The Tropics - The Engine of the Global Water Cycle

Precipitation is highest around the equator and drops as latitude increases.

The Tropical Band
Source - Earthobservatory. nasa.gov

Rain Forests
Rain forests is our name for places that have the highest rainfall. 
As the atmosphere over the Tropics have some of the highest water vapor concentrations, the most rainfall occurs in the latitude band created by the Tropic of Cancer and the Tropic of Capricorn.


Latitude and Precipitation

Precipitation & Latitude
Source - roperld.com

As expected, the precipitation (rainfall) is highest between the bands created by the two Tropics.
The amount of water vapor available in the tropical atmosphere is orders of magnitude greater than that available elsewhere. In other words, the water cycle in the tropics circulates greater volumes of water than the water cycle does in other parts of the Earth, recognizing the existence of Temperate Rain forests that are the result of local variations in atmospheric moisture content, local temperature and other factors.
The tropics are the engine of the global water cycle (aka, the Global Hydraulic Cycle).

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.

100 gallons of Water Added into Air Daily by 1 Big Tree

Trees, and all plants do to some extent, recycle some of the rainwater that falls on them rather than let it all flow away


The "Smoke" in Great Smokey Mountains National Park

Smokey Mountains National Park
Source - art.com

The Great Smokey Mountains contain over 187,000 acres of old growth forest - the largest such forest east of the Mississippi. It got its name because, from a distance, this forest-covered range appears to be exhaling plumes of smoke into the sky above it.
But this is not smoke. This forest is usually covered in a mist that is the result of water vapor transpired by the trees and the natural fog created by water-laden air from the Gulf of Mexico that cools rapidly as it moves onto the higher elevations of the Appalachian mountain range.
Scientists estimate that a quarter of the rain that falls on these trees is transpired back into the atmosphere by the forest.


The Amazon Rain Forest

The Amazon Rain Forest
Source - travelblog.com

Trees in the 55 million year-old Amazon Rain Forest cover over 1.4 billion acres.
The forest gets about 9 feet of rain every year.
Scientists estimate that fully half the rainfall is transpired back into the atmosphere by trees of the rain forest. 
Estimates vary widely, but every canopy tree in the rain forest is estimated to transpires more than 00 gallons of water every day, during the growing season - this translates into adding over 10,000 gallons of water transpired into the atmosphere over the course of a summer from ONE tree.

Global Warming Increases the Ocean in the Atmosphere

Increased water-holding capacity of the atmosphere and increased evaporation are together expected to increase the amount of water in the atmosphere.

Source - SuperStock.com

(1598R‑241586)

Water-Holding Capacity of the Atmosphere
On average, 2-3% of the molecules in the air are water molecules, with a maximum possible of 4% in warm and humid tropical locations.
However, as temperatures increase water vapor volume can exceed 4%, like in the inside of a boiling tea kettle.

Climate models, Earth-system science and satellite observations all conclude that as carbon dioxide levels increase, the air will heat up and water vapor volume in the air will increase beyond the maximum of 4%. 


Surface Evaporation
Evaporation is a continuous surface phenomenon that maintains an equilibrium between molecules leaving a water surface as vapor and vapor molecules being added to a water surface as liquid. As higher temperature implies more kinetic energy, there is more evaporation at higher temperatures


7% vs. 1-3% increase in water fluxes
Atmospheric studies and Earth System Science predict that for every degree Kelvin increase in surface water temperature, the ocean of water in the atmosphere will increase by 7%, while precipitation will increase between 1% and 3% - Source: Science, Vol 317, pages 233-235

Planes may increase Precipitation

Motion through clouds can expand air and, through this expansion, the air is cooled to such a low temperature that vapor condenses to liquid and even forms ice crystals.


Water Droplets seen in Clouds That Planes Penetrated

Source - http://www.youtube.com/watch?v=pYWAR2OiiNg&feature=mfu_in_order&list=UL


As reported in the LA Times (2 July 2009 issue), a study has been completed by the National Center for Atmospheric Research in Boulder, Colorado, that links planes to precipitation.
The study focused on satellite images taken over a four-hour period of a cloud layer over Texas. The images revealed a series of holes and canals in the cloud layer, that upon analysis, were shown to be made up primarily of liquid water droplets.
Scientists measured the holes and canals, and through a review of FAA records, identified a full array of aircraft that were involved.


Expanding Air Enables Flight and Condensation
Computer models built as part of the study showed that the very same mechanisms that keep airplane aloft had cooled the air to the extent necessary to condense water vapor. Cooling of air was the result of ambient air being forced to expand around propeller tips and over a jet wing.


Condensation Nuclei NOT Needed
Typically, an increase in collisions between water vapor molecules and condensation nuclei is required to convert vapor molecules to liquid molecules. But, as this research shows, rapid air expansion by motion of airplanes can produce precipitation without requiring condensation nuclei.