HOW PRECIPITATION FORMS While everyone has experienced precipitation, few people understand how it forms the different names for precipitation. Let's discuss the different kinds of precipitation and how they happen. Of course, precipitation falls often in the form of rain. Have you ever wondered why some rain is gentle with small misty raindrops, while other rain is literally a downpour with giant raindrops? Why do you see some clouds that look like rain is coming, but they don't rain at all? Actually, the average raindrop inside a cloud is only 20 micrometers in diameter. This is very small. The raindrops you feel are nowhere near that small; there are reasons for this that we will discuss. The average raindrop you feel will be about 2 millimeters in diameter. This is literally 10,000 times the size of the average condensation nucleus. If you measure the size of a cloud droplet, a raindrop will be 100 times its size. The size of a raindrop can be as little as 0.2 millimeters or as large as 2.5 millimeters in diameter. This is why you feel such a difference when you feel drizzle as opposed to a downpour. You can imagine that a raindrop will grow in size inside the cloud until it becomes too heavy. Friction caused by the air itself will cause the raindrop to break up, with several smaller droplets falling to the earth. In other words, while cloud droplets have a negligible fall velocity, raindrops do not and they will fall. The aerosol that causes water vapor to condense must be hygroscopic. The term hygroscopic means it loves to attach to water and absorb it. Salt particles above the ocean are very hygroscopic, so these will attract water vapor at the lowest humidity possible. Other particles are not as hygroscopic as salt. There is an equation called the Stoke's drag law, which calculates the drag force and ultimately the velocity of any raindrop. The calculation indicates that the drag force and velocity of a raindrop is directly proportional to the diameter of the sphere, which is essentially the sphere of any raindrop. What you would see from this calculation is that it might take hours or days, if ever, for a cloud droplet to fall from the cloud to the ground. This is because the friction caused by air keeps it suspended within the cloud.
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