Relative humidity at negative temperature

[astro2cosmos]

as we know at zero degree temp. water condense to form ice. so what will be the relative humidity of a chamber at a temperature below zero degree temp.?
will it be nagative or what?

 

[russ_watters]

Celsius is a relative temperature scale and the negative doesn't have any deeper meaning other than it was decided to use 0 as the freezing temp of water because it is convenient. Air below 0C can still hold some water vapor, but only a little.

 

[Bill_K]

For a liquid-gas system, the amount of vapor present is called the vapor pressure. If the system is in equilibrium, any more vapor would cause liquid to condense, any less would cause liquid to evaporate. We then say the air is saturated. Likewise for a solid-gas system.

Relative humidity is defined as the amount of vapor in the air compared to the maximum, so if the system is in equilibrium, the relative humidity is one hundred percent.

 

[nucleus]

“as we know at zero degree temp. water condense to form ice.”

Water melts at zero degrees C; however, it can be in a liquid form at temperatures as low as –40 degrees. See super cooling:

http://en.wikipedia.org/wiki/Supercooling

As for relative humidity: it is defined as the ratio of the partial pressure of water vapor (in a gaseous mixture of air and water vapor) to the saturated vapor pressure of water at a given temperature. In other words, relative humidity is the amount of water vapor in the air at a specific temperature compared to the maximum water vapor that the air is able to hold without it condensing, In other words it is a percentage so it can’t be negetive, but the dew point temperature can be.

There are other measures of moisture like absolute humidity and dew point temperature.

As I type this, the air temperature in Vostok Antarctic is –58C and the dew point temperature is –62C which gives a RH of 42%.

http://www.wunderground.com/global/stations/89606.html?MR=1