly much faster than a warm front. Cold fronts are preceded by thunderstorms, tornadoes, hail or sometimes only a brisk wind and overcast sky. All cold front, strong or weak, will deliver some sort of weather change. cold fronts move in cycles that generally last 5-7 days.
WARM FRONTS: A warm front is the movement of a warm air mass. It is formed when a mass of warm air overtakes a cold air mass and moves over it. Warm fronts are much gentler in nature than are cold fronts, and tend to move slower too. That means that the weather that they can bring tends to last a little longer. The warm air, which is in the shape of a mound, overtakes the cooler air, in the form of a wedge. It slowly slides up the ramp of cool air. The precipitation caused by a warm front is usually milder than the types of precipitation brought about by a cold front. As the front passes, skies will clear and temperatures will usually rise. The passage of a warm front is almost always followed by the approach of a cold front.
STATIONARY FRONTS: A stationary front is neither a warm nor a cold front. It is an almost motionless boundary between two air masses. Almost any type of weather is possible. Cloudy skies accompanied by precipitation are the most common results. The weather may be stab e and stay the same for quite some time, even weeks! Stationary fronts are especially common during the summer months. Eventually, stationary fronts will dissipate, as the air masses on either side of the front will acquire similar characteristics. If the stationary front does not dissipate, usually a warm or a cold front will form.
OCCLUDED FRONTS: When a moving cold air mass meets a moving warm air mass it will overtake it, slide underneath, and push the warmer air mass aloft. After that the cold air wedge will drag the warm mound of air along with it. The two air masses stay locked together, forming an occluded front. An occluded front generally brings precipitation, but usually not HEAVY precipitation.