Sun+Atmosphere = A Moisture Pump

An energy imbalance in the atmosphere moves air, and the moisture it contains, from the tropics to higher latitudes.

Energy for Atmospheric Circulation

Sunlight
Source - digitalart.org

The most energy from the sun, falls on the Earth's equator. The least energy from the sun falls on the polar regions. 
This difference in energy creates an energy potential that moves air (mass) from the equatorial region to the polar regions through a series of complex steps and air movement patterns.

Source - geology.wmich.edu

The Tropics - A Moisture Source

As temperatures rise, more water evaporates and is contained in the atmosphere. The most water, thus, exists in the atmosphere over the tropics and equatorial region, while less moisture exists in the air over the poles. 

As the air moves from the warmer to the cooler areas, this air acts like a moisture pump. While a number of global and local influences impact this pump's operation, the tropics are a dominant source of moisture to higher latitudes.