The most common factor in the formation and location of deserts is distance from sea. Air moving over seas and oceans collects water vapor evaporated from the surface by the sun’s heat. As the moisture-laden air crosses the coast and blows over the land, it rises and cools, causing the water vapor to condense and fall as rain or snow. The air gradually loses its moisture and moves on. Unless it crosses a major lake or river system, where more sun-powered evaporation can take place, the air’s water vapor content is not replenished, and the farther it travels from the coast, the drier it becomes. Consequently, regions located deep inland, in the centers of continents, generally have dry climates; wherever winds may blow from, they have generally lost their moisture by the time they arrive. In some cases the dryness is severe enough to maintain deserts. Since these deserts are found toward the middle of major landmasses, they are termed continental deserts.