Chapter 18: Water in the Atmosphere

1. Chapter 18: Water in the Atmosphere 18.1: Humidity and Condensation 18.2: Clouds 18.3: Precipitation

2. 18.1: Humidity and Condensation Water vapor is everywhere and it is always on the move. • Characteristics of Water • It is unique because it exists in all three states of matter, depending on its temperature. • Solid at 0oC or below, • Liquid between 0oC-100oC, and a • Gas (water vapor) at 100oC or above.

3. 18.1: Humidity and Condensation 1. Condensation-the change from water vapor to liquid water. Ex. dew, fog and clouds. 2. Condensation releases heat and evaporation absorbs heat.

4. 18.1: Humidity and Condensation • Humidity • Specific Humidity- the actual amount of water vapor in the air at a given time and place. • Air is saturated when the rate of condensation equals the rate of evaporation.

5. 18.1: Humidity and Condensation 3. Relative Humidity- How near the air is to its maximum capacity for holding water vapor. See example on page 392. 4. Humidity is measured using a psychrometer.

6. 18.1: Humidity and Condensation • Condensation • Dew Point- The temperature at which saturation occurs and condensation begins. • Two conditions are needed for water vapor to condense: 1) there must be materials for the water vapor to condense on and 2) air must cool to or below its dew point.

7. 18.1: Humidity and Condensation • If temperatures are 0oC or less dew turns to frost. • Fog forms when a cold surface cools the warmer moist air above it.

8. 18.2: Clouds Clouds form when air cools to its dew point. • Types of Clouds • Four main types- low, middle, high and clouds of vertical development. • If air movement is horizontal then the clouds form in layers called stratiform. If the air movement is mainly vertical, clouds grow upward in great puffs called cumuliform.

9. 18.2: Clouds 3. Stratus and Strato-describe clouds that form in layers and are low. 4. Cumulus and Cumulo- describe clouds that grow upward. They are puffy with flat bases. 5. Cirrus and Cirro- describe high, feathery ice clouds.

10. 18.2: Clouds 6. Alto-describes clouds located between 2000 and 7000 meters. 7. Nimbus and Nimbo- refer to dark rain clouds.

11. 18.2: Clouds • Cloud Formation • Condensation Level- atmospheric level at which condensation occurs. • Adiabatic Lapse Rate- the rate at which air cools as it rises.

12. 18.2: Clouds 3. Cumulus clouds- Form when moist air rises and cools to its dew point. 4. Stratiform clouds- Form when the air is stable and the cloud can't move up or down.

13. 18.3: Precipitation Precipitation-is any form of water that falls from a cloud to the earth's surface. • How Precipitation Forms • Condensation droplets collide and combine to form bigger droplets. • Ice crystals form in clouds that are below freezing.

14. 18.3: Precipitation 3. Sleet- Frozen raindrop 4. Freezing Rain- is rain that freezes when it hits the ground. 5. Hail-Is a frozen raindrop that grows by colliding with other drops.

15. 18.3: Precipitation B. Measuring Precipitation • The National Weather Service uses a rain gauge to measure rain in hundredths of an inch. 2. The measurement represents what the depth of water would be if it did not soak into the ground, flow away or evaporate.

16. 18.3: Precipitation • Where does this occur? • The main cause of precipitation is the rising and cooling of moist air. 2. Near the equator and across mountain ranges are great areas to find precipitation.

17. 18.3: Precipitation 3. There is little or no precipitation where air sinks and warms. 4. Also air that sinks on the downwind side of mountain is very dry. See pg. 404.

18. 18.3: Precipitation D. Weather Modification • Implementing methods of changing the weather. 2. Rain Seeding- Droplets of frozen carbon dioxide or silver dioxide are dropped into a supercooled cloud. • Hail forming.