Why Does It Rain?

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson we'll learn the main causes of rain, including why low pressure systems bring stormy weather and how collisions of warm and cold air influence rainfall. Then, we'll look at the impact of geological structures on rainfall.

What Is Rain?

What a gloomy day outside! You check the weather and there's about a 70% chance of rain. You grab your best rain boots and umbrella, and head out into the soggy world. Rain is liquid precipitation that falls from the clouds. But, have you ever thought about how rain forms? Today, we're going to explain why it rains, and what scientific principles are at work.

Rain occurs when liquid precipitation falls from the clouds

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
clouds in low pressure system

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
cold front

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.

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