Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Rain Sheets, why?



On Tue, 27 May 1997, Brad Shue wrote:

Here in Arkansas, we had some pretty nasty rain tonight. A few grad
students and myself were sitting around watching the lightning and the
rain after a cookout. Why does the rain come down in sheets came up.

Cool!

Did you know that this phenomena works in miniature? While making
water-mist rainbows while watering the garden last year, I noticed just
this "rain curtain" effect in the falling mist. This with a metal
"squirtgun" type garden hose attachment. When the trigger is gently
pulled, the water comes out as a radial spray of extremely fine droplets
(rather than as a powerful jet.) I held the sprayer up high, and pointed
it upwards. (The slight wind kept me from wetting myself!) The "sheets"
were very visible with the sun in the foreground and the mist-cloud in
front of me. To improve the display, I pulled the hose up onto our front
porch, which is built across a sunken driveway and has a 15ft drop to the
ground. The "sheets" were visible most of the way down.

The best we could come up with was that the larger rain droplets reach
terminal velocity and cause a wave-like disturbance in the wind from the
storm and other droplets get caught in this initial phase. Much like the
Bernouli demonstration, the smaller droplets get pulled into the area
behind the larger drops because of the reduced pressure.

My theory is similar but not identical. The droplets in a volume are not
packed with identical initial density, so the more-dense volumes of
air/rain will tend to outrun the less dense parts. It appears to be not a
question of droplets falling through air, but of volumes of air being
dragged downwards by arrays of droplets, and less-dense volumes of
air/droplets being displaced. The garden-hose rain "sheets" almost
resemble a sort of Bernard cell, with fast, dense vertical sheets of
droplets moving through columns of low-density air/droplet "fluid."
Perhaps the rain "sheets" appear for the same reason as Bernard cells,
salt "fingers", etc. If one were to photograph the falling rain with a
camera which was also falling, then maybe the rain sheets would be seen as
parts of a sort of convection cell array. Or maybe they resemble the
large-scale structure of the universe, with "bubbles" of low density which
are surrounded by high-density walls. If Bernoulli effect causes denser
rain to fall faster and attract more rain to itself, then Bernoulli effect
takes the place of gravity, and causes the "universe" of raindrops to
condense into a bubble/walls structure.

Might it be that a vertical sheet-like shape can move downward faster than
another shape? If true, then I'd expect energy considerations to
automatically assemble the arrays of droplets into falling sheets. But I
thought that a falling rod-shape would move even faster, and so I would
expect the droplets to assemble themselves into rain-spears rather than
rain-sheets.

Anyway, the whole thing seems to be an example of "emergent behavior" in a
population of adjacent objects. This type of thing is often impossible to
explain except through appeals to "attractors" and complexity concepts.
The raindrops simply "know" to do it.

......................uuuu / oo \ uuuu........,.............................
William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623
EE/Programmer/Science exhibit designer http://www.eskimo.com/~billb/
Seattle, WA 98117 billb@eskimo.com SCIENCE HOBBYIST web page