1. March 13, 2012 Get out your homework to be checked for completion
Go over homework problems
Humidity
March 13, 2012

2. 1. How many calories are needed to change 50. 0 grams of ice at -15
1. How many calories are needed to change 50.0 grams of ice at °C to steam at °C?
2. Calculate the number of calories given off when 32.0 grams of steam cools from °C to ice at °C.
3. Calculate the calories released when 10.0 g of steam at °C are converted into ice at minus 20.0 °C.
4. Calculate the calories required to change 10.0g of ice at -10.0˚C to 10.0g of steam at 110.0˚C.

3. The amount of water vapor in the air
Humidity
The amount of water vapor in the air

4. Relative Humidity = Actual amount of water vapor in the air
Amount the air is capable of holding

5. Mixing Ratio Actual amount of water vapor in the air
mass of water vapor (grams)
mass of dry air (kilograms)

6. Amount of water vapor the air
mixing ratio
Actual amount of water vapor in the air
Relative Humidity =
Amount of water vapor the air
is capable of holding

7. Saturation Mixing Ratio
Amount of water vapor the air
is capable of holding
Saturation Mixing Ratio
When the mixing ratio = 100%

8. saturation mixing ratio
How near the air is
to saturation
mixing ratio
Relative Humidity =
saturation mixing ratio
Amount of water vapor the air
is capable of holding

9. It’s relative to the TEMPERATURE
So why is it called “Relative” humidity?
It’s relative to the TEMPERATURE
Look at the table at the top of page 481 in your book

10. Water Vapor Content at Saturation (g)
Amount of Water Vapor Needed to Saturate a Kilogram of Air at Various Temperatures
Temperature (°C)
Water Vapor Content at Saturation (g)
-20
0.75
-10 2
3.5 5 10 7 15 20 14 25 30
26.5

11. Sample Problem:
If the temperature is 25°C and there are 10 grams of water vapor per 1 kg of air in the atmosphere, what is the relative humidity?
50%

12. 15 g/kg of water vapor Sample Problem:
If the relative humidity is 75% and the temperature is 25°C, how many grams per kilogram of water vapor is currently in the atmosphere?
15 g/kg of water vapor

13. Water Vapor Content at Saturation (g)
Practice Problem:
If the relative humidity is 37.7% and there are 10 g/kg of water vapor in the atmosphere, what is the current temperature?
Temperature (°C)
Water Vapor Content at Saturation (g) 10 7 15 20 14 25 30
26.5
30°C

14. Dew Point The temperature at which a parcel of air becomes saturated
Measures the actual moisture content of the air.
Relative humidity measures how near the air is to saturation.