Rainfall in the Tropics

Earlier this week we looked at the radiation budget of the Earth, that is the difference between the incoming energy from the Sun and the outgoing infra-red energy lost to space. The most striking feature of this budget is that, in the tropical regions of the Earth there is a surplus of energy whilst outside the Tropics there is a deficit.

The excess of energy within the Tropics has a fundamental impact on the types of weather systems that form in this part of the world; with more energy available to the atmosphere it is no surprise that tropical weather systems are more intense than those in the extra-tropics. The most obvious way that this can be seen is by looking at rainfall amounts. It rains far more in the Tropics than elsewhere on the globe.

One way that the large amounts of energy from the Sun entering the Tropics is manifested is in evaporating water from the ocean surface, and from the soil and vegetation over land. This increases the amount of water vapour in the atmosphere available to form clouds and thus rain as it condenses. The map in figure 1 shows the global mean precipitation rate (in mm per day). Although there is some heavy rain outside the Tropics, mainly over the western North Pacific and North Atlantic, it is clear that all of the really wet parts of the Earth are in the Tropics. Much of this rain in the Tropics falls in intense thunderstorms.

Figure 1. Annual average precipitation rate (in mm per day). This estimate is based on information from rain gauges over land and satellite measurements over the oceans.© 2010 UCAR

Figure 2: Annual mean sea surface temperature (degrees C), averaged from 1971-2000. Note the very warm water in the tropical west Pacific and the cold “tongue” along the equator in the east Pacific.© US Climate Prediction Center