Intra-tropical convergence zone
The intertropical or intertropical convergence zone ( ITC for I nter T ropic C onvergence or ITCZ for I nter T ropical C onvergence Z one ), also called Kalmengürtel , equatorial depression and in the Atlantic Doldrums , is a few hundred kilometers wide depression near the equator in the area of the trade winds that meet from north and south . It is characterized by convection phenomena and usually strong cloud cover . This means that the general calm in this part of the oceans is interrupted several times a day by downpours and thunderstorms with stormy and strongly rotating gusts. Overland, the severity of the storm depends on the local humidity.
The ITC arises in the tropics , where the zenith of the sun moves between the tropic and the tropic . The solar radiation in the tropics is generally very high due to the steep angle of radiation . However, where the sun is at its zenith, the irradiation is highest. This is where the surface of the earth or the water surface is heated the most. Heated surfaces release the energy into the air as radiant heat. As a result, the air masses expand ( thermal expansion ), become lighter and rise. The vertical ascent creates a "suction" near the ground, an area with less "lower" air pressure. This is why one speaks of a heat dip (T). Where the zenith is currently, with a time delay of 3 to 4 weeks, due to the relatively strongest warming, an extensive heat depression (in the broadest sense) develops almost parallel to the latitude around the whole earth. This is the equatorial low pressure channel , above which you can see the cloud band of the ITC on satellite images .
The heated air rises due to its lower density ( convection ). Because the air pressure decreases with increasing altitude, it expands adiabatically and cools down. When the temperature falls below the dew point , massive cloud formations form, due to the falling water vapor capacity of the air and the mostly high humidity . Strong zenital precipitation is the result. During the condensation at high altitude, the stored thermal energy is released again, which was supplied to the air below during evaporation through radiant heat . Since during the evaporation process, thermal energy is invested in changing the state of aggregation (see evaporative cooling) and the air therefore does not heat up as much as it would without absorbing water vapor, one speaks of the heat energy contained in water vapor-saturated air and not measurable with the thermometer of "latent heat".
While air masses are set in motion parallel to the earth's surface by local pressure differences and accordingly always flow from a high pressure to a low pressure area, the vertical transport of the air masses is driven by temperature-related differences in density . The air masses heated by the heated surface of the earth are not displaced and driven by a high pressure area, but tend to rise themselves, leaving behind a low pressure area on the earth's surface, which in turn sucks in near-surface air from other regions.
At altitude, the air flows sideways (i.e. north and south). As a result of the expansion, ascent and lateral flow of the air masses at great heights, both the air density and the air pressure near the ground drop sharply. As a result, a zone of stable low pressure areas spanning the entire globe is formed , both vertically and horizontally, which is known as a low pressure channel.
Since air pressure differences are equalized by mass flows, air masses flow horizontally from the north and south below, which is known as convergence . Horizontal air movements are called wind. The intra-tropical convergence creates the trade winds, which are relatively constant in direction and strength.
By the Coriolis force , an image force , the trade winds are deflected on the northern hemisphere and the southern hemisphere in their direction of movement, which is why the resulting winds that trade-winds , both on the north and also have in the southern hemisphere an east component (trade winds and southeast trade). Due to the strong convection, the troposphere has its greatest height in the ITC and the tropopause is also higher.
In the area of ITC, the Walker circulation also acts , which u. a. is partly responsible for the El Niño phenomenon . However, there is often no wind in the equatorial low pressure channel, which is why it was problematic for sailing seafarers to pass through the Kalmen , while in the trade zone they could rely on constant winds. The intra-tropical convergence zone forms in the area of the greatest warming of the earth's surface. Therefore it follows the zenith of the sun with a delay of almost a month.
The zenith of the sun runs parallel to the latitude and shifts over the course of the year. Only in the tropics can the sun stand exactly vertically over you during its midday high (at the zenith ) and only at certain times of the year. Since the earth moves in an orbit around the sun and its axis of rotation is inclined at an angle of 23.5 °, the position of the zenith changes continuously. During the equinox around March 19-21, the sun is at its zenith over the entire equator . During the northern summer solstice on June 21st, it is at its zenith over the tropic . During the second equinox around September 22nd to 24th, the zenith is above the equator for a second time in the same year. During the winter solstice on December 21, it reaches the tropic . This wandering of the zenith of the sun has far-reaching effects on the thermal air circulation and the shift of the entire trade fair cycle over the course of the year.
Land surfaces are warmed up more strongly by solar radiation than ocean surfaces and therefore give off significantly more heat energy to the air in the months in which the zenith of the sun moves over them. The different warming of land and sea areas has a strong influence on the extent of the relocation of the ITC over the course of the year. The uneven distribution of land areas on earth means that the central position of the ITC is approximately 5 ° north latitude. Over the Pacific and Atlantic Oceans, it shifts only a few degrees over the course of the year, but over South America it shifts significantly, especially in southern summer, because of the larger land mass to the south and the rainy seasons accordingly . Since the Indian Ocean is surrounded on three sides by large land masses, the shift over the resulting Asian-African monsoon area is particularly pronounced. To the north of India, due to the effects of the Himalayas and the highlands of Tibet, to the north even the tropic is exceeded.
The course of the ITC and its seasonal change thus also influence the climate zoning . Without the influence of the land masses, the zoning of the climatic zones would be much more similar to a global belt pattern parallel to the latitude.
When the ITC crosses the equator and shifts north after March 21 and south after September 23, a secondary ITC is created by the evaporation of the rich zenital precipitation in the rainforest areas in the equatorial zone .
Effects of the ITC
The vertically rising humid air masses lead to strong cloud formations ( cumulonimbus ), torrential showers and thunderstorms . Since the humid air with its high content of gaseous water vapor cools down considerably on ascent to very high altitudes, its water vapor capacity decreases progressively. The relative humidity continues to increase. If the dew point is not reached, condensation occurs. In the process, condensation heat escapes , which additionally accelerates the rise of the air masses. The condensation of water vapor produces enormous amounts of liquid water due to the size of the convection currents, which leads to heavy rains. Both the zenith rains in the always humid and alternately humid tropics and the almost year-round midday rain in the always humid tropics are thunderstorms.
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