On Thu, 15 Jun 2000, Glenn Holmes wrote:
> One birding point that intrigued me was that in the open and low
> humidity environments encountered at most sites, bird calls carried much
> longer distances than what I'm familiar with here in tropical north
> Queensland.
The openness is a probably a big factor but the humidity probably isn't.
Attentuation of sound by air depends on frequency, temperature humidity
and air pressure, and the relationship isn't straightforward.
At the frequencies used by most passerines propagation will be better at
higher humidities, other things being equal. In a very open environment
a passerine call might carry 20-30% further at 80% relative humidity
than it does at 20%.
Propagation also improves when relative humidity is extremely low -
say less than 5% - but I doubt this is often encountered.
In many environment relative humidity and temperatures both change
through the day and both affect propagation - you can't make any general
statements about when sound propagation will be best (although some
books do) - you have to do the maths for the specific values.
In many situations scattering will overwhelm attenuation. One source
of scattering is intervening objects such as vegetation, another is
atmospheric turbulence such as caused by wind.
Its probably alrgely reduced scattering you noticed. Although there
are several other factors which affect propagation or audibility, e.g
the level of background noise in the relevant frequency range.
Andrew taylor
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