Clouds and Precipitation

1. Clouds and Precipitation Section 11.3

2. Cloud Formation • Condensation nucleus – a small particle in the atmosphere around which water droplets form; < .001 mm in diameter • A cloud is visible when the number of these droplets gets large • Stable air resists rising • Air mass stability determines the types of clouds that form and weather patterns

3. Stable Air • Stability of air depends on how the temperature of the air mass changes compared to the atmosphere • Fair weather clouds form under stable conditions

4. Unstable Air • The atmosphere is considered unstable when air masses continue to rise because the air mass is less dense than the surrounding air • Unstable clouds produce thunderstorms

5. Atmospheric Lifting • Clouds form when moist air rises, expands, and cools • Convective lifting occurs when air rises when it is heated and becomes warmer than the surrounding air • Describe Orographic lifting and convergence.

6. Atmospheric Lifting • Orographic lifting occurs when an air mass is forced to rise over a topographic barrier; Sierra Nevadas and heavy rain • Convergence occurs when air flows into the same area from different directions; common at mid-latitudes and equator

7. Types of Clouds • Cloud shapes are due to differences in how clouds form • Cloud formation occurs at different altitudes • First classified in 1803 • Classified by altitudes formed and shape • Low (<2000 m), Middle (2000 m – 6000 m), and High (>6000 m)

8. Low Clouds • Cumulus – puffy, lumpy looking clouds • Stratus – a layered, sheetlike cloud that covers most of the sky in an area; fog that has lifted

9. Middle Clouds • Altocumulus and altostratus are made up of ice crystals and water droplets due to lower temps • Altocumulus – white or gray in color and form large round masses or wavy rows • Altostratus clouds are gray and form sheets of clouds • Sometimes produce mild precipitation

10. High Clouds • Made up of ice crystals • Cirrus – have a wispy indistinct appearance • Cirrostratus – a continuous layer that can cover the sky; vary in thickness

11. Vertical Development Clouds • Cumulonimbus clouds can reach into the tropopause; the top is composed entirely of ice; • These giant clouds can produce torrential rains, strong winds, and hail

12. Precipitation • All forms of water that fall from the clouds to the ground • Rain, snow, sleet, and hail • Coalescence occurs when cloud droplets collide and join together to form a larger droplet • Rain is precipitation that reaches Earth’s surface as liquid; between 0.5 – 5 mm

13. Snow, sleet, and hail • Ice crystals form in clouds with a temp. below freezing causing snow to fall • Sleet forms when air currents move droplets to move up and down between freezing and non-freezing air • If this up and down motion occurs over a large area then hail forms (large ice pellets)

14. The Water Cycle • 97% of Earth’s water is in the oceans • The constant movement of water between the atmosphere and Earth’s surface • Radiation causes evaporation of water • Water rises and condenses into clouds • Water falls to Earth as precipitation • Cycle continues and nourishes living things