Part 2. Water in the Atmosphere

Chapter 7

Precipitation Processes

Precipitation ProcessesNot all clouds precipitate Precipitation requires rapid cloud drop growthThe 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

(.000002 mph)

(.02 mph)

(.6 mph)

(15 mph)

(.04 in)

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

Collisions cause coalescenceLarger 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.

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.

The Bergeron Process

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)

Forms of PrecipitationSnow 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

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

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

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.

Hail Formation

Updrafts that carry hailstones high into the cloud

Annual hail frequency

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

Measuring PrecipitationStandard raingages• Sparse network

Doppler Radar Precipitation Estimates

Snow MeasurementAccumulated snow measured• Water equivalent of snow = 10 to 1 ratio

Automated snow pillows• Convert weight to water equivalent

Cloud SeedingDry ice method

Silver iodide method