The ocean can supply enormous amounts of water vapor. Why is it the case then that some of the driest places on Earth are near the ocean? The important ingredients in order to have a desert next to the ocean are to have the following: cold ocean water, lack of traveling mid-latitude cyclones/tropical systems, and a mountain range that blocks moisture from moving in from other regions. Each of these reasons is discussed additionally below. When the temperature of water is cold, then much less moisture will evaporate from the ocean surface as compared to warm ocean water. Cold salty water does not promote much in the way of evaporation. Even with an ocean full of water, that water is not drinkable and only puts a limited amount of moisture in the air. Some moisture is provided to the air. There are places on earth that are very rainy, snowy and have wet conditions that are next to cold water. Coastal Alaska is an example. Thus, there must be more to the story that just cold water. Mid-latitude cyclones supply a lifting of the air and precipitation. These storm systems can bring rains even to dry places under the right circumstances. Locations on Earth that are dominated by high pressure, such as the belt of sub-tropical high pressure between the equator and mid-latitudes, are not significantly influenced by mid-latitude cyclones. In cases with cold ocean water, tropical storms such as hurricanes will not be able to develop and prosper over cold ocean water. Thus, the high pressure belt limits lifting to the air and the cold ocean water eliminates the possibility for tropical system development. There is an additional reason that makes places that are near cold water and within the high pressure belt even drier. Moisture can evaporate from a warm ocean and travel thousands of miles from the source region before the moisture is precipitated out of the air. A factor that can stop the traveling of the moist air over long distances is the presence of a very high mountain range that blocks the moisture transport. This will result in the moisture being precipitated out of the air as it rises up the windward side of the mountain range. Thus, a location that has all three factors (cold ocean water, in high pressure belt, influenced by a long linear mountain chain) can be extraordinary dry. Examples of locations that have these three factors are portions of the west coast of South America. This region is influenced by a cold ocean current, is within the high pressure belt and the Andes Mountains blocking any Atlantic moisture from making it to the west coast. Some places are so dry that they very rarely receive any precipitation. The Atacama Desert in Chile is a classic example. |