Raindrops don’t get much larger than a quarter inch because when they get to falling 20 miles an hour (which is how fast they go at this size) the air resistance starts breaking them up. Low clouds might release larger drops for a short time — we’ve all felt the big splat of first drops or drops right after thunder. But basically, this is as big as they can be because the aerodynamics of their trip through the air won’t let them get larger. Raindrops also have a lower end of size and don’t get much smaller than a half a millimeter because they don’t fall very well below that size. This is drizzle-size drops, and they only travel 4 miles an hour, maybe a little more. At that rate, falling from a cloud that’s just a mile up, they’d take something like fifteen minutes to fall. That’s a long time, and if the air is the least bit dry, they just won’t make it at all; they’ll evaporate first. So raindrops have a size. If we divide everything in the universe into piles, according to its size, and if every pile has things that are ten times as tall, wide, and long as the next, (or one tenth as tall, wide, and long) we have about 42 piles all together. Meteorology magnitudes We might ask ourselves how many of these piles contain objects relevant to the study of weather. Let’s take a look: Near the middle is our starting pile, things that are close to one meter tall, wide or long, like your own body. We call that the zero [0] pile. Your umbrella is in this pile, Along with your raincoat And the weather balloons sent up by the National Weather Service to spy on the atmosphere’s secrets. Puddles are in this size range. One tenth of a meter is about 4 inches, and that’s about the size of a softball. We call this the [-1] pile, because it’s going down in size. This is the size of the largest hailstones on record. It is also the width of a train track, and you may be interested to know that my husband’s dad, a pilot, said that he could feel the bumps in the air when he flew over train tracks. They are wide enough to cause updrafts on a hot day. It is also the size of a junco, which is a harbinger of winter. (More on that another day.) One hundredth of a meter, one centimeter, is the next pile size downwards. We call it the [-2] pile. It is a more general size range for hail. Hail is often 2 or 3 centimeters. The largest raindrops are at the low end of this size range, because 5 millimeters is 1/2 centimeter. One thousandth of a meter, one millimeter, is the [-3] pile. Here is the main collection of raindrops. And here are the drizzle drops. They are less than a millimeter, but not 10 times less; about half a millimeter. Large, dendritic (branched) snowflakes are also in this size range. One tenth of a millimeter is the next size range. [-4] Newborn snowflakes, just getting big enough to fall, are in this size range. Medium-sized flakes belong to either [-3] or [-4] Heavy fog droplets are in this size range.
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