What Causes Rain?


We all know rain comes from clouds, but how do those clouds form? What causes it to rain? Well, it all starts with water. It’s one of the most unique substances in that it can exist naturally on earth in three different states: solid, liquid, and gas. Among other things, our planet has a special way of filtering water out of the atmosphere and reusing it. You might be thinking how can the Earth reuse water? Well believe it or not, every time it rains we’re actually getting soaked with the same water from millions of years ago, the same water that dinosaurs drank! Our planet has the same water from when it first formed roughly 4.5 billion years ago. 

How is this possible? It all lies in what’s known as the hydrologic cycle. This cycle is responsible for transporting water from the ground to the air and back into the ground again. Water is transported into the air through two main processes: evaporation (such as from a body of water like the ocean) and transpiration (which is evaporation of water from plants). Did you know that of the transpired water passing through a plant, only 1% is actually used in the growth process of the plant. The remaining 99% is passed into the atmosphere!

After water molecules undergo transpiration or evaporation, they are transported into the air where they can rise up, high into the sky and cool down. As the air filled with water molecules cools, it reaches the dew point (which is pretty much what it sounds like, it’s a point the temperature can reach where dew is able to form!). This is known as the condensation stage. During this stage, all of those water molecules are losing energy and start slowing down. They bump into each other and collide, forming droplets of water. What does that all look like? A cloud! Clouds are physically made up of millions of small water droplets. When a bunch of those water droplets start collecting, they grow and grow until they’re too heavy to stay in the air. This is where we head into the precipitation stage. The heavy water droplets start falling and eventually reach the ground as rain.

Excess rain that doesn’t get absorbed into the ground is called runoff. Rivers and lakes are a direct result of runoff! In most cases, the water in these rivers and lakes returns back into the ocean where the hydrologic cycle begins anew. For more information on the hydrologic cycle, you can visit www.weather.gov and https://www.usgs.gov.

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