On Wed, 23 Oct 2019 18:58:48 -0400, Ralph Mowery
wrote:

In article ,
says...
How much humidity does sub zero air contain????? Not very much is how
much. The colder it gets outside the LESS likely the system is to ice
up.

At +40c 1kg of air at 100% RH holds 49.8 grams of water.
At -40C that same Kg of air can only hold 0.1 grams of water.
At 0c it can hold 3.8grams - at 20c it can hold 15 grams, and at -20c
only 0.75 grams



All that is fine and dandy . However if it is raining and the
temperature is slightly below 0 deg C you will probably ice up very
fast. That hapens a lot in the area I live in. Middle of North
Carolina.
Actually it will ice up faster at 3 or 4 above than it will at zero.