Air is not a good conductor of heat.  An object which is above the ambient
air temperature will transfer heat to the air immediately surrounding it,
thus heating the air.  If the air is still, a thermal gradient will develop
.. the air closest to the hot object will be hot and the further away you
move the closer the temperature will be to the ambient air temperature.
Because of the high thermal resistance of air, the hot object will loose
heat very slowly.

If the air is moving, however, cooler air will be brought into contact with
the hot object and heat will be transferred away from the hot object at a
higher rate.  The faster the air moves, the faster cool air will reach the
hot object and the faster heat will transfer from the hot object to the air.

Of course in real life the air does not sit still even if there is no wind.
Convection will cause heat loss in the absence of wind but the flow of air
due to convection will generally be small compared to the flow of air at
wind speeds which are interesting to measure.

Evaporative cooling is an entirely different effect.  It has to do with the
heat required for water to transition from the liquid phase to the vapor
phase (the vapor phase has higher entropy).  When water evaporates from a
body of liquid water, the remaining liquid water looses heat and its
temperature will drop below the ambient air temperature unless an equal
amount of heat is added.

Water has nothing to do with the function of a hot-sensor anemometor except
for the effects that humidity has on the heat capacity and thermal
conductivity of the air (which could be compensated for by additional
computations if the humidity is known).

Hmm ... does anyone out there have an idea how much error humidity could
introduce if you just went with this approximation for King's law:

S = A * [(P - D) / (TS - TA)] ** 2
where:
S = air speed
A = full-scale calibration constant
P = power dissipated by the airspeed sensor
D = "still-air" (S = 0) power dissipation
TS = temperature of the airspeed sensor
TA = ambient temperature
** = "to the power"

The formula came from the article here:
http://devel.penton.com/ed/Pages/magpages/may2598/ifd/0525ifd.htm
(scroll down past the first article to the one titled "Low-Power Thermal
Airspeed Sensor")

Better yet, does anyone have King's law (full version .. not approximation)
handy?

Ken


>Several people mentioned windchill as a possible effect with the reference
>temp. sensor in a hot-sensor anemometer. I was not aware that wind chill
>worked on dry objects (hence the theory behind wet-bulb/dry-bulb humidity
>sensors). Can someone please tell me why it is a consideration in this
>case? Are you concerned that water might condense onto the sensor?
>
>Sean