Air masses are very large quantities of air that have approximately the same temperature and humidity throughout. They are created over sea (maritime), large areas of land (continental) and over the poles (arctic and Antarctic). These air masses are further broken down into warm (tropical) and cold (polar) types. So a polar continental mass would be one made up of a uniform mass of cold air formed over a large land area, while a tropical maritime mass would be one made up of warm air formed over an ocean.
Typically, air masses formed over oceans contain more water vapour that those formed over continents. Warm air masses also contain more water vapour than cold air masses. Thus a tropical maritime air mass has warm humid air, while a continental polar mass has cold dry air. Polar air masses also create higher pressure beneath them as the cold air is denser. Tropical air masses create lower pressures beneath them because the warm air is less dense.
Air masses are pushed or dragged along by the winds. As they are moved, they take on some of the character of the surfaces over which they travel. The cold air masses from the higher latitudes are continually moving down towards the lower latitudes. Meanwhile, the tropical masses are moving up from the middle latitudes. The junctions between different air masses are called fronts. The fronts that mark the boundaries between the polar air masses and the tropical air masses are called polar fronts. These regions of contact are where unsettled weather occurs.
Unsettled weather with stormy winds and rain is brought about by the depressions, or low-pressure regions, that the weather forecasters talk about on TV. These are formed when a tropical air mass bulges into a polar air mass, thus buckling the polar front. The warm tropical air mass, being less dense, rises over the cooler, dense polar air. This rising air causes the air pressure to be reduced under the bulge. The cold dense air races towards the area of lower pressure and, because of the Earth’s spin, begins to spiral around it. The spin of the Earth on its axis makes the cold air move anticlockwise in the northern hemisphere and clockwise in the southern hemisphere.
As the front of warm air continues to ride up and over the cold air, the pressure drop is greatest immediately below the advancing tip of the warm front. This marks the middle of the depression. Meanwhile, the cold air front swings around behind the warm air bulge, undercutting it.
In time, the cold front catches up with the warm front and cuts off the bulge of warm air above it from the main part of the tropical air mass. When this happens, what is known as an occluded (closed-off) front is formed, and the air pressure below starts to rise. The isolated region of warm air gives up its heat to the cold air and mixes with it, so the pressure bellow rises again.
The opposite of a depression is a high-pressure region, or anticyclone. These come about in areas where the air is descending. Descending air warms up and tends to absorb water droplets – so it removes any clouds from the sky. Areas of high pressure usually produce stable weather with clear skies, leading to hot sunny days in summer and cold days with hard frost in winter.