Cyclones - (n.)
Weather is typically described by a weather map, showing lines of equal pressure (isobars), boundaries between air masses (fronts), and direction of motion. A commonly seen map, in mid-latitudes, describes a large cyclonic eddy in the air, which forms when a tongue of warm, moist tropical air invades cold, dry polar air masses. The center of the cyclone is a region of low pressure (indicated by the letter L on maps). There are two fronts (air-mass boundaries): a warm front and a cold front. An observer on the ground sees first the warm front, as the cyclone moves eastward. Air pressure drops, temperature rises with the arrival of the tropical air, and so does humidity. Cloudiness increases and a rainstorm may follow. The reason for the rain is that warm air surrounded by cold air must rise, and as it rises, it cools and loses its ability to hold water. The condensation of the water vapor within the rising air releases heat, so that the air warms and keeps rising until condensation stops. A few days later the cold front arrives, restoring cooler, drier weather. The cyclones are separated by high pressure centers (marked by the letter H). Air moves clockwise around these H centers (anticyclonic circulation), while it flows anti-clockwise within the cyclones in the northern hemisphere. The reason that the air moves in eddies, rather than in straight paths, is the rotation of the Earth. The great centers of cyclone creation in the northern hemisphere are the Aleutian Low and the Iceland Low. Cyclones are extremely common in the bad-weather belt in the sub-Antarctic, with the worst region being the Drake Passage.