Relative humidity fall crocus

Rodger Whitlock
Mon, 16 Aug 2004 08:19:38 PDT
On 12 Aug 04 at 15:54, wrote:

> Jane McGary wrote:

> >True; but the nights, at least at my home (which is about 1000 feet
> >higher in elevation than Dave's) are in the fifties F (10-15 degrees C),
> >and the humidity has mostly been below 20%

> This is not an attempt to pick on Jane at all, just a chance to
> bring forward a better way to express humidity.
> When Jane says 20% humidity, I ask myself at what temperature is
> this occcuring?  Relative humidity varies throughout the day, 100%
> or close if dew is formed overnight and dropping as the temperature
> rises during the day, assuming no airmass change.
> Dew point is a better measure of humidity (absolute) than RH and is
> not expressed as a percentage, but rather the temperature at which
> the condensation rate exceeds the evaporation rate.

So far, so good.
> The case for 100% or near 100% relative humidity overnight is common
> for many locations when winds are light and cloud cover is not
> extensive. Without the sun to heat it up, the air temperature in
> many areas areas cools until it reaches the dew point. At this point
> further cooling is difficult, because a whole lot of heat is
> released by the water vapor condensing into liquid (dew), or ice
> (frost) if it is cold enough.

You overlook the phenomenon of radiative cooling. In still, clear
air on a cloudless night, superficial temperatures can drop
significantly below air temperature as heat radiates into the sky.
Remember that the universe as a whole has an average temperature of
a few degrees Kelvin, and that's more or less the sky temperature on 
a clear night.

Dew and hoarfrost are often due to radiative cooling. When you notice
that hoarfrost hasn't formed under the canopy of trees, you are
seeing the effect of radiative cooling.

Hence, the dewpoint may be well below the *air* temperature, yet dew 
(or frost) still forms overnight.

> With overnight lows in the 50's, Jane's dew point is probably in the
> 50's and with a daily maximum RH of 100%.

At this point, your logic has gone off the rails by missing the very 
real possibility of radiative cooling causing dew.

Remember that Jane's up in the Cascade foothills. A thousand feet may 
not sound like much, but makes the possibility of radiative cooling 
even more likely than at sea level because there's significantly less 
atmosphere above her. Or so it seems to me.

Question for Jane: feeling slightly light-headed? Can I interest you 
in an oxygen tank?

Rodger Whitlock
Victoria, British Columbia, Canada
Maritime Zone 8, a cool Mediterranean climate

on beautiful Vancouver Island

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