re: heating oil / diesel fuel questions
6 feb 2000
ben williams wrote:
>as i understand 'wind chill', it only refers to the felt effect on skin.
here's the scoop from page 8.15 of the 1993 ashrae hof:
the wind chill index (wci) is an empirical index developed from 
cooling measurements obtained in antarctica on a [dry] cylindrical flask
partly filled with water... the index describes the rate of heat loss
from the cylinder by radiation and convection for a surface temperature
of 91.4 f, as a function of ambient temperature and wind velocity. as
wci = (10.45-0.447v+6.686sqrt(v))(91.4-ta)/1.8 (69)
where v and ta are in mph and degrees f... the 91.4 surface temperature
was chosen to be representative of the mean skin temperature of a resting
human in comfortable surroundings.
a number of valid objections have been raised about this formulation.
cooling rate data from which it was derived were measured on a 2.24 in.
diameter plastic cylinder, making it unlikely that wci would be an
accurate measure of heat loss from exposed flesh, which has different
characteristics than the plastic (curvature, roughness, and radiation
exchange properties) and is invariably below 91.4 f in a cold environment.
moreover, values given by the equation peak at 56 mph, then decrease
with increasing velocity.
nevertheless, for velocities below 50 mph, this index reliably expresses
combined effects of temperature and wind on subjective discomfort. for
example, if the calculated wci is less than 1400 and actual air temperature
is above 14 f, there is little risk of frostbite during brief exposures
(1 h or less), even for bare skin. however, at a wci of 2000 or more,
the probability is high that exposed flesh will begin to freeze in 1 min
or less unless preventative measures are taken to shield the exposed skin
(such as a fur ruff to break up the wind around the face).
rather than using the wci to express the severity of a cold environment,
meteorologists use an index derived from the wci called the equivalent
wind chill temperature. this is the ambient temperature that would produce,
in a calm wind (defined for this application as 4 mph), the same wci as
the actual combination of air temperature and wind velocity. equivalent
wind chill temperature teq,wc in degrees f can be calculated by:
teq,wc = -0.0818(wci) + 91.4 (70)
where teq,wc is expressed as a temperature (and frequently referred
to as a wind chill factor), thus distinguishing it from wci, which
is given either as a cooling rate or as a plain number with no units.
for velocities less than 4 mph, equation (70) does not apply, and
the wind chill temperature is equal to the air temperature.
equation (70) does not imply cooling to below ambient temperature,
but recognizes that, because of wind, the cooling rate is increased
as though it were occurring at the lower equivalent wind chill
temperature. wind accelerates the rate of heat loss, so that the
skin surface is cooling faster toward the ambient temperature...