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Science Forum

  1. 27 Mar '16 13:59 / 3 edits
    Earlier this morning, I was surprised it clearly drizzled for a few minute while there was not a cloud in sight. My mother had a good looked and agreed with me that this was indeed the case.
    I tried looking this phenomenon up to find the explanation of the cause only to find I personally find I am unsure of that explanation.
    https://en.wikipedia.org/wiki/Serein_%28meteorology%29
    "...for others the phenomenon is simply non existing[3] and should be considered equivalent to rain falling from a distant cloud, when there is a strong vertical wind shear between the cloud itself and ground, while the sky is apparently clear.."
    In this case if that were true, the wind would have had carried the rain many km horizontally while it was windless at ground level were I was. Do you think that is the most likely explanation or can you come up with a better one?
  2. Standard member DeepThought
    Losing the Thread
    27 Mar '16 15:09
    Originally posted by humy
    Earlier this morning, I was surprised it clearly drizzled for a few minute while there was not a cloud in sight. My mother had a good looked and agreed with me that this was indeed the case.
    I tried looking this phenomenon up to find the explanation of the cause only to find I personally find I am unsure of that explanation.
    https://en.wikipedia.org/wiki/Sere ...[text shortened]... re I was. Do you think that is the most likely explanation or can you come up with a better one?
    Given the words "wind shear" I'd imagine that the rain falls from clouds being blown one way but falls through air being blown the other, so we get twice the speed between the rain drops and the clouds - whether that's an adequate explanation or not I couldn't say.
  3. 27 Mar '16 15:44
    Originally posted by humy
    Do you think that is the most likely explanation or can you come up with a better one?
    Clouds can be seen when there are a large number of small droplets. As the coalesce into bigger droplets they become rain. I believe that if they are high enough or cold enough they can be snow flakes or hail stones.
    Different conditions result in clouds taking on many different looks.

    Rain is probably less visible than clouds, and I can imagine that a scenario where rain forms quite quickly would result in you never seeing dense clouds. Further, there may have been clouds prior to the rain which mostly turned into rain and by the time it got to the ground there were not clouds left.
  4. 27 Mar '16 15:48
    Under certain conditions rain can also take a while to fall, so its also possible that the
    cloud the rain fell from dissipated before the rain reached the ground.

    Particularly when the raindrops are very small [or made so by wind sheer] and thus have
    a significant tendency to float.
  5. 27 Mar '16 16:16 / 4 edits
    Originally posted by twhitehead

    Rain is probably less visible than clouds, and I can imagine that a scenario where rain forms quite quickly would result in you never seeing dense clouds.
    perhaps that could work by having a kind of 'invisible rain cloud' that is invisible because most of the droplets are less than half the wavelength of visible light in diameter thus don't scatter the light thus are not visible. But then there has to be some larger droplets for rain to form. But perhaps if the continuous process they go through from tiny invisible droplets too small to scatter light to rain drops is fast enough so that, at any one given moment, there is insufficient density and number of the intermediate size droplets to scatter enough light to make us see the cloud from the ground?

    There might be a possible way to test such a hypothesis;
    such invisible rain clouds, if they exist, may not have most of their droplets of sufficiently small diameter to prevent them scattering invisible UVB light thus they might be clearly seen with a special UVB camera in areas of the sky were you see with your naked eye no clouds. Then it is a matter of somehow showing rain sometimes comes from them.
  6. 27 Mar '16 17:14
    Originally posted by humy
    But perhaps if the continuous process they go through from tiny invisible droplets too small to scatter light to rain drops is fast enough so that, at any one given moment, there is insufficient density and number of the intermediate size droplets to scatter enough light to make us see the cloud from the ground?
    I actually don't know what quantity of droplets is required to see a typical cloud. It is probably quite large. I think it perfectly reasonable to think a low enough density cloud that is too sparse to be seen can still form rain under certain conditions.

    Cloud droplets form around particles in the atmosphere. Presumably when cloud formation is slow or there are a lot of particles (dust) then more small droplets form, but if cloud formation is fast with fewer dust particles then possibly fewer droplets form but they grow larger faster.
  7. Standard member sonhouse
    Fast and Curious
    28 Mar '16 11:40
    Originally posted by humy
    Earlier this morning, I was surprised it clearly drizzled for a few minute while there was not a cloud in sight. My mother had a good looked and agreed with me that this was indeed the case.
    I tried looking this phenomenon up to find the explanation of the cause only to find I personally find I am unsure of that explanation.
    https://en.wikipedia.org/wiki/Sere ...[text shortened]... re I was. Do you think that is the most likely explanation or can you come up with a better one?
    One thing: It may have been zero wind at the surface but since wind itself is invisible, you don't know what the wind was at altitude, say 3,000 meters up or so. A horizontal wind up there could have brought rain from some other place and start it dumping on you and it would look to you like a cloudless rain which it was FOR YOU but not at the origin of the rain.
  8. 28 Mar '16 22:04 / 1 edit
    Originally posted by sonhouse
    One thing: It may have been zero wind at the surface but since wind itself is invisible, you don't know what the wind was at altitude, say 3,000 meters up or so. A horizontal wind up there could have brought rain from some other place and start it dumping on you and it would look to you like a cloudless rain which it was FOR YOU but not at the origin of the rain.
    yes, that's true. It could have been that.
  9. Standard member sonhouse
    Fast and Curious
    30 Mar '16 13:42 / 1 edit
    Originally posted by humy
    yes, that's true. It could have been that.
    Then again, it could have been aliens wanting to help out your dry conditions, you know, just like they built the pyramids
  10. Standard member apathist
    looking for loot
    30 Mar '16 15:44
    Lawn sprinkler.