The tropics (ancient Greek tropaí = tropic circles) are a climate zone, which is located between the northern and southern tropic. Tropics do not mean tropical rainforests per se. The tropical rainforest is only one of five vegetation zones within the tropics. In addition, a distinction is still between desert, thorn-bush savanna, dry savanna and wet savanna.
The ratio of precipitation and evaporation rate significantly determines which vegetation zone is formed. If the precipitation exceeds the evaporation rate, one speaks of a humid month (lat. Umidus = damp). If the evaporation rate exceeds the precipitation, one speaks of an arid month (Latin aridus = dry). Arid and humid months occur in all five vegetation zones of the tropics. Decisive for the vegetation zone is here again the appropriate ratio.
Number of arid months and the corresponding vegetation zone:
10-12 arid months = desert
8-10 arid months = thorn-bush savanna
5-8 arid months = dry savanna
3-5 arid months = wet savanna
0-3 arid months = Tropical Rainforest
in reverse:
9-12 humid months = Tropical Rainforest
7-9 humid months = wet savanna
4-7 humid months = dry savanna
2-4 humid months = thorn-bush savannah
0-2 humid months = desert

Expansion of the tropics:

Characteristics of the tropics

Change from humid and arid climate
high radiation intensity of the sun
Year-round high evaporation rate
high humidity during the humid months
small fluctuation range of the temperature
Sun is in the zenith almost all year round
Daytime climate instead of seasonal climate
nutrient-poor soil
immense biodiversity (apart from the very arid areas)
Relatively stable climate