Monday, January 7, 2013

Whats the deal with hail?

My airplane question buddy had a few more questions that I felt were perfect for this blog. So here we are again.

Where does hail come from?



To start answering this question, first we have to answer another question he had "how do clouds stay in the air? How liquid/hail stays up there?

Another reason (besides explaining science to kids) I love this is because every question I've done so far, I've ended up learning something in researching the answers.

Why dont clouds fall from the sky?

The main reason water particles can stay in the sky without falling, is because the smaller the droplet of water, the slower it falls. You can try this yourself by dumping a bucket of water at the same time you spray a misting water bottle. (the water bottle is not spraying steam, its not a gas its still tiny tiny water droplets.

The reason this happens, is because as you make something bigger (scale it up) its volume and surface area get bigger, but the volume grows faster than the surface area. Surface area is the amount of space on the outside surface of something. (imagine the outside of a water balloon) And volume, is all the space inside something (imagine all of the water in a water balloon). You can see this when you are filling a balloon. Putting in a lot of something, only makes it grow a small amount. The last couple of breaths in blowing up a balloon you can see this very well (the same size breath you put in at the beginning, only makes it grow a tiny amount compared to your first couple of breaths)

So because big droplets only grow their surface area a little while their volume grows a lot, they end up falling faster. This is because how fast something falls depends on how much volume it has, and how much surface area it has (because the more surface area you have the more air is going to bump into you on the way down) and the more volume something has, the more momentum will be generated. The more momentum is generated, the more air particles you can shove out of your way, and pick up more speed.

So now that we know tiny droplets fall very slow, there has to be something else keeping them up there right? Otherwise they would just keep falling, but very slowly. They are kept up by something we call updraft.

Hot air rises (this probably deserves a post on its own, but I'll do it quick here). Heat is just particles jiggling and bumping into each other. So the faster things bump into each other, the more they will space out. Try getting a bunch of friends standing REALLY close to each other, then tell everyone to DANCE AND MOSHPIT!!! Soon you will find everyone spaced out a bit.

Because gravity pulls on everything (air included) air included. Air with particles closer together (high pressure) will be pulled harder than air with particles further apart (low pressure). So the hot air (because its particles are more spaced apart) will be shoved out of the way by the colder air, which pushes it up.

So these updrafts are just hot air pushing up. The hot air gets there because sunlight warms the ground, which warms the air above it. This is why you will notice there are less clouds at night than during the day.


How does hail stay in the sky long enough to get big, and not just fall when its small?

So now that we know how rain droplets are kept up in the sky, how do they turn into hail? Obviously cold is required here. Hail happens when rain drops get too heavy to be held up by updrafts and fall. The temperature in the clouds needs to be below freezing to start the process. On the way down, the droplet freezes, and it runs into other rain droplets on the way down, that stick to it like sticking your tongue to a metal pole when its really cold outside.

This would only result in tiny pea sized hail though, so how does it get bigger sometimes? It doesnt happen very often, but when it does, its because of very strong updrafts  During very strong storms, winds can reach 100 miles per hour. It takes 90mph of updraft to form a baseball sized piece of hail.

So the updraft pushes the tiny ice chunk back up (collecting more droplets and getting bigger) until the air pressure evens out again, then it falls back down (collecting more droplets and getting bigger). It does this a bunch of times, until its size and weight are too much for the updraft to push back up, and it continues falling until it smashes into earth.


So in the end, hail is a bit more complicated than most people think, there are a lot of things that have to happen just right (or wrong depending on your point of view) to get hail.

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