Part 2. Water in the Atmosphere

1. Part 2. Water in the Atmosphere Chapter 7 Precipitation Processes

2. Precipitation Processes • Not all clouds precipitate • Precipitation requires rapid cloud drop growth • The average precipitation drop is 1 million times larger than the average cloud droplet • Terminal velocity = the velocity with which a drop fall • Terminal velocity is very low for cloud droplets • Terminal velocity is much higher for precipitation

3. (.6 mph) (.02 mph) (.000002 mph) (.04 in) (15 mph)

4. Raindrops form from cloud droplets • Collision-coalescence = Large precipitation form from the collision and a coalescence of separate cloud droplets • Larger collector drops are the ones that develop into raindrops

5. Collisions cause coalescence Larger collector drop falls faster than the the smaller cloud droplets. The collector drop will run into and coalesce with many cloud droplets, causing the collector drop to grow. However, air pressure sweeps many small droplets out of the way of the collector drop.

6. Growth of snow takes place in clouds of both comprised of supercooled water and ice • Bergeron process = ice crystals grow because air that is at the saturation vapor pressure over water is supersaturated over ice. This causes the ice crystals to grow by deposition, forming snowflakes.

7. The Bergeron Process

8. Cumulonimbus clouds: the top is all ice particles (fuzzy cloud margins), the middle is a mix of ice and liquid droplets, and the bottom has all liquid drops (sharp margins)

9. Forms of Precipitation • Snow forms by the Bergeron process • Lake-effect snows form when cold air (below freezing) blows over warmer water. Places like Buffalo, Cleveland and western Michigan get much lake-effect snow. • Topography influences snow: snow can fall at higher elevations even if it is raining at lower elevations

11. Raindrop shape is altered by air resistance as it falls. The air resistance can flatten rain droplets and even break them apart.

13. Totals are liquid water totals. Snow accumulations are melted to water for inclusion in this map.

14. Graupel = extensively rimed ice crystals • Hail = concentric layers of ice around graupel • Hail forms in updraft and downdraft interactions in thunderstorms. Each concentric ring corresponds to another updraft in the storm. • The Great Plains has the highest frequency of hail in the U.S.

15. Hail Formation Updrafts that carry hailstones high into the cloud

16. Annual hail frequency

17. Sleet = ice (snow) crystals that melt in an atmospheric inversion layer as they fall and then refreeze in a colder layer near the surface • Freezing Rain = supercooled raindrops that freeze upon contact with a surface that is below 32°F (0°C) Sleet formation

18. Measuring Precipitation • Standard raingages • Sparse network

19. Doppler Radar Precipitation Estimates

20. Snow Measurement • Accumulated snow measured • Water equivalent of snow = 10 to 1 ratio • Automated snow pillows • Convert weight to water equivalent • Cloud Seeding • Dry ice method • Silver iodide method