Lesson 16 - Precipitation Processes

what is precipitation?

• () or () particles falling from the () that reach the surface

• most () don’t precipitate

• () alone cannot create precipitation

• solid or liquid particles falling from the atmosphere that reach the surface

• most clouds don’t precipitate

• condensation alone cannot create precipitation

why don’t most clouds precipitate?

• cloud droplets must grow () times their original () to form precipitation, takes longer than () minutes

• cloud drops are about ()% the radius of a ()

• cloud droplets must grow 1,000,000 times their original volume to form precipitation, takes longer than 15 minutes

• cloud drops are about 1% the radius of a raindrop

if it does not reach the ground, what is it?

• () (remember that strong () motion may exist even below virga)

• virga (remember that strong vertical motion may exist even below virga)

what are the two precipitation processes?

• ()-()

• (…) process ( () deposition)

• collision-coalescence

• ice crystal process (vapor deposition)

what is collision-coalescence? where is it common and what clouds?

• the most () precipitation process in () clouds (tops > -() degrees C)

• common in (): “warm rain” from isolated cumulus clouds over () or ()

• also common in () stratiform clouds ( () commonly produces droplets)

• the most prevalent precipitation process in warm clouds (tops > -15 degrees C)

• common in tropics: “warm rain” from isolated cumulus clouds over land or ocean

• also common in marine stratiform clouds (nimbostratus commonly produces droplets)

what size of droplets are needed to reach raindrop size and why?

• need a () of droplet sizes to reach raindrop size

• () drops fall faster than () droplets

• the (), ()-falling drops () (merge) with () droplets

• strong () in () clouds may enhance and prolong this process (making () raindrops)

• need a range of droplet sizes to reach raindrop size

• large drops fall faster than small droplets

• the larger, faster-falling drops coalesce (merge) with smaller droplets

• strong updrafts in cumulonimbus clouds may enhance and prolong this process (making large raindrops)

what is the ice-crystal growth process? (bergeron-findeisen)

• occurs in () cloud ( () and () )

• between () degrees C and () degrees C (below ~(),000’ MSL) ice and water particles () ( () cloud)

• between () degrees C and () degrees C, cloud is mainly () () water

• occurs in supercooled cloud (water and ice)

• between 0 degrees C and -40 degrees C (below ~25,000’ MSL) ice and water particles coexist (mixed cloud)

• between 0 degrees C and -15 degrees C, cloud is mainly supercooled liquid water

what happens in a mixed cloud?

• () crystals grow as the () water droplets shrink

• because () for an ice crystal surface is () than that for a water droplet

• () diffuses toward the ice ( () growth)

• ice crystals grow as the supercooled water droplets shrink

• because SVP for an ice crystal surface is less than that for a water droplet

• vapor diffuses toward the ice (rapid growth)

what is the diffusion of water vapor?

• temperature is around () to () degrees C (most efficient) (() degrees C)

• SVP over ice is () than over water because it takes more () for a water molecule to escape from solid ice than it does from liquid water and it takes () molecules to escape the ice surface

• temperature is around -10 to -20 degrees C (most efficient) (-15 degrees C)

• SVP over ice is less than over water because it takes more energy for a water molecule to escape from solid ice than it does from liquid water and it takes fewer molecules to escape the ice surface

by molecular motions, the water vapor will therefore diffuse from () to () concentrations

• (), () because molecules are always () around randomly and colliding, which naturally () them until they are evenly () out

• the () crystals will grow while () shrink (the () droplets are constantly () into vapor and the () crystals are constantly () that vapor out of the air and () it onto themselves)

• higher, lower because molecules are always moving around randomly and colliding, which naturally mixes them until they are evenly spread out

• the ice crystals will grow while drops shrink (the liquid droplets are constantly evaporating into vapor and the ice crystals are constantly pulling that vapor out of the air and freezing it onto themselves)

what are the two ways ice crystal growth is increased by?

• ac…

• ag…

• accretion

• aggregation

what is accretion?

• also known as () where ice crystals () with () drops, and () of ice form on the ()

• extreme case: () (small hail)

• also known as riming where ice crystals collide with supercooled drops, and beads of ice form on the crystals

• extreme case: graupel (small hail)

what is aggregation?

• where falling () falling near () degrees C level () and () together

• falling ice particles may (), providing additional surfaces for ()

• in () and () latitudes, most precipitation begins as () higher up in the atmosphere

• where falling snowflakes falling near 0 degrees C level collide and stick together

• falling ice particles may fracture, providing additional surfaces for freezing

• in middle and high latitudes, most precipitation begins as snow higher up in the atmosphere

what are the six precipitation types?

• r

• s

• h

• f

• i

• g

• rain

• snow

• hail

• freezing rain

• ice pellets (sleet)

• graupel

what is rain?

• drop size: >() mm diameter (drizzle: <() mm) - larger drops ()

• RA = () rain from () clouds

• SHRA: () from () clouds ( () drops, usually () accumulation)

• intensity determined by () rate; (light rain is <() inch ( () mm) per hour)

• drop size: >0.5 mm diameter (drizzle: <0.5 mm) - larger drops flatten

• RA = continuous rain from stratiform clouds

• SHRA: showers from cumuliform clouds (bigger drops, usually faster accumulation)

• intensity determined by accumulation rate; (light rain is <0.1 inch (2.5 mm) per hour)

what is snow?

