Definition of the tropics

How to define the tropics?

If you define the tropics according to the solar orbit, the tropical zone is the area over which the sun is at its zenith twice a year. This is the area between the turning circles, which thus represent the border of these solar tropics, i.e. between each 23.5° northern and southern latitude around the equator. Above the tropics, the sun is vertical only once a year, namely at the time of the so-called solstice. The consequence of the sun’s position, which deviates only slightly from the vertical over the year, is a high level of irradiation throughout the year with correspondingly high temperatures. However, since the heat of the day comes mainly from direct solar radiation, it cools down noticeably at night. The temperature differences between day and night are relatively large, especially when compared with the very small differences in the average daily temperatures occurring during the course of a year. This sequence of climate is therefore called diurnal climate, which is very characteristic of tropics.

Another common classification of climate zones is by average temperatures. Here, the equator is the hottest climate zone on earth with average temperatures of 25°C to 27°C. This is mainly due to the length of the day, which only varies between 10.5 and 13.5 in the tropics.


The delimitation of the areas can also be based on the tropical climate. Thus, the climate of the inner tropics near the equator is called always humid tropical climate, which has high precipitation due to the average annual precipitation of about 2000mm. Whereas the outer tropical zones are called summer humid tropics and have a winter dry season. This precipitation is the zenithal rain following the peak of the sun, resulting from the rising air masses within the migrating intratropical convergence.


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