<P> Another convention utilizing these symbols is the indication of modification or transformation of one type to another . For instance, an Arctic air mass blowing out over the Gulf of Alaska may be shown as "cA - mPk". Yet another convention indicates the layering of air masses in certain situations . For instance, the overrunning of a polar air mass by an air mass from the Gulf of Mexico over the Central United States might be shown with the notation "mT / cP" (sometimes using a horizontal line as in fraction notation). </P> <P> Arctic, Antarctic, and polar air masses are cold . The qualities of arctic air are developed over ice and snow - covered ground . Arctic air is deeply cold, colder than polar air masses . Arctic air can be shallow in the summer, and rapidly modify as it moves equatorward . Polar air masses develop over higher latitudes over the land or ocean, are very stable, and generally shallower than arctic air . Polar air over the ocean (maritime) loses its stability as it gains moisture over warmer ocean waters . </P> <P> Tropical and equatorial air masses are hot as they develop over lower latitudes . Those that develop over land (continental) are drier and hotter than those that develop over oceans, and travel poleward on the western periphery of the subtropical ridge . Maritime tropical air masses are sometimes referred to as trade air masses . Monsoon air masses are moist and unstable . Superior air masses are dry, and rarely reach the ground . They normally reside over maritime tropical air masses, forming a warmer and drier layer over the more moderate moist air mass below, forming what is known as a trade wind inversion over the maritime tropical air mass . Continental Polar air masses (cP) are air masses that are cold and dry due to their continental source region . Continental polar air masses that affect North America form over interior Canada . Continental Tropical air masses (cT) are a type of tropical air produced by the subtropical ridge over large areas of land and typically originate from low - latitude deserts such as the Sahara Desert in northern Africa, which is the major source of these air masses . Other less important sources producing cT air masses are the Arabian Peninsula, the central arid / semi-arid part of Australia and deserts lying in the Southwestern United States . Continental tropical air masses are extremely hot and dry . </P> <P> A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena . In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front . The air masses separated by a front usually differ in temperature and humidity . Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines . Warm fronts are usually preceded by stratiform precipitation and fog . The weather usually clears quickly after a front's passage . Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift . </P>

Which type of air masses are associated with dry cool weather