Chapter 7

Precipitation

An ordinary cloud droplet is extremely small (~20 micrometers)

• 100 times smaller than an average raindrop

• if in equilibrium… the size of the droplet does not change

- # of molecules condensing onto the droplet are exactly equal to # evaporating

- saturation vapor pressure = equilibrium vapor pressure

Precipitation Processes

Growth of Cloud DropsGrowth of Cloud DropsAtmospheric Atmospheric vertical winds vertical winds and eddies can and eddies can keep small and keep small and light cloud light cloud droplets and droplets and condensation condensation nuclei aloft.nuclei aloft.

As the cloud As the cloud droplets knock droplets knock and join together, and join together, they grow larger they grow larger and their weight and their weight increases, causing increases, causing them to fall as them to fall as precipitation.precipitation.

To keep a droplet in equilibrium….

• more water vapor molecules are needed around it to replace those molecules that are constantly evaporating from the surface

• smaller droplets have greater curvature and a more rapid rate of evaporation

• this process is called the curvature effect

• smaller droplet require an even greater vapor pressure to keep them from evaporating away

• therefore, when air is saturated with respect to a flat surface, it is unsaturated with respect to a curved droplet of water, which leads to the evaporation of the droplet

• to keep tiny cloud droplets in equilibrium with the surrounding air, the air must be supersaturated

Vapor Pressure & SaturationVapor Pressure & Saturation

Smaller drops have Smaller drops have greater curvature and greater curvature and require greater vapor require greater vapor pressure to keep pressure to keep water molecules from water molecules from evaporating away.evaporating away.

As the drop size As the drop size increases, the increases, the required relative required relative humidity (RH) for humidity (RH) for equilibrium decreases.equilibrium decreases.

If the required RH is If the required RH is exceeded, the drop exceeded, the drop will grow.will grow.

How do tiny droplets of less than 1 micrometer grow to the size of an average cloud droplet?

• hygroscopic particles allow for condensation to occur at relative humidity's less than 100 percent

Example

- when condensation begins on a salt particle, it dissolves forming a solution

-the salt ions bind closely to the water molecules making evaporation difficult

- this condition reduces the equilibrium vapor pressure

- this is known as the solute effect

• A droplet containing a CCN (such as salt) can be in equilibrium with its environment at RH < 100%

Condensation and precipitation

• most clouds cannot produce precipitation (through condensation)

• this process is too slow to produce precipitation

• it would take several days for this process to produce rain under ideal conditions

• observations show that clouds can develop and produce precipitation in under an hour

• it takes about 1 million average size cloud droplets (20 micrometers) to produce one average size rain drop

• must be other processes at work

Collision and coalescence process

• Occurs in clouds with tops warmer than -15ºC

• some cloud droplets must be larger than others in order for collisions to form a raindrop

• larger drops may form on CCN or through random collision of droplets

• as cloud droplets fall air slows the drops descent

• the amount of air resistance depends on the size of the drop and the rate of the fall

• the speed of the falling drop increases until the air resistance equals the pull of gravity

• at that point the drop reaches its terminal velocity

Collision & Coalescence ProcessCollision & Coalescence Process

Both a) collisions Both a) collisions that join together that join together small cloud droplets small cloud droplets b) coalescence that b) coalescence that attaches faster and attaches faster and larger droplets with larger droplets with smaller slower smaller slower droplets work droplets work together to together to assemble nearly 1 assemble nearly 1 million cloud million cloud droplets into a droplets into a raindrop large raindrop large enough to fall to enough to fall to earth.earth.

Both a) collisions Both a) collisions that join together that join together small cloud droplets small cloud droplets b) coalescence that b) coalescence that attaches faster and attaches faster and larger droplets with larger droplets with smaller slower smaller slower droplets work droplets work together to together to assemble nearly 1 assemble nearly 1 million cloud million cloud droplets into a droplets into a raindrop large raindrop large enough to fall to enough to fall to earth.earth.

Collision & Coalescence ProcessCollision & Coalescence Process

Warm Clouds

• clouds that have above freezing temperatures at all levels

• precip forms by collision coalescense

• Example: Next Slide

• Rain that falls is called warm rain

• most important factor in prduction of raindrop is the clouds liquid water content

• Other significant factors:

-range of droplets sizes

-cloud thickness

-updrafts of the cloud

-electric charge of the droplets and electric field of cloud

Warm Cloud ProcessesWarm Cloud Processes

Collision and Collision and coalescence coalescence operates in operates in warm clouds (> warm clouds (> 15° C) to 15° C) to produce rain, produce rain, and is affected and is affected by the clouds by the clouds liquid water liquid water content, droplet content, droplet sizes, cloud sizes, cloud thickness, thickness, updrafts, and updrafts, and drop electrical drop electrical charges.charges. Figure 8.5Figure 8.5

Ice-crystal Process

• called the Bergeron process

• extremely important in mid and high latitudes

• air below freezing

• cloud are called cold clouds

• Example: next slide

• ice crystals form on ice nuclei

• number of ice nuclei available in the atmosphere is small

• deposition nuclei - allows water vapor to deposit as ice directly onto their surfaces

• freezing nuclei - promote the freezing of supercooled liquid water

• contact nuclei - cause freezing after being immersed in liquid

Ice Crystal ProcessIce Crystal ProcessCold clouds may drop Cold clouds may drop below –40° C before below –40° C before small droplets freeze into small droplets freeze into ice embryos that can ice embryos that can serve as condensation serve as condensation nuclei.nuclei.

