1. Chapter 9: Weather Factors Section 4: Water In the Atmosphere

2. The Water Cycle Water Cycle – the movement of water between the atmosphere and Earth’s surface. ◦ Evaporation ◦ Condensation ◦ Precipitation ◦ Surface runoff

3. The Water Cycle Evaporation- water molecules in liquid water escape into the air as water vapor.

5. Humidity Humidity - a measure of the amount of water vapor in the air. Relative Humidity – is the percentage of water vapor that is actually in the air. ◦ Compared to the maximum amount of water vapor the air can hold at a particular temperature.

6. Measuring Relative Humidity Relative humidity can be measured with an instrument called a psychrometer. ◦ (sy KRAHM uh tur) ◦ 2 thermometers ◦ Dry bulb ◦ Wet bulb

7. How Clouds Form Clouds form when water vapor in the air condenses to form liquid water or ice crystals. Condensation – molecules of water vapor in the air become liquid water. Dew point – the temperature at which condensation begins. ◦ Above freezing - water droplets ◦ Below freezing – ice crystals

8. How Clouds Form The Role of Particles ◦ For water vapor to condense, tiny particles must be present so the water has a surface on which to condense. ◦ In cloud formation, most of these particles are salt crystals, dust from soil and smoke. ◦ These tiny particles are called nuclei.

10. Types of Clouds Cirrus Clouds Cumulus Clouds Stratus Clouds

11. Cirrus Clouds Wispy, feather Only at high levels with low temperatures ◦ Above 6 kilometers Made of ice crystals

12. Cumulus Clouds Look like fluffy, rounded piles of cotton Form less than 2 kilometers above ground May extend upward as much as 18 kilometers

13. Stratus Clouds Form in flat layers Cover all or most of the sky A uniform dull, gray color

14. Altocumulus and Altostratus Middle level clouds; higher than regular cumulus and stratus clouds, lower than cirrus clouds. Between 2 and 6 kilometers above the ground

15. Fog Fog – clouds that form at or near the ground Air above the ground is cooled to the air’s dew point. Near bodies of water, low-lying marshes, mountain slopes