Why Is Rain So Surprisingly Gentle?

[Jim B.]

So I am 8 years old. And my family and I are taking a vacation to the Sears Tower in Chicago, IL. It is a purported 110 stories tall. And the whole trip over, all my cousin and I can talk about is throwing some solid object over the tower. My parents tell me they would never allow that, since a solid object thrown over a place so high would come down to earth with the force of a bullet. Finally we reach a compromise. My parents tell me they will allow me to spit over the Sears Tower. But they also warn me spit at that level will just evaporate before it reaches the ground. (Long-story-short, we finally reach the Sears Tower, and go to the top, only to find it is completely enclosed. What a bummer.)

Anyways, it is years later. And something has perplexed me all these years. Rain clouds are probably hundreds of feet in the air. Probably even taller than the Sears Tower, no? Yet rain doesn't come down with the force of bullets. Why not?

Thank you in advance to all who reply

[beowulff]

Terminal Velocity.

[Drum God]

Whoa. A first, I was struck by an 8-year-old using the word "purported". Reading further, I realized that you were eight years old, but now you're somewhat older.

Anyway, it doesn't matter how high above the surface of the Earth a falling object begins its fall. It eventually reaches terminal velocity and will not go faster. It can fall from hundreds of feet, the top of the Sears Tower, or miles up. Once terminal velocity is reached, that's it -- it goes no faster.

So, the velocity is terminal, but the impact of a raindrop on your head is not terminal for you.

[seodoa]

It's also worth noting that rain's terminal velocity is so high because raindrops are not very aerodynamic. They flatten as they fall and do not form that tidy teardrop shape we are all so familiar with. More info.

For more information about rain and terminal velocity, look here.

[panamajack]

Originally Posted by Drum God

Anyway, it doesn't matter how high above the surface of the Earth a falling object begins its fall. It eventually reaches terminal velocity and will not go faster. It can fall from hundreds of feet, the top of the Sears Tower, or miles up. Once terminal velocity is reached, that's it -- it goes no faster.

With the proviso that terminal velocity is dependent on fluid density and gravitational force, which are not constant through the atmosphere. At altitudes of a few miles, it makes little difference. But falling from really really high an object will have a higher 'terminal velocity' since air density drops off faster than gravity.

This doesn't contradict the basic point. Once it gets low enough the object does reach the same terminal velocity as if it fell from the lower height (not necessarily exactly since it must lose some of the energy it had when it was falling faster higher up).

[Anaglyph]

Also, in contrast to hailstones, if raindrops get too big they fall apart during their fall

[Harmonious Discord]

The sears tower observation area is enclosed. Mostly I'd say to protect people on the ground from falling pennies and bodies killing them.

[Snarky_Kong]

Originally Posted by Harmonious Discord

The sears tower observation area is enclosed. Mostly I'd say to protect people on the ground from falling pennies and bodies killing them.

A penny wouldn't kill anybody. They did this on Mythbusters. A penny at terminal velocity will sting like a bitch, but no deaths. I doubt falling bodies was a consideration.

[Uncommon Sense]

Originally Posted by Harmonious Discord

The sears tower observation area is enclosed. Mostly I'd say to protect people on the ground from falling pennies and bodies killing them.

Also, probably to protect the deck observers from high winds.

[Harmonious Discord]

Originally Posted by Snarky_Kong

A penny wouldn't kill anybody. They did this on Mythbusters. A penny at terminal velocity will sting like a bitch, but no deaths. I doubt falling bodies was a consideration.

Was it a new lighter penny or the older heavier copper penny?

[YamatoTwinkie]

Originally Posted by Harmonious Discord

Was it a new lighter penny or the older heavier copper penny?

The old copper penny was only ~23% heavier than the new one, so I doubt it would make too much difference anyway.

[davekhps]

Originally Posted by Snarky_Kong

A penny wouldn't kill anybody. They did this on Mythbusters. A penny at terminal velocity will sting like a bitch, but no deaths. I doubt falling bodies was a consideration.

I would guess that falling bodies was probably very much a consideration-- the (then) tallest building in the world with an open observation deck would be a suicide magnet.

Well, that, and the wind turbulence generated by an open-air deck probably wouldn't be much fun for the tourists.

[gigi]

Originally Posted by Drum God

So, the velocity is terminal, but the impact of a raindrop on your head is not terminal for you.

What makes some raindrops hit so much harder than others? Wouldn't they have all hit this terminal velocity by the time they fall from cloud height?

[Lemur866]

Wind.

[Hampshire]

If it were true that small objects falling from a great height had the force of a bullet we would all be in some serious trouble during a hail storm.
While large size hail can do some considerable damage (golf ball size and larger), the bullet size stuff is far from lethal.

(And how were you going to attempt to throw something "over" the tower?)

[DSYoungEsq]

Originally Posted by Hampshire

If it were true that small objects falling from a great height had the force of a bullet we would all be in some serious trouble during a hail storm.
While large size hail can do some considerable damage (golf ball size and larger), the bullet size stuff is far from lethal.

(And how were you going to attempt to throw something "over" the tower?)

He meant over the parapet/railing.

[gigi]

Originally Posted by Lemur866

Wind.

Straight down? What about when it just pours really heavily? It's wind?

[Lemur866]

Well, the more water falling the heavier the rain is. I thought you meant individual raindrops hitting harder. Raindrops have terminal velocity depending on their size, and so larger drops have a higher terminal velocity. But large drops break up into small drops, so there's a maximum drop size depending on how much water is falling.

Aside from that, terminal velocity is velocity relative to the air, not to the ground. If a given mass of air moves up or down or sideways it will carry along any raindrops in that mass of air. So during heavy winds you can add a sideways vector to the speed of the raindrop relative to your face.

[gigi]

Cool, thanks!