# Hammering Rain = Downdraft? **URL:** \ **Category:** Factual Questions\ **Created:** [May 3, 2003, 8:45pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907 "2003-05-03T20:45:10Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Zenster](https://avatars.discourse-cdn.com/v4/letter/z/cdc98d/32.png) [@Zenster](https://boards.straightdope.com/u/Zenster)\ **Post date:** [May 3, 2003, 8:45pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/1 "2003-05-03T20:45:10Z")
I was watching the rain come hammering down this morning and immediately thought that it appeared to be exceeding terminal velocity. Some questions: 1. Can rain sometimes exceed terminal velocity? 2. If so, are downdrafts primarily responsible for this? 3. If so, does anything else speed rain beyond terminal velocity? ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [May 3, 2003, 9:38pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/2 "2003-05-03T21:38:11Z")
Terminal velocity is defined as whatever it is in fact so it is a contradiction to say it can be exceeded just like it makes no sense to say my car can go faster than it maximum speed. Other than that I doubt a downdraft will make much difference. probably drop size makes much more difference. ---
**Author:** ![Zenster](https://avatars.discourse-cdn.com/v4/letter/z/cdc98d/32.png) [@Zenster](https://boards.straightdope.com/u/Zenster)\ **Post date:** [May 3, 2003, 10:19pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/3 "2003-05-03T22:19:04Z")
Good point **sailor** , but the question remains; When the rain is really pouring down hard, is this because there is also a downdraft assisting its acceleration? Of course, droplet size has something to do with it, but there seems to be times when the rain is a falling much harder than seems justified. It doesn’t seem likely that air density comes into play because cold air is denser and might therefore interfere with noticeable acceleration of the rain. Although, it also stands to reason that cold air sinks and is part of a downdraft. It’s why I’m curious about this. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 3, 2003, 10:44pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/4 "2003-05-03T22:44:12Z")
Also I might add in a heavy heavy rain, if you watch there seems to be moments of heavy then heavier rain, sometime in small patches. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [May 3, 2003, 10:47pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/5 "2003-05-03T22:47:30Z")
Downdraft is something that happens for a moment in a limited space. Air does not disappear so, if there was a substantial current of air flowing down it would generate a lot of wind sideways. Vertical movement of air is tiny when compared to horizontal movement. Even in a tornado. Air moves up or down mostly if it encounters obstacles but over open spaces it does not. I do not think what you saw had much if anything to do with downdrafts. ---
**Author:** ![jackelope](https://avatars.discourse-cdn.com/v4/letter/j/a6a055/32.png) [@jackelope](https://boards.straightdope.com/u/jackelope)\ **Post date:** [May 3, 2003, 11:58pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/6 "2003-05-03T23:58:45Z")
**sailor** , terminal velocity isn’t actually the fastest that it’s possible for something to go; it’s the speed at which the force of gravity’s acceleration on an object is equal to the resistance of the medium through which it’s travelling. I.e., if an object (such as a raindrop) is freefalling through a medium (air), its terminal velocity is the speed at which it will stop accelerating and maintain its speed. If something falls from high enough, like from a raincloud, that’s the speed it’ll be travelling when it lands. I think **Zenster** ’s question was whether it’s possible for a raindrop to be falling at a higher speed than its terminal velocity. Can’t help you on that one. However, I will point out that larger drops have a faster terminal velocity than smaller ones. Terminal velocity is dependent upon the ratio of surface area to volume (and mass); as a raindrop gets bigger, its weight (volume) increases with the _cube_ of its diameter, while its surface area increases with the _square_ of its diameter. So big, fat drops will fall considerably faster than fine drizzle; **Zenster** , is it possible that the rain appeared to be falling unusually fast because the drops were bigger than you usually see? (Disclaimer: The above is based on some knowledge of physics, and _not_ on knowledge of meteorology. Weather does strange stuff sometimes.) ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [May 3, 2003, 11:59pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/7 "2003-05-03T23:59:18Z")
**Zenster** what you are seeing is an increase in the _rate_ of rainfall. For example, 1 inch is falling per hour as opposed to 1/4 of an inch per hour. The wind may be associated with the “drag” of the raindrops ~ IIRC one of my professors related that the wind coming down with the rain is being “pulled” down by the drops. Therefore, if there are more drops there is more wind. Now, if the rain is coming at you _sideways_ ~ then I’d say yes, the wind is blowing it. Sometimes winds can blow rain under the flashing around my chimney. (grrrr). Hope that helps. 🙂 ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [May 4, 2003, 12:52am UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/8 "2003-05-04T00:52:55Z")
> [@](#): > > \*Originally posted by jackelope \* > **sailor** , terminal velocity isn’t actually the fastest that it’s possible for something to go; it’s the speed at which the force of gravity’s acceleration on an object is equal to the resistance of the medium through which it’s travelling. I.e., if an object (such as a raindrop) is freefalling through a medium (air), its terminal velocity is the speed at which it will stop accelerating and maintain its speed. If something falls from high enough, like from a raincloud, that’s the speed it’ll be travelling when it lands. It makes no sense. The definition of terminal velocity is “the fastest the object will fall” so, by definition, it cannot fall faster. The TV can be diffrent depending of different factors (density, etc) but given everything, once you have a TV that is the max. By definition. ---
