1. Clouds

2. RECAP • Dew - condensation on cold surfaces. • Frost - deposition on very cold surfaces. • Haze - condensation on small airborne particles. • Fog – saturated wet haze, visibility <1 km. • Radiation fog – driven by radiation cooling • Advection fog – driven by air movement • Upslope fog - topography driven • Evaporation fog – cold air above warm body of water.

3. Clouds • Definition: visible aggregate of tiny water droplets or ice crystals suspended in air • Classification: four major cloud groups according to the height (of the cloud base). Within each group there are different types identified by their appearance • High (cirrus: Latin for “wisp of hair”) • Cirrus, Cirrostratus, Cirrocumulus • Middle (alto, from the Latin altus: “high”) • Altostratus, Altocumulus • Low (stratus, Latin for “layer”) • Stratus, Stratocumulus, Nimbostratus • Clouds with vertical extension (cumulus: ”heap”) • Cumulus, Cumulonimbus

4. What are the important things to know • Composition: recall that in the troposphere the temperature drops with altitude. • High clouds (16,000 – 43,000 ft in midlatitudes) are made mostly of ice • Middle clouds (6,500 – 23,000 ft in midlatitudes) are composed of water droplets and some ice crystals (higher up or when the temperature is low) • Low clouds (surface to 6,500 ft in midlatitudes) are composed primarily of water droplets except in very cold weather when it snows • For reference: commercial aircraft flies around 30,000 ft.

6. Basic Cloud Types

7. High Clouds • Form at high altitudes (above 16,000-20,000 ft). • Composed mostly from ice crystals. • Appear white and very thin. • Cirrus clouds (this transparency’s background): • thin wispy clouds • blown by strong high altitude winds (jetstream) • typically move from west to east (?) • point to fair, pleasant weather

8. ???cumulus types • Small, rounded puffy, in parallel waves or bands • Cirrocumulus (high) • “mackerel sky” • Altocumulus (middle) • Stratocumulus (low)

9. ???stratus types • Cirrostratus (high) • Halo around sun/moon • Altostratus (middle) • Sun dimly visible • Stratus (low) • No sun, like thick fog

10. Cumulonimbus cloud • Thunderstorm cloud with a very dark base. • Large vertical extent (several km thick). • The top may be flattened by winds (anvil). • It can occur as a single isolated cloud or a wall of clouds. • All types of precipitation is possible: strong showers, snow, hail. Lightning is very likely. Tornadoes are possible. • They are associated with large amount of energy release, strong up- and down-drafts.

11. Other types of clouds • Clouds driven by topography: some sort of a barrier present: • Cap clouds: wind over a mountain or • Lenticular clouds: formed over a mountain • Banner cloud: downwind of an isolated mountain top. • Mammatus clouds: typically under cumulus clouds. • Condensation trails from jet aircrafts: adding water vapor, providing condensation nuclei, cooling of the air flowing over the wing. • Very high altitude clouds: • noctilucent clouds: 75-90 km above the surface; • Where is the water coming from? - meteorites, CH4 chemistry.

12. Cap cloud

13. Lenticular clouds The clouds form on the lee side of the mountains in the crests of the mountain wave. Can be confused with UFOs

14. Banner cloud

15. Observing the Earth from Space • Geostationary satellites: • Orbit the equator at the same rate as the Earth spins at about 36000 km above the surface. • Remain above a fixed spot on the Earth surface. • Provide continuous monitoring of a specific region • Polar-orbiting satellites. • Provide global coverage of the planet, move slightly westward on each pass • Have a lower orbit (~850 km). • Observe the area straight beneath them.

16. Satellite observations • Visible wavelengths: • Use the reflected sun light from the top of the clouds. • The albedo of the clouds gives information about the thickness of the cloud. • No temperature information about the clouds. • Infrared observations: • IR light emitted from the clouds. • Provide T information about the clouds. • This can be converted into altitude information. • Using different wavelengths in the IR one can obtain 3D information about the structure of the clouds.

17. Name that cloud! Cirrocumulus Altostratus Cirrostratus Altocumulus