At very low At very low temperatures, vapor can temperatures, vapor can also condense as ice onto also condense as ice onto nuclei formed by:nuclei formed by:a) deposition a) deposition b) freezing b) freezing c) contact,c) contact,primarily in the glaciated primarily in the glaciated region of the cloud.region of the cloud.

Molecules from Water to IceMolecules from Water to Ice

Ice crystals have lower saturation Ice crystals have lower saturation vapor pressures than liquid droplets, vapor pressures than liquid droplets, creating a gradient of high to low creating a gradient of high to low water molecules from liquid to ice water molecules from liquid to ice that encourages ice growth.that encourages ice growth.This growth is critical to the ice-This growth is critical to the ice-crystal precipitation process.crystal precipitation process.

Ice Particle ChangesIce Particle Changes

As ice crystals fall and collide with super As ice crystals fall and collide with super cooled drops, they get bigger by accretion.cooled drops, they get bigger by accretion.

Falling icy matter is called graupel, and Falling icy matter is called graupel, and aggregation describes the joining of two aggregation describes the joining of two ice crystals into snowflakes.ice crystals into snowflakes.

Ice Crystal GrowthIce Crystal Growth

Ice crystal growth Ice crystal growth is the dominant is the dominant cause of cause of precipitation in precipitation in nimobstratus and nimobstratus and cumulonimbus cumulonimbus clouds, both of clouds, both of which have lower which have lower liquid content than liquid content than warm-layered warm-layered clouds such as clouds such as stratus.stratus.

Cloud SeedingCloud Seeding

Artificial seeding, such as Silver Iodide, and natural seeding, such as Artificial seeding, such as Silver Iodide, and natural seeding, such as cirriform ice crystals, are available to increase the number of cirriform ice crystals, are available to increase the number of condensation nuclei and encourage precipitation.condensation nuclei and encourage precipitation.

Forms of precipitation

• Rain

-drizzle (diameters smaller than 0.5mm)

-virga (rain evaporating prior to reaching the surface)

• Snow

-flurry (light snow from cumulus clouds)

-squall (intense snow shower)

-blizzard (heavy snow and winds > 30knots)

• Sleet (snow that partially melts and freezes again into ice)

• Freezing Rain (supercooled liquid water that reaches the ground

• Hail

Evaporating RainEvaporating Rain

Rain falling Rain falling into low into low humidity air humidity air below will below will cause the cause the drops to drops to decrease in decrease in size, possibly size, possibly evaporating evaporating into streaks of into streaks of dry air as in dry air as in this virga.this virga.

Snowflakes & SnowfallSnowflakes & Snowfall

Snowflakes are crystalline structures that can have plate, Snowflakes are crystalline structures that can have plate, column, dendrite, or needle forms.column, dendrite, or needle forms.

Air temperature and humidity determine crystal form, and Air temperature and humidity determine crystal form, and dendrite is the most common habit.dendrite is the most common habit.

Sleet & Freezing RainSleet & Freezing Rain

Environmental Environmental temperatures may temperatures may reveal a warm zone reveal a warm zone between two between two freezing layers.freezing layers.

Snow falling into Snow falling into the warm zone will the warm zone will melt and either melt and either a) fall as rain and a) fall as rain and refreeze on contact refreeze on contact with the ground, or with the ground, or b) refreeze and fall b) refreeze and fall as sleet.as sleet.

Sleet & Freezing RainSleet & Freezing Rain

Four vertical temperature profiles are shown to illustrate the phase change Four vertical temperature profiles are shown to illustrate the phase change that a snowflake may experience in its path toward earth's surface.that a snowflake may experience in its path toward earth's surface.

Rime & Freezing RainRime & Freezing Rain

Rime, a granular ice, Rime, a granular ice, accumulates when super cooled accumulates when super cooled fog droplets touch a frozen fog droplets touch a frozen surface.surface.

Freezing rain creates incredible Freezing rain creates incredible strain on branches and other strain on branches and other structures, resulting in costly structures, resulting in costly damages.damages.

Snow Grains & Snow PelletsSnow Grains & Snow Pellets

Flat and long snow grains fall as frozen drizzle, too small to bounce Flat and long snow grains fall as frozen drizzle, too small to bounce or shatter.or shatter.

Snow pellets are larger, and have a rounded layer of rimed ice that Snow pellets are larger, and have a rounded layer of rimed ice that creates air bubbles and a bounce.creates air bubbles and a bounce.

Hailstones & DamageHailstones & Damage

Updrafts in a towering cumulus cloud recirculate graupel Updrafts in a towering cumulus cloud recirculate graupel through an accretion and freezing process that produces large through an accretion and freezing process that produces large heavy hailstones.heavy hailstones.HailstreaksHailstreaksSuch storms cause regular property damage, but only 2 U.S. Such storms cause regular property damage, but only 2 U.S. deaths in the 20th Century.deaths in the 20th Century.

HailstreaksHailstreaks

Coffeyville HailstoneCoffeyville Hailstone

Regular and polarized light images of the 14 cm diameter Regular and polarized light images of the 14 cm diameter hailstone that fell in Kansas in 1970.hailstone that fell in Kansas in 1970.

Hailstorms cause severe damage to crops and other structures.Hailstorms cause severe damage to crops and other structures.Farmers name for it?Farmers name for it?

Aurora HailstoneAurora Hailstone

Previous hailstone record holder in the U.S.Previous hailstone record holder in the U.S.17.8 cm in diameter17.8 cm in diameterJune, 2003June, 2003

Vivian Hailstone

Largest hailstone on record in the U.S.Largest hailstone on record in the U.S.20.32 cm in diameter, 2 lbs in weight20.32 cm in diameter, 2 lbs in weightJuly, 2010July, 2010