**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\ **Post date:** [May 4, 2003, 3:03am UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/9 "2003-05-04T03:03:45Z")
Is terminal velocity defined relative to the air or the ground? IANAP but if a raindrop was fell until it reached terminal velocity and later entered a denser patch of air, where it would have a lower terminal velocity, wouldn’t it slow down since it is going faster than the (new) terminal velocity? I don’t know whether (PI) this happens in practice. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [May 4, 2003, 12:16pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/10 "2003-05-04T12:16:15Z")
> [@](#): > > \*Originally posted by sailor \* > Downdraft is something that happens for a moment in a limited space. Air does not disappear so, if there was a substantial current of air flowing down it would generate a lot of wind sideways. Vertical movement of air is tiny when compared to horizontal movement. Even in a tornado. Air moves up or down mostly if it encounters obstacles but over open spaces it does not. I do not think what you saw had much if anything to do with downdrafts. Actually, vertical air movements can be _very_ significant. Mountain waves, for instance (that’s a current generated by air movement over mountaintops), can have vertical velocities of hundreds or even thousands of feet per minute and will maintain themselves so long as the wind blows over the peak at a given velocity. Of course, mountains can be defined as objects. Thunderstorms, however, can occur anywhere, even over relatively featureless plains or open water. With thunderstorms you can get extremely violent up and down drafts, which a reason aircraft try to avoid such storms. Small planes have been literally ripped apart by the violence of such air currents, and large planes can experience severe physical damage. When a such a severe downdraft impacts the ground you get a “microburst”. Rain caught in a microburst would, indeed, fall faster and land harder than rain outside of the effect. Microbursts don’t last long, but there are other air currents within thunderstorms that do and they would account for at least some of the lighter/heavier rainfall effects. As for how you measure terminal velocity… well, there’s two ways to consider speed in the air. There’s “airspeed”, which is your rate of travel compared to the air mass you are moving through. Then there is “groundspeed”, which is your speed relative to the surface of the planet. Now, when folks are skydiving for fun, the vertical air movement component of the air mass they are traveling through is pretty close to nil, so relative to the ground those two different speeds are so close as to make no difference (120 mph is a typical figure given, although higher speeds are possible if the skydiver is falling feet first, as an example, which provides less air resistance). But if an air mass is, for example, moving upwards at 20 mph then, while the terminal vertical _airspeed_ remains 120 mph the _groundspeed_ drops to 100 mph (you still need a parachute, of course). On the flip side, if the air you’re falling through is doing _down_ at 20 mph then your groundspeed is 140 mph. There have been a couple instances of folks falling through thunderstorms. Survival is quite iffy - folks have been entombed in an inch or more of ice on the way down - but those few who have survived report being flung _upward_ within the storm, in other words, the vertical up movement exceed the down movement generated by gravity. Survival is complicated not only by cold and ice formation but also because thunderstorms can reach up 30,000-50,000 feet, meaning if you get flung up to the “anvil” - the top part of the cloud formation - you may die from lack of sufficient oxygen. Which is wandering a little off the subject. The point of this ramble is that yes, within storms you can have extremely strong vertical air movements. Any rain within a downdraft will fall faster relative to the ground. Yes, this certainly could account for the “hammering rain” effect, as can larger raindrops. > [@](#): > > _Originally posted by shade_ > IANAP but if a raindrop was fell until it reached terminal velocity and later entered a denser patch of air, where it would have a lower terminal velocity, wouldn’t it slow down since it is going faster than the (new) terminal velocity? I don’t know whether (PI) this happens in practice. Yes, it does. The excess energy turns into friction and heat and the rain drop would slow to the new terminal velocity figure. In the case of rain drops falling through several tens of thousands of feet this wouldn’t be a huge temperature difference, not enough to boil the water, although it could melt small hail/frozen rain back to liquid form. ---
