Rain occurs when liquid precipitation falls from the clouds
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High and Low Pressure Systems
Rain is ultimately caused by differences in air pressure. Air with high pressure sinks and stays near the ground, while low pressure air rises.
Although we might think of air as weightless, this is not true. All matter, even invisible gases have weight. The more air on top of you, the greater the air pressure, or the weight of the air in the atmosphere. So, low pressure systems have less air above them, causing them to rise. High pressure systems typically are near the surface, with lots of air on top of them, causing them to sink.
Think of it like a light and heavy object falling into a pool. High pressure means there are more molecules in a given area, which makes the air heavier. Heavier objects, like a nail, would sink in a pool. On the other hand, low pressure air has less air molecules in the same amount of area, so this type of air rises. Think of low pressure air like a float in water - it is less dense than water, so it rises to the top of the pool.
Air Movement
So, what does all this air pressure stuff have to do with rain? Well, when air rises, it cools in the upper atmosphere. Like we mentioned, the molecules in cold air are packed closely together. This makes less room for water vapor, or water in its gaseous form. When the water vapor gets cold, it condenses into liquid water. The liquid water combined with dust particles in the atmosphere form clouds. When the clouds become saturated, or containing the maximum amount of water they can hold, rain droplets form. And, when the cloud can no longer hold anymore water, the water falls from the sky as precipitation.
So, when a low pressure system enters the area, the low pressure air rises into the atmosphere and the high pressure air stays near the surface. As the air rises, it cools and forms clouds. Eventually the clouds become saturated and rain falls.
Clouds form over the Eastern United States due to a low pressure system
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Temperature Changes
Changes in temperature can also cause precipitation. Cold air is denser than warm air, and sinks towards the Earth. When a cold front, or a cold body of air, moves into an area, it forces the warm, less dense air to rise. As the air rises, it cools, water condenses forming clouds, and eventually falls as precipitation.
Cold air depicted in blue forces the warm air upward where it forms clouds and rain
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Geological Features and Rainfall
Now that we know the basic principles of rainfall, let's look at why different areas of the Earth get more or less rain.
If you had to pick the city with the most rainfall in the United States, you might think of Seattle, Portland or other cities in the Pacific Northwest. These weather patterns have to do with the mountains surrounding the area. As warm, moist air blows inland from the Pacific Ocean, it is forced up the mountain face. As the air rises, it cools and water condenses producing rain. This rainy side receiving air from the ocean is called the windward side of the mountain, and gets lots of precipitation.
However, the other side of the mountain, the leeward side, gets very little rain at all. This side of the mountain is often a desert, like the Great Basin in Nevada, Utah and Arizona, which are on the leeward side of the Cascade Mountains.
The windward side of mountains receives more rain
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Lesson Summary
Rain is liquid precipitation falling from the clouds. Rain occurs when air rises into the upper atmosphere and cools. The cool temperatures cause water vapor to condense into water droplets, which fall from the clouds as rain when the air becomes saturated. Air pressure contributes to this process.
Low pressure systems bring stormy weather, since the low pressure air is less dense, causing it to rise into the clouds, where water vapor condenses to form rain. Changes in temperature can also cause rain. Cold air is denser than warm air. When a cold front enters an area it forces warm air into the clouds, where water vapor condenses into rain. Geological barriers also alter rainfall patterns, like mountains whose windward side gets more rain than the leeward side that gets less rain.
