1. Water in the Atmosphere

2. Water enters the atmosphere by EVAPORATION.
The movement of water between the atmosphere and Earth’s surface is called the WATER CYCLE.
Water enters the atmosphere by EVAPORATION.
EVAPORATION is the process by which water molecules in liquid water escape in to the air as water vapor.
Water vapor also enters the air via plants.

3. Water Cycle

5. HUMIDITY is the amount of water vapor in the air.
The percentage of water vapor in the air compared to the maximum amount the air can hold is called the RELATIVE HUMIDITY.
Warm air can hold more water vapor than cool air.

6. For instance at 10 degrees Celsius 1 cubic meter can hold 8 grams of water vapor.
If there were 8 grams of water vapor in that cubic foot then the relative humidity would be 100 percent.
If the air held 4 grams of air the relative humidity would be 50 percent.

7. IT’S NOT THE HEAT IT’S THE HUMIDITY
Even on a hot day you can feel comfortable if the air is dry.
If the temperature and relative humidity are high it is usually uncomfortable.
When the relative humidity is high, evaporation slows down.
Evaporation has less of a cooling effect on the body.

8. Measuring Relative Humidity
Relative humidity can be measured with a psychrometer.
A psychometer has 2 thermometers, a wet bulb and a dry bulb thermometer.
The wet bulb is moistened with water and air is then blown over both thermometers.
Because the wet bulb thermometer is cooled by evaporation, its reading drops below that of the dry –bulb thermometer.

9. If the relative humidity is high, the water on the wet bulb will evaporate more slowly and the wet bulb temperature will not change that much.
If the relative humidity is low the water on the wet bulb will evaporate rapidly and the wet bulb temperature will drop.
The relative humidity can be found by comparing the temperatures of the wet and dry bulb thermometers using a chart like the one on page 546.

10. How Clouds Form
Clouds of all kinds form when water vapor in the air becomes liquid water or ice crystals.
The process by which molecules of water vapor become liquid water is called CONDENSATION.

11. The temperature at which condensation begins is the dew point.
If the dew point is below the freezing point the water vapor changes directly into ice crystals.
When you look at a cloud you are looking at millions of tiny ice crystals or water droplets.
For water vapor to condense, tiny particles must be present i.e. dust particles, smoke, salt crystals.
Some time water vapor condenses on to blades of grass. This is called dew.
Frost is ice that has been deposited directly from the air onto a cold surface.
Clouds form whenever air is cooled to it’s dew point and particles are present.