Wikipedia:Reference desk/Archives/Science/2019 August 12

= August 12 =

Avoiding raindrops
I'm not very scientific but do like [over]thinking trivial things. Interested to know if this approach is foolish or not.

I live in the UK, where it rains fairly often. When it does, it's often a drizzling sort of rain (rather than torrential) and there's usually a bit of wind (rather than a still day or blustery). Under such circumstances, I reason that walking very close to buildings (eg a parade of shops, or office blocks) must be about 1/3 drier than walking on an open pavement, even if there are no awnings etc, because only the rain falling directly overhead or blown from the other direction than the building will make me wet, while one side will be more protected.

Would that seem right?

Incidentally, if the rain is heavier or quite persistent, I'd do the opposite, as the risk of getting drenched by poor guttering etc is quite a big one. --Dweller (talk) Become old fashioned! 11:53, 12 August 2019 (UTC)


 * I walk in light rain often. When it starts, you can look at the sidewalk. If it is drier close to the building, there is less rain reaching the ground closer to the building. I usually find that one side of the road is drier close to the building and the other side of the road is not. 199.164.8.1 (talk) 13:37, 12 August 2019 (UTC)
 * Wind direction could be a factor. ←Baseball Bugs What's up, Doc? carrots→ 14:55, 12 August 2019 (UTC)
 * Wind direction definitely makes a difference, but even when there is no wind and the light rain is falling vertically, I usually walk close to a building if possible because the rain seems less heavy there. I expect someone can explain why, and point us to a reliable source. One possibility is an updraft from the warm exterior of the building.   Dbfirs  18:47, 12 August 2019 (UTC)
 * Rain can only fall perfectly vertically in a vaccum. Wind resistance will cause drops to drift. Now, it is an easy thought experiment. Imagine looking down from above. Place thousands of raindrops at random starting locations. Draw a circle around each one. That is how far each drop might drift due to wind resistance. If you want to include wind, move the circles in the direction of the wind. Any circle that overlaps a wall has a probability of landing on the roof or hitting the side of the wall. If so, it won't make it to the ground. Notice that drops that are close to the wall have a lower probability of making it to the ground that drops that are not close to the wall. You expect to see less drops near the wall that hit the ground. In real-world experience, you see that less drops hit the ground close to the wall. 199.164.8.1 (talk) 19:21, 12 August 2019 (UTC)
 * Rain has some information on this.
 * Another fact not to forget is that raindrops usually have electrical charge, so a wall will have some attractive (most of the time, I think) or repulsive effect. Drops attracted to the wall won't fall on the passer-by. I have no idea of the magnitude of the effect (could well be perfectly negligible). Gem fr (talk) 20:21, 12 August 2019 (UTC)
 * Most of these effects will be negligible and can be ignored. A Fermi approximation for these calculations would quickly show that many of the effects, like "attractiveness to the wall due to unbalanced charges", would not cause a deviation within significant figures and can safely be ignored.  Actual wind may have a measureable effect, but assuming perfect calm, I wouldn't expect the random tumblings of the water droplets to matter; by definition without wind, such random tumblings would cancel out over thousands of rain drops, and we can model the situation using perfectly vertical drops.  -- Jayron 32 15:10, 14 August 2019 (UTC)
 * I will accept it to be negligible (although I remember having done some modern variant of the Oil drop experiment, based on the fact that electrical force are NOT negligible in such instance), but not "it cancels out": when a drop hits the wall it is gone, no drop will come from the wall side into the air to balance and cancel out the effect. Gem fr (talk) 09:45, 15 August 2019 (UTC)
 * Also, most walls aren't completely flat, but have some protrusions, like window sills and window A/C units, that will take out some raindrops. Thus, not only will drift toward and away from the wall be significant, but also parallel to it, if that makes the drops hit those protrusions.


 * The updrafts mentioned previously might also blow the raindrops away from the wall. SinisterLefty (talk) 02:54, 13 August 2019 (UTC)

Thank you, that's just marvy. --Dweller (talk) Become old fashioned! 08:12, 13 August 2019 (UTC)


 * Dweller, alternatively you could replace science with Zen philosophy; some pertinent advice from Hagakure (Japan, circa 1716):
 * "There is something to be learned from a rainstorm. When meeting with a sudden shower, you try not to get wet and run quickly along the road. But doing such things as passing under the eaves of houses, you still get wet. When you are resolved from the beginning, you will not be perplexed, though you still get the same soaking".
 * Alansplodge (talk) 12:10, 15 August 2019 (UTC)
 * Thanks, . In a Quentin Tarantino version of Japan, I'd hope to find a wizened old Master who might teach me, probably after several years of abuse, how to walk between the raindrops. --Dweller (talk) Become old fashioned! 12:14, 15 August 2019 (UTC)


 * There is no such thing as bad weather, just unprepared clothing. If you have decided to step out of your comfort zone you might as well have a good enough reason to do so regardless of it being night or day, with a drizzle or a storm, windy or calm and not worrying about things being pleasant or scorching hot or blistering cold. (All of that can happen in parts of Canada over a weekend, especially during April and October.) But if you are still interested in figuring out how to walk or run on a pavement or next to a building in the rain, under sunshine or not, here is a fairly mathematical answer by MinutePhysics - Walking or Running in Rain. You should however avoid thunder storms in open fields, meadows, deserts, etc. and definitely not stand around with a big metallic rod in such circumstances because no amount of mathematics or zen can cure getting struck by lightning. --Zailagu (talk) 07:04, 17 August 2019 (UTC)


 * Proper clothing can certainly help, but at extremes it can be difficult to be entirely comfortable no matter what you are wearing. Either the temperature or humidity is likely to be too high or low at some point on your body. And dealing with zero-visibility white-out conditions requires more than goggles, something along the lines of a sonar or radar system. And wind that makes it impossible to walk is another example of "bad weather". SinisterLefty (talk) 18:08, 17 August 2019 (UTC)