Humidity and Dew Point
"it's not so much the heat, it's the humidity!"
Humidity: amount of water vapor in air
•Specific humidity: mass of water vapor per mass of air
(units: g/kg)
• Saturation: when something can't absorb or dissolve any more... (applies to many things, not just air)
•Relative humidity: water vapor content
water vapor capacity
(sketch the following graph)
Questions
Can hot air or cold air hold more water?
If hot air with a certain amount of water vapor is cooled, what happens to its RH?
If cold air with a certain amount of water vapor is heated, what happens to its RH?
When is RH an issue for indoor air quality?
Predict: Draw a graph of daytime temperature and time of day, and guess what the graph should look like.
Now assume this air has a constant amount of water vapor in it. Sketch a second line for RH.
Interesting tidbit: water vapor is less dense than air.
"this humid air is so thick!" <-- is this statement accurate?
humid air is more likely to rise than dry air.
Dew Point
The temperature (for a certain amount of humidity) where saturation can be reached.
Example: Air is at 20C (68F) with 70% RH. How cold must this air get to reach saturation?
What will happen when the temp cools further?
(RH cannot go above 100%)
Examples
If the temperature outside is 30C (86F) and...
Dew point is: Is the RH high or low?
10C (50F)
25C (77F)
30C (86F)
(today's weather forecast)