Mountains

Mountains make a barrier for moving air. The wind pushes air, and clouds in the air, up the mountain slopes. The atmosphere is cooler at high elevations, and there is less of it: lower pressure makes it hard for lowland animals to get enough air to breathe. Dense masses of warm, moist air that move up and over a mountain swell as the air pressure confining them drops away. The air becomes colder in the same way as a pressurized spray can's contents become colder when the can's pressure drops rapidly. (The phrase that describes this phenomenon is adiabatic expansion.) Water that existed as a gas under the high pressure and temperature of the flatlands now condenses into cool droplets, and clouds form over the mountain. As the cloud continues to rise, droplets grow and grow, eventually becoming too heavy to float in the air. The clouds dump rain, and snow, on the mountain slopes. After topping the crest, however, the clouds may have no more moisture to rain on the other side of the mountain, which becomes arid. This rain shadow is best illustrated in the Sierra Nevada mountains of California, where tall redwood forests cover the ocean-facing side of the mountains, and Death Valley lies in the rain shadow.