Weather systems of tropical and equatorial zones show some basic differences from those of middle and high latitudes. Weakness of the Coriolis force close to the equator and lack of contrast between air masses conspire to prevent development of strong, clearly defined fronts and intense wave cyclones of the types that characterize higher latitudes. Nevertheless, there is no lack of intense atmospheric activity in the form of convective cells, for the high specific humidity of maritime air masses in low latitudes guarantees an enormous reservoir of energy in the latent form. This same energy source powers the most formidable of all storms-the tropical cyclone.
Fundamentally the arrangement consists of two rows of high-pressure cells, one or two cells to each land or ocean body. The northern row lies approximately along the Tropic of Cancer; the southern row, along the Tropic of Capricorn. Between the subtropical highs lies the equatorial trough of low pressure. Toward this trough converge the northeast and southeast trade winds. For this reason the trough is called the Intertropical convergence zone. At higher levels in the troposphere, the airflow is almost directly from cast to west in the form of persistent tropical easterlies.
One of the simplest forms of weather disturbance is an easterly wave, a slowly moving trough of low pressure within the belt of tropical easterlies. These waves occur in a zone 5° 30° latitude over oceans both north and south of the equator, but not over the equator itself. The wave is simply a series of indentations in the isobars to form a shallow pressure trough. Note that it travels westward, perhaps 200 to 300 mi (325 to 500 km) per day. Airflow tends to converge on the eastern, or rear, side of the wave axis. This causes the moist air to be lifted and to break out into scattered showers and thunderstorms. The rainy period may last for a day or two.
Another related disturbance is an equatorial wave, or weak equatorial low, which forms in the centre of the equatorial trough. Although the normal airflow is from east to west, there develops an eddy in the tropical easterlies. Here, air flow is locally reversed and tends to run opposite to the main stream. The result is a weak low-pressure centre into which moist equatorial air masses converge, causing rainfall from many individual convectional storms within the low. Several such weak lows are shown on the world weather map, lying along the equatorial trough. Because the map is for a day in July or August, the trough is shifted well north of the equator.
Another distinctive feature of tropical weather is the occasional penetration of powerful tongues of cold polar air from the middle latitudes into very low latitudes. These are known as polar outbreaks and bring unusually cool, clear weather with strong, steady winds moving behind a cold front with squalls. The polar outbreak is best developed in the Americas. Outbreaks which move southward from the United States over the Caribbean Sea and Central America are called northers or nortes; those which move north from Patagonia into tropical South America are called pamperos or friagems.
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