1. What is Fog?
Fog is essentially a dense cloud of water droplets, or cloud, that is close to the ground.
Fog forms when warm, moist air travels over a cold surface.

2. Humidity
Chapter 18

3. Humidity Humidity is the amount of water vapor in the air.
Two methods used to estimate water vapor
content in the air.
1. Relative humidity
2. Dew-point temperature

4. Review
Evaporation

5. Review Water Vapor Formation Water vapor gas molecule
Water vapor molecules move from liquid water to air above water.
Eventually water vapor molecules leaving water will equal water vapor molecules entering the water

6. Measuring Humidity 1. Relative humidity or % RH Facts:
The amount of water vapor in the air is affected by temperature and air pressure
Warm air holds more water vapor than cold air.
What is relative humidity?
The ratio of the water vapor present in the air compared to the maximum amount of water vapor that air can hold

7. Measuring Humidity 1. Relative Humidity (% RH)
100 marbles = 100 water vapor molecules = 100 % relative
humidity
At 100% relative humidity the air is said to be saturated.

8. Measuring Humidity 2. Dew Point What is Dew Point?
Air temperature at which the air is fully saturated with
water vapor molecules (100%RH). 
If the weather man/woman says the dew point is 72° -
Then condensation or dew will form on the ground at 72°

9. Measuring Relative Humidity
A psychrometer is used to measure relative humidity

10. Dry Bulb Temperature
Dry Bulb Temperature minus Wet Bulb Temperature