Does Rain really 'flatten' water...

[Alistair]

Is this phenomenon the result of:

a) the weight of the rain really squishing the waves
b) the rain absorbing some wind energy and therefore making it less wavy
c) magic
d) not really a phenomenon at all because it doesn't.

?

[Alistair]

Mr Uscoe, who may or may not be a physics teacher, replied to me but
probably meant to reply to everyone, viz:

--------

"The precise mechanics of the phenomena are uncertain but an enhancement
of near-surface turbulence and the damping of short surface gravity
waves has been observed in laboratory experiments."

S.A. Thorpe, Dynamical processes of transfer at the sea surface,
Progress in Oceanography, Volume 35, Issue 4, 1995, Pages 315-352, ISSN
0079-6611, 10.1016/0079-6611(95)80002-B.
(http://www.sciencedirect.com/science/article/pii/007966119580002B)


It seems to be relevant to the interpretation of satellite sea surface
radar data:

http://scholar.google.co.uk/scholar?q=rain+sea+surface+roughness&btnG=&hl=en&as_sdt=0%2C5

----------

There does seem to be something going on. A proper study is found here:
"The Effect if Rain in Calming the Sea"
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0485(1992)022%3C0404%3ATEORIC%3E2.0.CO%3B2

... full of equations and diagrams and serious science. "The major part
of the damping is attributed to turbulence; enhanced viscous decay due
to small scale random and unresolved motions. Short waves, less than
25cm in wavelength, are most severely damped."

[Henry Law]

On 13/11/12 09:18, Alistair wrote:
> Mr Uscoe, who may or may not be a physics teacher, replied to me but
> probably meant to reply to everyone, viz:

> "The precise mechanics of the phenomena are uncertain but an enhancement
> of near-surface turbulence and the damping of short surface gravity
> waves has been observed in laboratory experiments."

What a wonderful place this newsgroup is. I'm greatly indebted to Mr
Uscoe, and to you as his emanuensis, for the things I've learned here
(not least that "gravity waves" are quite different from "gravitational
waves").

--

Henry Law Manchester, England

[mruscoe]

On 13/11/2012 09:18, Alistair wrote:
> Alistair wrote:
>> Is this phenomenon the result of:
>>
>> a) the weight of the rain really squishing the waves
>> b) the rain absorbing some wind energy and therefore making it less wavy
>> c) magic
>> d) not really a phenomenon at all because it doesn't.
>>
>> ?
>>
>
> Mr Uscoe, who may or may not be a physics teacher, replied to me but
> probably meant to reply to everyone, viz:
>

At some point Thunderbird has has its reply to group button relabeled as
followup, and I clicked on reply instead.

I'm not a physics teacher, but I've spent too much with environmental
science over the last year.

If you are trying to measure wind speed all over the world's oceans, by
interpreting the patterns of surface waves measured using radar because
you haven't got wide enough coverage of surface weather stations,
understanding the effects of rain on the sea roughness is important.

[Peter Ford]

Very interesting. Do you know if there are any studies from eg bulk
viscosity models suggesting how the rain might modify the actual
windspeed? I know that you can't uncorrelate them in reality because
the weather systems bringing rain will bring different wind patterns,
but it seems relevant to eg an overcast windy day when it starts
raining.

Thanks,
Peter

[Carl]

On 13/11/2012 10:41, mruscoe wrote:
> On 13/11/2012 09:18, Alistair wrote:
>> Alistair wrote:
>>> Is this phenomenon the result of:
>>>
>>> a) the weight of the rain really squishing the waves
>>> b) the rain absorbing some wind energy and therefore making it less wavy
>>> c) magic
>>> d) not really a phenomenon at all because it doesn't.
>>>
>>> ?
>>>
>>
>> Mr Uscoe, who may or may not be a physics teacher, replied to me but
>> probably meant to reply to everyone, viz:
>>
>
> At some point Thunderbird has has its reply to group button relabeled as
> followup, and I clicked on reply instead.

Don't you hate it when some program stylist decides you must learn a new
way to use a standard tool 'cos "it's time they did some sort of an
update"? How many such updates actually add value (to admittedly free &
excellent software), I wonder?

>
> I'm not a physics teacher, but I've spent too much with environmental
> science over the last year.
>
> If you are trying to measure wind speed all over the world's oceans, by
> interpreting the patterns of surface waves measured using radar because
> you haven't got wide enough coverage of surface weather stations,
> understanding the effects of rain on the sea roughness is important.

Everyone who's spent time afloat recognises the wave-suppressing effect
of heavy rain. That's different from the wind & wave-stimulating effect
of an approaching storm, where the falling rain creates a down-draft of
wind which blows across the water, sometimes creating very severe
conditions.

Big waves form by the merging of smaller waves. Rain drops disrupt
small waves, as already mentioned, which may be one contributory factor
to overall wave reduction.

A water surface is only tenuously held flat under its very low density
fluid blanket (air) by gravity, viscosity & surface tension. It takes
only tiny local pressure variations to pluck that surface into ripples &
shear between wind & water will catch & keep energising these. Smaller
waves get overtaken by larger waves which absorb their energy & grow
thereby.

When the air is full of rain, I believe this tends to reduce the
relative wind speed (except at the edges of the storm). It also
increases the mean density of the fluid above the water surface. The
energy of the impacting raindrops stirs up the water surface into much
smaller scale of turbulence, mixing the water surface to diffuse &
dissipate the energy otherwise concentrating into the wavelets. I
should suppose, also, that the impacts of raindrops disrupting & beat
down wave peaks by much more than they affect the troughs?

Just a few thoughts -
Carl


--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find: tinyurl.com/2tqujf
Email: ca...@carldouglas.co.uk Tel: +44(0)1932-570946 Fax: -563682
URLs: carldouglas.co.uk & now on Facebook @ CarlDouglasRacingShells

[Henry Law]

On 13/11/12 14:58, Carl wrote:
> Everyone who's spent time afloat recognises the wave-suppressing effect
> of heavy rain.

Not on our canal; we hide under the bridges. Pigeon s**t is then a
hazard, though.

[Henry Law]

On 13/11/12 10:41, mruscoe wrote:
> At some point Thunderbird has has its reply to group button relabeled as
> followup, and I clicked on reply instead.

I thought it was just me for a week or two. Why did they do that?? At
least your reply was to a valid address; "m...@nowhere.invalid" and
similar posters can't help you out by re-posting your article.

[Carl]

On 13/11/2012 16:48, Henry Law wrote:
Pigeon s**t is then a
> hazard, though.
>

Flattens the water nicely, doesn't it? And slicks down the unruly hair too.

Carl

[Rebecca Caroe]

Why has nobody mentioned reducing surface tension?
or am I barking [up the wrong tree]?