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

10
(No Transcript)
11

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

12
(No Transcript)
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 32F
  (0C)

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