• large () possible just above () degrees C

• SN = () snow

• typical of () clouds and () (> () in)

• SHSN = () snow (), typical of clouds with some ()

• enhanced () of lakes (()-() snow)

• large aggregates possible just above 0 degrees C

• SN = continuous snow

• typical of stratiform clouds and accumulation (> 4 in)

• SHSN = intermittent snow showers, typical of clouds with some convection

• enhanced downwind of lakes (lake-effect snow)

what is the lake-effect snow?'

• localized snow that forms when (), () air moves over a large body of () water, picking up () and () before dumping heavy snow over ()

• 1. the pickup: () air blows across a relatively (), unfrozen ()

• 2. the cloud building: the () water () into the () air, rapidly creating a () band of clouds and heavy ()

• the dump: as the () pushes these clouds over the () land () of the lake, the snow gets squeezed out, often causing massive, blinding () in () bands

• localized snow that forms when cold, dry air moves over a large body of warmer water, picking up moisture and heat before dumping heavy snow over land

• 1. the pickup: arctic air blows across a relatively warm, unfrozen lake

• 2. the cloud building: the warm water evaporates into the cold air, rapidly creating a thick band of clouds and heavy snow

• the dump: as the wind pushes these clouds over the cold land downwind of the lake, the snow gets squeezed out, often causing massive, blinding snowstorms in narrow bands

what is hail?

• () or () chunks of ()

• only forms in () clouds, greater than ()” diameter

• can weigh more than () lb and exceed ()”

• round or irregular chunks of ice

• only forms in CM clouds, greater than 0.2” diameter

• can weigh more than 1 lb and exceed 5”

what is freezing rain?

• () rain that () onto objects

• What does a sounding look like during freezing rain? - a highly specific “()” or “()” structure

• () hazard

• liquid rain that freezes onto objects

• What does a sounding look like during freezing rain? - a highly specific “nose” or “sandwich” structure by a weather balloon

• aviation hazard

what are ice pellets (sleet)?

• ()drops that ()

• raindrops that freeze

what is graupel?

• () with rime ()

• crystals with rime accretion

what are the 4 types of precipitation: soundings?

• snow:

• freezing rain:

• rain:

• sleet:

explain snow

• Sounding Profile: The temperature line remains entirely () () (<()°C) from the () all the way to the ().

• The Process: There is no () layer to cause (). () crystals grow via the () process and fall to the ground intact as ().

• Sounding Profile: The temperature line remains entirely below freezing (<0°C) from the cloud all the way to the surface.

• The Process: There is no warm layer to cause melting. Ice crystals grow via the Bergeron process and fall to the ground intact as snowflakes.

explain freezing rain

• Sounding Profile: A () structure with a () warm nose aloft and a very () freezing layer at the surface.

• The Process:

  1. Snow falls into a deep layer of () air (>()°C) and completely () into raindrops.

  2. It then hits a critically () freezing layer (<()°C) at the surface.

  3. Because the cold layer is so (), the drops do not have time to () in the air. Instead, they become () and () instantly upon hitting the ground.

• Sounding Profile: A sandwich structure with a deep warm nose aloft and a very shallow freezing layer at the surface.

• The Process:

  1. Snow falls into a deep layer of warm air (>0°C) and completely melts into raindrops.

  2. It then hits a critically shallow freezing layer (<0°C) at the surface.

  3. Because the cold layer is so thin, the drops do not have time to refreeze in the air. Instead, they become supercooled and freeze instantly upon hitting the ground.

explain rain

• Sounding Profile: The temperature is () freezing (>()°C) for the entire () half of the atmosphere, all the way down to the ().

• The Process: Precipitation may start as () high up, but it melts completely into () early in its () and stays liquid.

• Sounding Profile: The temperature is above freezing (>0°C) for the entire lower half of the atmosphere, all the way down to the ground.

• The Process: Precipitation may start as snow high up, but it melts completely into rain early in its descent and stays liquid.

explain sleet

• Sounding Profile: A () structure with a () warm nose aloft and a () freezing layer at the surface.

• The Process:

  1. Snow falls into a () layer of warm air (>()°C) and partially melts.

  2. It then falls into a deep, () layer (<()°C) near the ground.

  3. The partially melted droplets have enough time to () into hard, translucent () pellets before hitting the ground.

• Sounding Profile: A sandwich structure with a shallow warm nose aloft and a deep freezing layer at the surface.

• The Process:

  1. Snow falls into a shallow layer of warm air (>0°C) and partially melts.

  2. It then falls into a deep, cold layer (<0°C) near the ground.

  3. The partially melted droplets have enough time to refreeze into hard, translucent ice pellets before hitting the ground.

what two things need to happen for freezing rain to occur at surface or aloft?

• () falling

• () () levels

• falling snow (), then () on impact

• precipitation falling

• multiple freezing levels

• falling snow melts, then refreezes on impact

what warns of freezing rain above?

• presence of (…) (or () ) warns of freezing rain above

• presence of ice pellets (or sleet) warns of freezing rain above

what are some characteristics of freezing rain?

• () storms can be devastating (damage to () and () lines, () outages, () accidents, () delays)

• freezing rain is common in () band ahead of () front

• most common in the north() and north() U.S.

• freezing rain is almost () on () surfaces

• ice storms can be devastating (damage to trees and power lines, power outages, automobile accidents, aviation delays)

• freezing rain is common in narrow band ahead of warm front

• most common in the northwestern and northeastern U.S.

• freezing rain is almost invisible on smooth surfaces

what is the appearance of ice pellets?

• relatively () balls of (); () (rimed crystal) has a () () appearance

• relatively clear balls of ice; graupel (rimed crystal) has a milky white appearance