1. Weather Part 3
Humidity

2. Remember:
Water takes longer to warm and cool because of its high heat capacity.
Lots of solar energy is needed to change the state of water!
Solid  Liquid  Gas
Ice  Water  Vapour

3. Consider This:

4. Consider This:
When liquid water from the lake evaporates, it exists as vapour.
We cannot see it.
It is in the space between the lake and the clouds.

5. Consider This: But the air cannot hold all of the vapour.
Some will condense back into a liquid, in the form of clouds, fog, mist, or rain.

6. Consider This:
As the distance from the lake increases, the air temperature cools.
Warm air can hold more water vapour that cool air.

7. Saturated Air
When the air contains the maximum amount of water vapour possible, at that temperature.
Water will continue to evaporate until the air is saturated.
Example:
At 20°C 1 kg of saturated air has 15g of water vapour.

8. Saturated Air If air becomes cooler or if more water evaporates:
Some of the vapour will condense.
Water droplets form around particles in the air.
Ex: dust, salt
Droplets also form on the surface of solids.
Ex: dew on grass

9. Humidity The amount of water vapour in the air.
Coastal regions tend to have a higher humidity than regions inland.
Summer tends to have higher humidity.
Seems “sticky” when it is hot and humid.
Does bad things to your hair!! (haha)

10. Humidity
Warm air can hold more vapour than cool air.

11. Absolute Humidity The actual amount of water vapour in the air.
Expressed in grams of vapour per kilogram of air.

12. Relative Humidity
The percentage of water vapour in the air compared with the amount the air can hold (at that temperature) if it was saturated.

13. Relative Humidity Relative Humidity Demo

14. Think About It:
What does a weather reporter mean by saying “the relative humidity is 62 percent”?
The amount of water vapour in the air is 62% of the amount that would be in the air at that temperature if the air was saturated.

15. Consider This:
The relative humidity inside the house is low during the winter.
Why?
The air is much warmer in the house than outside.
Warm air can hold more.
With a low relative humidity, no wonder your skin feels dry!

16. Dew Point
The temperature at which the air is saturated so vapour condenses into liquid.
Condensation begins to occur.
Clouds form, precipitation may happen.

18. Consider This:
Cold cans of pop get condensation on them (dew) because temp of can is cooler than temp of air.
As air becomes cooler around the can, the vapour is the air begins to condense.
This creates the drops on the can or the puddle underneath it.

19. Humidex The humidex scale was created by Environment Canada.
It takes into account both the temperature and the relative humidity.
It indicates how warm a given temperature will feel to the average person when relative humidity is taken into account.

21. Consider This:
On a summer day when the relative humidity is high, you feel much hotter than when the temperature is the same, but it is less humid.
A weather forecast may indicate that the temperature is 35°C but feels like 40°C.

22. Consider This: Why does it feel warmer?
An increase in relative humidity reduces the amount of perspiration (sweat) from evaporating and in return cooling your body, causing you to feel hotter!