Hobbies And Interests
Prevailing Wind
Rain shadows are most pronounced in those areas in which the windward side of the mountain range does receive precipitation. For this to occur, air must be saturated with moisture; for instance, it may travel with an extensive "fetch" --- the distance wind blows over a body of water. Steady winds blowing in the same direction ("prevailing") with a long fetch accumulate substantial moisture. Long-fetch prevailing winds drawing off the Pacific are responsible for the rain --- and rain shadows --- all along the Pacific coast, from the Cascade Range and Sierra Nevada all the way down to the Andes, where a rain shadow produces the Atacama Desert, the driest place in the world.
Topographic Features
For a rain shadow to occur, the prevailing wind must be stopped relatively abruptly by a mountainous slope with quickly rising elevation. If the conditions are right, rather than stopping the wind, the slope actually deflects the wind in an upwards path. Glider pilots --- and birds --- take advantage of this phenomenon, known as "ridge lift," to stay aloft for long periods of time. Where meteorology is concerned, ridge lift is also called "orographic lift" --- from "oro," meaning "mountain" --- and has the effect of directing a stream of moisture-laden air upwards.
Windward Side: Rising Air
As the air is forced upwards, the pressure decreases, cooling the mass of air and decreasing the amount of moisture that it can contain. As a result, this moisture begins to condense out of suspension, forming droplets of water that fall as rain or snow on the windward side of the mountains. This accounts for the heavier rainfall on that side; it also means that, as the front continues moving, it has been robbed of its moisture, and little exists to replenish it --- by the time it reaches the leeward slope, there's not much precipitation left.
Leeward Side: Sinking Air
On the leeward side of the mountain range, the air current begins to drop in elevation again; as atmospheric pressure increases, the air is warmed, increasing the amount of moisture that it can hold. This has two principal effects: firstly, it decreases the likelihood of any precipitation occurring, producing the rain shadow phenomenon. Secondly, however, the greater moisture capacity of the descending air increases evaporation on the leeward slope, thereby making it even more arid.