[Weather] Acoustic rain gauge

[Robert Lawson]

I am no engineer by any means, but could Doppler measuring devices or 
even using optics as with lasers to measure the amount of mass that 
passes through a give plane of dimension per unit time?

Robert W4RL 



Dale Chatham wrote:
> There are a number of problems with this approach, I think.
>
> 1.  Not all raindrops are the same mass.  One could not, therefore, 
> assume volume = #hits * raindrop size.  One could, I suppose, measure 
> the energy of each raindrop, calculate its mass from the energy.
> 2.  Not all raindrops fall at the same angle.  This is probably a rather 
> serious limitation, as I don't see how you could normalize on angle 
> without some sophistication of the instrument.  One could take the 
> arccos(angle)*result of calculation in #1.
> 3.  Not all raindrops fall at the same velocity.  Wind blown rain is 
> likely faster than wind not acted on by rain.  Larger drops probably 
> fall faster than smaller drops.  (Mass is the cube of radius of a drop, 
> speed goes as the cross sectional area, wich is a square function).
>
> So, in order to successfully make such a measurement, one would, it 
> seems to me, need to accurately measure (or derive from measurements) 
> the following:
>
> Raindrop mass.
> Raindrop speed.
> Raindrop direction.
>
> In order to have meaningful results, one would also have to sample 
> across an area sufficient to ensure that it was representative of the 
> whole.  I'd think the proper area would be of the order of ten square 
> inches to a square foot.
>
> One might be able to simplify the problem somewhat by collecting rain 
> and allowing it to drip from the reservoir, counting drops.  This would 
> simplify the problem by ensuring equal sized drops with equal velocity 
> falling vertically.  However, one would not be able to measure rainfall 
> rate instantaneously, as one can do to sme degree with the tipping bucket.
>
>
>
> Frédéric wrote:
>   
>> Le jeudi 26 avril 2007 12:55, Matthew Smith a écrit :
>>
>>   
>>     
>>> Measurement of rainfall by this means is not - as far as I can see -
>>> feasible.  Rainfall is about volume (per area); an acoustic transducer
>>> could only really measure the product of the velocity of an incoming
>>> drop multiplied by its mass (will the real physics guys help me out
>>> here?)  These "hits" would not contain the data to measure the
>>> volume/area.
>>>     
>>>       
>> Tell me if I am wrong, but because of the air, and because a drop is not 
>> solid, its speed only depends on its mass, no? I mean, falling from 100m or 
>> 1000m, it will reach the same speed (if a physician could confirm...) So, 
>> measuring the energy of the sound can give its mass, so its volume.
>>
>> Air temperature and pressure are parameters to take into account... ut if 
>> there is wind, the drop will not produce the same sound, as it will not hit 
>> the surface with the same angle.
>>
>> Not very easy, so :o/
>>
>>   
>>     
>
>
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