20
Sep
10

A need to know

Last week, I was caught in a long downpour while riding my bike. I really dont like to ride in the rain, so I hid under a bridge. As I waited for the rain to pass, I wondered about rain… Its strange how the minds wonders about things. I thought it strange too, that I have been drenched so many times by rain, yet I know so little about it.

What I dont know about rain?

– How long does it take for a drop of rain to fall to land? 10mins? … can we have a 10 minute rain to land forecast before it actually hits?

– What makes the raindrops big or small?

– Why is rain not a constant trickle of water from the sky? Or a great big splash as it comes down all at once?

…. curious minds wanna know…

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3 Responses to “A need to know”


  1. 1 kevsteppe
    November 2, 2010 at 5:57 pm

    -How long does it take for a drop of rain to fall to land?

    Apparently terminal velocity of a rain drop is ~10meters/second. So rain from a cloud at about 3000 meters altitude takes 100 seconds to land – about 1.5 minutes.

    -Why is rain not a constant trickle of water from the sky?

    Surface tension causes water to form drops or beads when not in a container of some kind nor in sufficient quantity to be continuous.

    -Or a great big splash as it comes down all at once?

    The amount of water in the air as a gas is water vapor. The amount of water vapor the air can hold is called dew point. When that limit is reached (amount of water >= what the air can hold), liquid water starts to precipitate in the form of tiny raindrops — a cloud. If the drops are small enough, they will “float”. As the tiny raindrops run into eachother, they will start to merge. If they get big enough, the weight outstrips the surface area, it stops floating and falls — rain. Since not all the raindrops hit that size at the same time, it doesn’t come down all at once.

    – What makes the raindrops big or small?

    If there is a lot of water in the cloud, then more rain drops can merge before hitting the ground, meaning bigger drops. If there is an upward air current (warm air rising into the cloud from below) that will tend to keep the rains floating longer leading to larger drops. Note that in the tropics there is more evaporation from oceans (more water in clouds) AND there tend to be water air updrafts off the land (more time to get large drops), which partly explains why heavy intense rainfall is more common in the tropics than elsewhere.

    -kevsteppe svmilagros.wordpress.com

  2. 2 kevsteppe
    November 2, 2010 at 6:07 pm

    oh…. other planets/moons may have other kinds or rain
    Titan, the moon of Saturn, has occasional methane rain. Venus probably has sulfur rain (that must smell …umm… yeah, smell). Jupiter may have neon rain – perhaps colorful? And on planets very close to other stars, it’s possible to get iron rain. That would hurt, but then, it’s too hot to survive anyway.

  3. 3 funvinyldecals
    November 7, 2010 at 10:46 am

    Hey Kev,

    Thanks! How do you know these stuff? You must have been curious like me and went out to get these answers.
    Thanks for sharing!


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