# raindrops **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2009, 5:07pm UTC](https://boards.straightdope.com/t/raindrops/516597 "2009-11-06T17:07:02Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![davidt](https://avatars.discourse-cdn.com/v4/letter/d/51bf81/32.png) [@davidt](https://boards.straightdope.com/u/davidt)\ **Post date:** [November 6, 2009, 5:07pm UTC](https://boards.straightdope.com/t/raindrops/516597/1 "2009-11-06T17:07:02Z")
Is it true that it is impossible for two raindrops to touch prioir to hitting the ground? ---
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [November 6, 2009, 5:17pm UTC](https://boards.straightdope.com/t/raindrops/516597/2 "2009-11-06T17:17:21Z")
That makes no sense to me. Dropping through a perfect vaccuum they woudl never “catch up” to each other; but if the wind blows them sideways they could certainly join up I woudl think. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [November 6, 2009, 6:08pm UTC](https://boards.straightdope.com/t/raindrops/516597/3 "2009-11-06T18:08:33Z")
During drop formation, the proto-drops are lifted and dropped several times in the cloud, getting bigger thru mergers until they are too heavy to be kept aloft. You can see this directly in larger hailstones that are made up of dozens of smaller hailstones. It would be easier for rain drops to merge than hailstones because there is less “bounce”. OTOH, a really large drop, once it starts falling, will break up due to air pressure. ---
**Author:** ![pan1](https://avatars.discourse-cdn.com/v4/letter/p/e495f1/32.png) [@pan1](https://boards.straightdope.com/u/pan1)\ **Post date:** [November 6, 2009, 6:25pm UTC](https://boards.straightdope.com/t/raindrops/516597/4 "2009-11-06T18:25:17Z")
> [@davidt](#): > > Is it true that it is impossible for two raindrops to touch prioir to hitting the ground? no. Wind will move the rain drops, often into each other. If there was no wind, we’ll exclude updrafts too just for simplicity, two raindrops falling a couple inches apart would never touch. If a big raindrop and a little rain drop fell from the exact same spot a few milliseconds apart: If the big one falls first, the little one might catch up - but only because the big one has a greater drag against the air. They’d both be close to the same shape, but the big one has a greater surface to interact with the air, slowing it down slightly more than the little one. If the little one falls first, they’ll only meet at the point of impact with the ground. Galileos cannonballs didn’t have far enough to fall to notice the air-drag effect, but raindrops can fall for miles…plenty of time for the little one to catch up a few milliseconds. If they fell in a vacuum and for whatever reason didn’t instantly evaporate to fill the vaccum, they’d never meet. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [November 6, 2009, 6:59pm UTC](https://boards.straightdope.com/t/raindrops/516597/5 "2009-11-06T18:59:03Z")
> [@pan1](#): > > If the big one falls first, the little one might catch up - but only because the big one has a greater drag against the air. They’d both be close to the same shape, but the big one has a greater surface to interact with the air, slowing it down slightly more than the little one. What makes you think this? I am fairly sure a large raindrop falls faster than a small one. ---
**Author:** ![BobArrgh](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bobarrgh/32/2881_2.png) [@BobArrgh](https://boards.straightdope.com/u/BobArrgh)\ **Post date:** [November 6, 2009, 8:28pm UTC](https://boards.straightdope.com/t/raindrops/516597/6 "2009-11-06T20:28:14Z")
> [@RedSwinglineOne](#): > > What makes you think this? I am fairly sure a large raindrop falls faster than a small one. Really? I am pretty certain that Galileo proved that objects fall at the same rate, regardless of their respective masses. The acceleration due to gravity is 9.8 m/s^2 (meters per second per second) at sea level. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [November 6, 2009, 8:41pm UTC](https://boards.straightdope.com/t/raindrops/516597/7 "2009-11-06T20:41:29Z")
They accelerate at the same rate, but with raindrops were talking terminal velocity. From [here.](http://www.wonderquest.com/falling-raindrops.htm) > [@](#): > > At sea level, a large raindrop about 5 millimeters across (house-fly size) falls at the rate of 9 meters per second (20 miles per hour). Drizzle drops (less than 0.5 mm across, i.e., salt-grain size) fall at 2 meters per second (4.5 mph). ---
