1. Water In The Atmosphere The presence of water in the atmosphere is a major influence on the formation of clouds, rain, snow, lightning, fog, and most other weather events. This chapter will provide students with an understanding of the importance of atmospheric moisture and its effects on weather and climate.

2. Heat Energy and Water How does water in the atmosphere mainly exist? water vapor It can also exist as a liquid or a solid. Water can change from a solid to a liquid to a gas by adding heat.

3. However, if I have a special kind of ice (like CO2), it can change directly from a solid to a vapor without becoming a liquid. This is called: Sublimation and an example is with dry ice. It can also happen with frozen water or snow if the air is dry, and its temperature is below freezing. Although some water enters the atmosphere through sublimation, most water gets into the atmosphere through evaporation.

4. When water evaporates and enters the atmosphere, the molecules move much faster than they did when they were in a liquid form. This is due to “hidden” heat within the molecules called: latent heat.

5. Example of Latent Heat: On a cold night when temperature is below freezing, a farmer can spray plants with water and they won’t freeze due to latent heat. http://www.youtube.com/watch?v=1PcnC WZP7I0http://www.youtube.com/watch?v=1PcnC WZP7I0

6. Humidity When we discuss how much of this water vapor is in the atmosphere, we are talking about: Humidity. As more water vapor is added to the air, the humidity increases. When the air has all the water vapor it can possibly hold at a given temperature, it is said to be saturated.

7. Warm air can hold more water than cold air. Therefore, as temperature increases, the amount of water it can hold also increases. When we discuss how close to saturation the air actually is, we call it: relative humidity. Relative humidity compares how much water is in the air to how much water the air can hold.

8. When they say on the news that the relative humidity is 90% -That Does NOT mean… that 90% of our atmosphere is water. -It Does mean… that the atmosphere can hold a certain amount of water and 90% of that amount is in the air.

9. Example Think of the atmosphere as a cup of water This cup can hold 10 ml of water. It is holding 5 ml of water. What % is it filled? This cup can hold 8 ml of water. It contains 6 ml. What % is it filled?

10. What is the Relative Humidity If a temperature can hold 10 grams of water and contains 5 grams? 50% If a temperature can hold 25 grams and contains 10 grams of water? 40% If a temperature can hold 30 grams and contain 30 grams? 100% saturated If a temperature can hold 20 grams and contains 30 grams? 150% supersaturated-probably raining (need enough nuclei for rain to condense on)

11. What would happen to the relative humidity if the amount of moisture stays the same but…. 1.The temperature drops? it would increase 2. The temperature increases? it would decrease See cup drawings on the board

12. Measuring Relative Humidity Relative Humidity is measured with a psychrometer. A psychrometer has 2 parts: Wet bulb- thermometer covered with a damp wick. Dry bulb- a normal thermometer. The person measuring relative humidity holds the psychrometer and whirls it around. Water evaporates from the wet wick. When it evaporates what does it pull with it? Latent heat. Therefore, what will happen to the temperature in the wet bulb thermometer? It will drop.

13. By looking at the difference is temperature readings between the dry bulb and the wet bulb, and by using that information and applying it to the chart, one can determine relative humidity. Another way to measure is with a hair hygrometer. It contains a bundle of hair that increases in length as humidity increases. To express the actual amount of moisture in the air, scientists use specific humidity. Specific Humidity= # of grams of H2O kg. of air.

14. Below are 3 regions of the earth and 3 different specific humidity readings. Match the specific humidity to its region. California 18 g/kg South Pole 1 g/kg Hawaii 10 g/kg

15. Dew Point We already said that warm air can hold more water than cold air. Therefore, if air is cooled until it reached the point of saturation, it is called its dew point. When temperature is lowered below dew point, water vapor from the atmosphere condenses into a liquid which we call dew.