Subfreezing Humidity?

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RustyDusty

RustyDusty

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I've always wondered this, and maybe one of you can answer this question for me. How is it possible for there to be a humidity level outside when it's below freezing? Or is ice considered humidity? It's 19 degrees here, with an 87% humidity level.
 
You are really asking about relative humidity. Air at different temperatures becomes saturated with moisture at different rates. So, warm air is able to be saturated with more mosture before it condenses into clouds/rain.... Cold air cannot retain as much moisture, therefore, you will normally see dew (which turns to frost) or clouds/sleet/snow. In other words... at 19 degrees, the air is saturated with moisture to 87% of its capacity. That is not the same amount of moisture as 70 degrees with 87%humidity.

When the air becomes saturated, you get dew... When air holding as much moisture that is can cools, then that extra moisture has to go somewhere (condensate on the dust in the sky- clouds- gets too much moisture- precipitation)
 
To finish answering your question: Humidity is water vapor, or water in a gaseous form. This type of water does not have the capacity to freeze... There is an exception to that, wich is the process of deposition, but there are requirements that must be met for this to occur.
 
That's why you feel dry in the winter... the relative humidity at the outdoor temperature is just about always pretty high, because the air can hold very little moisture. But that same concentration of water exists everywhere, so where it's 75 degrees indoors, your relative humidity is down at 10-20%.
 
rob_robinson87 said:
You are really asking about relative humidity. Air at different temperatures becomes saturated with moisture at different rates. So, warm air is able to be saturated with more mosture before it condenses into clouds/rain.... Cold air cannot retain as much moisture, therefore, you will normally see dew (which turns to frost) or clouds/sleet/snow. In other words... at 19 degrees, the air is saturated with moisture to 87% of its capacity. That is not the same amount of moisture as 70 degrees with 87%humidity.

When the air becomes saturated, you get dew... When air holding as much moisture that is can cools, then that extra moisture has to go somewhere (condensate on the dust in the sky- clouds- gets too much moisture- precipitation)
Click to expand...


What do you do for a living? I saw this thread and thought "there's something I can contribute to this forum" (I'm in the mechanical engineering field and a lot of my work involves HVAC systems design) but couldn't figure out how to explain Psychrometrics in layman's terms.

You pretty much summed it up ...well done :cheers:
 
I used to teach Middle School Science and "Weather and Atmosphere" was my favorite unit. That is the hard part at that age is to give the "big picture" without going so much into detail to blow their minds. :)

Now, I am an Instructional Designer working for a contractor to develop computer-ased training for the military. Funny thing is that it isn't much different.
 
There is also "Super saturation". While visiting my Grandparents in Oklahoma I have been in many a night were the humidity is 110% at night and 90 degrees F. Wish they had air conditioning or at lease electricity and running water.
 
Dang, FABO has diverse crowd......asking about weather on a car forum and BANG there ya go, answered
 
rob_robinson87 said:
You are really asking about relative humidity. Air at different temperatures becomes saturated with moisture at different rates. So, warm air is able to be saturated with more mosture before it condenses into clouds/rain.... Cold air cannot retain as much moisture, therefore, you will normally see dew (which turns to frost) or clouds/sleet/snow. In other words... at 19 degrees, the air is saturated with moisture to 87% of its capacity. That is not the same amount of moisture as 70 degrees with 87%humidity.

When the air becomes saturated, you get dew... When air holding as much moisture that is can cools, then that extra moisture has to go somewhere (condensate on the dust in the sky- clouds- gets too much moisture- precipitation)
Click to expand...
How does, if it does, tie into dew point? When I lived in AZ the official start of Monsoon season was when the dew point hit whatever the number was for 3 consecutive days.....
 
I farted. There's your humidity.
 
What is the dew point temperature?

The dewpoint temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature.

If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%.

If there is then further cooling of the air, more water vapor must condense out as even more dew, fog, or cloud, so that the dew point temperature then falls along with the air temperature.

While relative humidity is (as its name suggests) a relative measure of how humid the air is, the dewpoint temperature is an absolute measure of how much water vapor is in the air. In very warm, humid conditions, the dewpoint temperature often reaches 75 to 77 degrees F, and sometimes exceeds 80 degrees. No matter how hot the temperature gets, a dewpoint temperature of (say) 75 deg. F always represents the same amount of water vapor in the air.

During the summer, the dewpoint temperature -- not the relative humidity -- is usually a better measure of how humid it feels outside. It is also a good measure of how much "fuel" is available to showers and thunderstorms, with a higher dewpoint representing more water vapor available for conversion to rain.

I'm sure you will find this explanation all over Google its pretty standard therory used to help when investigating IAQ problems in commercial buildings.
 
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