There are some days when the rain falls peacefully and gently and nourishes the earth. But some other days the rain falls in a pouring rain Meteorologists like me call a downpour. Standing outside in one of those intense rainstorms makes you feel like you are suffocated in a heavy, wet towel. These storms can flood the land below them, causing great destruction.
What is causing this difference?
All rain comes from a combination of two things: humidity – usually in the form of clouds – and air currents moving upwards. When moist air rises through a cloud, the air cools down and the water in it turns into tiny raindrops.
This is the same thing that happens when you can See your breath on a cold evening. The temperature change from warm to cold causes water droplets to form in your breath.
In a cloud, these tiny raindrops are very light and float when the rising air pushes them upwards. But the higher they go, the bigger and heavier they get. Eventually they get so heavy that they fall to earth as rain.
Cold air storms are steady and slow
Cold air can hold much less moisture than warm air, so winter clouds don’t contain much water. you are rather thin and layered as puffy and big and full of water.
Since cold air likes to sink to the ground, it is difficult to get that air up quickly, so these winter clouds have only slight air currents upwards. As these slow currents pass through the thin clouds, which don’t contain much moisture, tiny raindrops form. Gravity pulls them down slightly against the airflow before they get too big. When the clouds are thin and the air is moving slowly, it’s nice and quiet.
Thunderstorms and large winter storms are quick and intense
Hard rainstorms occur when there is a lot of moisture in the air and the air moves upward very quickly. Summer storms are the perfect example.
The warm, moist air rises very quickly – like a hot air balloon – and can move as fast as 30 to 40 mph. The air also holds much more moisture than winter clouds – up to five times as much.
All of this creates very high, thick clouds that are full of moisture. Water droplets form quickly as the air moves up through the clouds. But because the wind is blowing up so quickly, the droplets can get huge before gravity pulls them down to the earth. When the weight of all the water droplets becomes too high for the wind, the wind flow collapses and all the raindrops in the cloud collapse at once. These are summer thunderstorms.
Thunderstorms can drop an inch, two, or even three inches of rain less than an hour. These sudden, heavy rains, called cloudbursts, can cause flash floods that can flood streams and roads, trapping people everywhere.
Fortunately, because thunderstorms are so violent and relatively small, they don’t last long. Once the rain falls from the clouds and crushes the upward currents of air, the clouds disappear and you often see a beautiful blue sky.
Of course, winter can also trigger some strong storms – especially over the warmer sea water. When strong winter storms drop a lot of heavy rain, the same principles apply: lots of moisture in the air, fast wind currents upwards and high clouds.
No two rainstorms are ever the same. Sometimes clouds can rain so hard that you feel like you are in the shower. Other times they just bring a nice peaceful drizzle. Whether you’re soaked or singing in the rain, you’ll know why.
This article is republished by The conversation under a Creative Commons license. read this original article.