**Author:** ![lee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lee/32/7455_2.png) [@lee](https://boards.straightdope.com/u/lee)\ **Post date:** [November 6, 2009, 8:55pm UTC](https://boards.straightdope.com/t/raindrops/516597/8 "2009-11-06T20:55:20Z")
Actually the rate at which raindrops fall is complicated. Size does affect terminal velocity, [but small drops which come from large drops breaking as they small may have the velocity of the original drop.](http://www.newscientist.com/article/dn17147-superterminal-raindrops-break-speed-limit.html) This affects rainfall estimation. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [November 6, 2009, 9:06pm UTC](https://boards.straightdope.com/t/raindrops/516597/9 "2009-11-06T21:06:52Z")
So, generally speaking, we probably have larger drops falling faster through drizzle size drops. The large drops become larger as they join with the small drops and pick up speed until they become too large and burst. The small drops then decelerate to their terminal velocities and the cycle repeats. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [November 6, 2009, 9:10pm UTC](https://boards.straightdope.com/t/raindrops/516597/10 "2009-11-06T21:10:38Z")
> [@BobArrgh](#): > > Really? I am pretty certain that Galileo proved that objects fall at the same rate, regardless of their respective masses. The acceleration due to gravity is 9.8 m/s^2 (meters per second per second) at sea level. Yeah, if you live on the moon or in an evacuated bell jar. Real objects do fall at different rates in air, if you haven’t noticed. A feather and a hammer really don’t reach the ground at the same time here in my living room. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [November 6, 2009, 9:30pm UTC](https://boards.straightdope.com/t/raindrops/516597/11 "2009-11-06T21:30:18Z")
Amazingly enough, unlike snowflakes, all raindrops are _almost exactly alike!_ ---
**Author:** ![lee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lee/32/7455_2.png) [@lee](https://boards.straightdope.com/u/lee)\ **Post date:** [November 6, 2009, 9:44pm UTC](https://boards.straightdope.com/t/raindrops/516597/12 "2009-11-06T21:44:34Z")
Well, they were measuring impact of the tiny drops that showed they were traveling significantly faster than the terminal velocity for that size of drops. The terminal velocity is not a magic number that drops of a certain size move toward. iIf they are traveling significantly faster than terminal velocity, I don’t think there is a force which applies extra braking to slow them down to their natural terminal velocity. My understanding is that it reflects the speed at which they stop accelerating as they fall through the air toward the ground. It may be that once the big drop breaks up the smaller drops remain at the speed they held on break up. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [November 6, 2009, 10:17pm UTC](https://boards.straightdope.com/t/raindrops/516597/13 "2009-11-06T22:17:09Z")
The air resistance is a braking force. Air resistance is trying to slow the object, the force of gravity is trying to speed it up. If you shoot a feather out of a cannon, it slows very quickly. I’m guessing that the drops they measured that were moving more quickly than expected had recently burst form a larger drop and had not yet had time to slow. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 6, 2009, 10:19pm UTC](https://boards.straightdope.com/t/raindrops/516597/14 "2009-11-06T22:19:02Z")
> [@lee](#): > > I don’t think there is a force which applies extra braking to slow them down to their natural terminal velocity. My understanding is that it reflects the speed at which they stop accelerating as they fall through the air toward the ground. Objects falling faster than their terminal velocity slow down until they reach terminal velocity (or the ground). Terminal velocity is the velocity at which drag force and gravitational attraction are equal in size. There’s no separate force for extra braking, but the same old drag force is greater when the speed is greater. That’s why you get poor gas mileage at 100 mph. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 6, 2009, 10:28pm UTC](https://boards.straightdope.com/t/raindrops/516597/15 "2009-11-06T22:28:57Z")
As for the OP: I can picture a case where raindrops might a_ctively seek to join each other_! If two raindrops in close proximity have opposite electrostatic charges, they will tend to attract each other.