Relative humidity is defined as follows:

RH=100*e/es

whe
e= partial pressure of vapour
es=saturation vapour pressure

es is tabled against dew point temperature.

Above 0°C no problem
Below 0°C (exactly 0.01°C is triple point of water) exist two tables
of saturation pressu
1) ews = saturation above water
2) eis = saturation above ice

Now my question a
1) eis or ews is used in RH formula?
2) real RH sensors measure 100*e/eis or 100*e/ews?

Thanks for any help!

fdix wrote:
Relative humidity is defined as follows:

RH=100*e/es

whe
e= partial pressure of vapour
es=saturation vapour pressure

es is tabled against dew point temperature.

Above 0°C no problem
Below 0°C (exactly 0.01°C is triple point of water) exist two tables
of saturation pressu
1) ews = saturation above water
2) eis = saturation above ice

Now my question a
1) eis or ews is used in RH formula?
2) real RH sensors measure 100*e/eis or 100*e/ews?

Thanks for any help!

Well, it's a good question, and I'm not sure there is a
one right answer. At temperatures cooler than the
melting point, water can exist in both liquid and
solid form, so some combination of Es over ice
and Es over water may be appropriate. If I
remember things correctly, though, those saturation
vapor pressures are over a flat surface, and liquid
water in the atmosphere invariably displays a
curved surface, and a curved surface is more likely
to lose liquid water through evaporation than a
flat surface, meaning the saturation vapor pressure
will be higher over the curved surface than over
the flat surface.

I'm not at all sure how a 'real' RH sensor works.

I'm not sure this helps, but...


Scott, really at notscape det not

According to your article in sci.geo.meteorology,
where you Fri, 13 Feb 2004 07:54:29 -0600 wrote ....

Below 0°C (exactly 0.01°C is triple point of water) exist two tables
of saturation pressu
1) ews = saturation above water
2) eis = saturation above ice

Now my question a
1) eis or ews is used in RH formula?
2) real RH sensors measure 100*e/eis or 100*e/ews?

Thanks for any help!

Triple point does not matter here
( gas / liquid / solid equil. at 610,6 Pa ).

There are two sat. preasure curves
for both ice and water, that cross each other at 0.00deg.
( water preasure above s / l equals. )

Vapor above ice climbs more quickly, so when freezing
the saturation is reached over ice while not yet over water.

Water can be overcooled to up to -10deg,
being liquid before ( e.g. dropletsin clouds).
Ice cannot be overheated.
Condensation below zero goes AFAIK into solid phase.

Note that table based calculating humidity from dry and wet
termometer temperatures below 0deg depends upon
state of water - there are two tables. (standard method)

I cannot guarantee it, but I bet
calculation should be done over ice.

remember things correctly, though, those saturation
vapor pressures are over a flat surface, and liquid
water in the atmosphere invariably displays a
curved surface, and a curved surface is more likely
to lose liquid water through evaporation than a
flat surface, meaning the saturation vapor pressure
will be higher over the curved surface than over
the flat surface.

As you have mentioned, saturation is defined over
flat surface. Small drops has preasure higher,
so the vapor can become oversaturated until
reach "drops saturation preasure" or
encounter sea salt particles, where as opposite
saturation can be reached at lower humidity.
Note that air over saturated NaCl has
about 75% rel. humidity, little more over wet NaCl
particles

--
"Libor the Wanderer"
Sorry for my english in case of such troubles.
As dialup user I am sorry for late response.
ForPrivateResponseRemoveDelAndThisFromAboveAddress .

Libor Striz Sat, 14 Feb 2004 15:24:38 +0100 wrote
....


Vapor above ice climbs more quickly, so when freezing
the saturation is reached over ice while not yet over water.

Oops, means the water vapor preasure ..... with temperature.

--
"Libor the Wanderer"
Sorry for my english in case of such troubles.
As dialup user I am sorry for late response.
ForPrivateResponseRemoveDelAndThisFromAboveAddress .