I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook. In the early 19th century, it was thought, among many, that clouds were made up of millions of little bubbles. It wasn't understood that solid water drops could actually float in the air. At some point, someone came up with the concept of what is called terminal velocity and with that came some understanding of clouds and rain.
Cloud drops are being tugged earthward by gravity, however, as the drop speeds up, the air pushes back on the drop. Eventually these two forces balance each other out and the drop doesn't go anywhere. It's now floating on air. If the air is not going up or down, which hardly ever really happens, the cloud drop will lazily drip towards the ground at about 2 feet per minute--its terminal velocity. Now, by comparison, if I jumped out of an airplane for some strange reason, my terminal velocity would be about two miles per minute.
Now a cloud drop is about 20,000 times smaller than a raindrop, which is about a sixteenth of an inch across. So raindrops have a higher terminal velocity than cloud drops. And most rain falls at around15 miles per hour. The bigger the drop, the harder the rain fall. So when cloud drops collide, thousands of them join together to make up raindrops.
The classic image of a raindrop falling is that of a teardrop, round at the bottom and pointed at the top. But high-speed photography has shown the shape to be, in fact, more like a cross between a pancake and a hamburger. So, what you see is not necessarily what you think you see when it comes to clouds and raindrops.
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