**Author:** ![jonpluc](https://avatars.discourse-cdn.com/v4/letter/j/fbc32d/32.png) [@jonpluc](https://boards.straightdope.com/u/jonpluc)\ **Post date:** [May 4, 2003, 6:21pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/11 "2003-05-04T18:21:48Z")
> [@](#): > > \*Originally posted by Broomstick \* > \*\*Actually, vertical air movements can be _very_ significant. > There have been a couple instances of folks falling through thunderstorms. Survival is quite iffy - folks have been entombed in an inch or more of ice on the way down - but those few who have survived report being flung _upward_ within the storm, in other words, the vertical up movement exceed the down movement generated by gravity. Survival is complicated not only by cold and ice formation but also because thunderstorms can reach up 30,000-50,000 feet, meaning if you get flung up to the “anvil” - the top part of the cloud formation - you may die from lack of sufficient oxygen. Which is wandering a little off the subject. > > \*\* Sorry for the complete hijack but could you cite these? not that i dont believe you but that i would be interested in reading about “riding a thunderstorm wave” so to speak. I was wondering if you supplied your own air and had a lightweight thermal protection could you like “ride a thunderstorm”? Talk about xtreme sport!!! ---
**Author:** ![jackelope](https://avatars.discourse-cdn.com/v4/letter/j/a6a055/32.png) [@jackelope](https://boards.straightdope.com/u/jackelope)\ **Post date:** [May 4, 2003, 8:39pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/12 "2003-05-04T20:39:50Z")
There’s a story briefly recounted [here](http://www.sky-fire.tv/index.cgi/thunderstorms.html) about glider pilots in a thundercould: > [@](#): > > Five German glider pilots made the mistake of flying into a thunderstorm over the Rhone Mountains in 1930. Fearing their fragile craft might break up, they bailed out. They all essentially became human hailstones encased in ice. All but one froze to death. I’d heard the story before, including the detail omitted here that they were wearing parachutes, which of course would be a lot more responsive to the updrafts inside a storm cloud. > [@](#): > > _Originally posted by sailor_ > **It makes no sense. The definition of terminal velocity is “the fastest the object will fall” so, by definition, it cannot fall faster. The TV can be diffrent depending of different factors (density, etc) but given everything, once you have a TV that is the max. By definition.** True; it’s the fastest an object can fall, but it’s not the fastest an object can go. For example, if you went high up in a hot-air balloon and fired a bullet from a gun straight down toward the earth, the bullet would immediately begin _slowing down_ until it was falling at its terminal velocity (assuming muzzle velocity was faster than TV, of course). ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [May 4, 2003, 9:11pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/13 "2003-05-04T21:11:27Z")
You’d do better with a pressure suit of some sort. I refer you to _The Man Who Rode the Thunder_ by William H. Rankin for a first hand account of falling through a thunderstorm, including the depressurization effects experienced at 30,000 and 40,000 feet. Howard Fried recounts his encounter with a thunderstorm (and possible tornado) [on this site](http://www.avweb.com/news/columns/182238-1.html). Scroll down to “Calm Before the Storm”. Even has pictures. You will be amazed that an airplane that mangled landed safely. Also on the Internet: [Search this site for the word “hailstone”](http://www.aarrgghh.com/no_way/home.htm) for an account of “human hailstones” from 1930 Germany. I personally didn’t find it terribly detailed or satisfactory. I also once read of an ultralight pilot who strayed too close to a thunderhead and was grabbed by the updraft. He was found some time later, dead, and he and his mangled aircraft were largely encased in ice. For better or worse, though, I don’t have a specific cite on that one. ---
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [May 6, 2003, 5:05pm UTC](https://boards.straightdope.com/t/hammering-rain-downdraft/172907/14 "2003-05-06T17:05:08Z")
Just so happened I encountered another thunderstorm survival story illustrating “vertical air movement” [on this site](http://www.avweb.com/news/columns/184352-1.html) > [@](#): > > I can’t remember which was louder – the hail hitting the airplane or that girl in the back seat screaming as we hit the severe turbulence. I do remember that as the rate of climb pegged out on the up side I jerked the throttle back so hard that the engine quit. I learned later in other thunderstorms that the engine may have quit because of water inundation, but at the time over Georgia, I didn’t care. Not only was the engine dead, but the speed had dropped off so much that the prop had stopped. > > So, there we were, with the prop stopped, but still climbing at about 3000 feet per minute. Then we hit the downdraft side and plummeted at about the same rate in the other direction. That was in a C172 - about 2600 lbs, give or take, assuming near gross weight. Heck of a “breeze” to fling that much weight upward at 3000 feet per minute. That’s upward travel at about 30 mph, without benefit of engine. Possibly more, since the instrument showing the upward travel “pegged out” or hit the maximum value it can indicate. Hey, I’m impressed.