Sometimes, it takes a young, inquisitive mind to raise an important weather question. Such was the case, recently, when Alex, an eight and half year old from New Zealand, queried, “Why doesn’t it rain in dry countries, when there is water all around there?”
This sets the stage for discussing “desert islands” and other places near oceans where rainfall is scarce. This may or may not have been on the mind of Samuel Coleridge in his “Rime of the Ancient Mariner.” But there really are places where the following holds true, “Water, water, everywhere, nor any drop to drink.”
Alex told me in her e-mail message that she knew all about the water cycle – water evaporates, forms clouds, clouds rain and the process keeps going on.
But, Alex couldn’t figure out why it doesn’t rain when there are oceans all around.
Water evaporates from water bodies, vegetation, puddles and other Earth-based sources all the time. Evaporation (the transfer of water into vapor) proceeds faster when it is warmer and drier. But evaporation is only one part of the water cycle equation.
Something has to process the water vapor and get it to return to its liquid or solid form. We see this transformation when clouds appear. The clouds can be high fliers (cirrus, filled with ice crystals in all seasons), cumulus (towering clouds), stratus (low-lying, flat clouds) or a myriad of other varieties that fill the skies from the Earth’s surface to the top of the troposphere (around heights of 8 to 12 miles above the Earth).
Condensation or deposition (the process of transforming water vapor directly to ice) requires either cooling of the air or a pressure reduction. Both occur when air rises vertically (cumulus type clouds). The process is slower, and may never even yield a cloud, when motions are more horizontal. Sinking air acts in the reverse, evaporating clouds.
Rising air occurs most often near low-pressure systems and weather fronts are present. Air often rises strongly when the ground or ocean surface is very warm (provided that there are no warm layers or inversions in the atmosphere above). Air also rises over mountains and where sea breezes occur. Yet, we know from experience, rainfall data and weather satellite images that clouds don’t occur everywhere.
In fact, there are some places where high pressure and sinking air dominates the weather almost year round. The regions along and either side of the Tropic’s of Capricorn and Cancer (both around 23 degrees from the Equator) are perhaps the most common. Notice the east-west bands of desert regions that straddle these two latitudinal lines. In between these two bands lies the meandering Inter-Tropical Convergence Zone (ITCZ), with its locally heavy rainfall regions (Fig. 1).
Another place where rainfall is scarcer is where ocean or lake temperatures are cold or the ground is snow-covered. One such place is the Atacama Desert that lies along Chile’s northwest coast. Here, average annual rainfall is almost zero. Southern California southward to western coastal Mexico is another dry place due to chilly ocean water temperatures.
Finally, atmospheric winds move the water vapor around a large, rotating sphere that moves in an annual passage around the Sun. The water vapor is not distributed in a uniform manner and the forces that create upward motion are not uniform either.
Put all this together and it starts to make sense that some places are rainier than others.
The result is a set of climate types that are based on rainfall and temperature. These two weather variables alone control vegetative growth and define many ecosystems. Known as the Koeppen Climate Classification System (Fig. 2), helps us to see why certain vegetative zones exist where they do.
So, desert islands or other dry places really are as likely to exist as places with an over-abundance of rainfall.
© 2010, H. Michael Mogil, All Rights Reserved
