1. Clouds are made up of:

A.Liquid water dropletsB.Ice CrystalsC.Water VaporD.A combination of liquid water, ice, and water vapor

2. The liquid water droplets and ice crystals which make up clouds are denser than the surrounding air molecules. Why don’t they fall to the ground?

A.They are attracted to each other by electrical forces and water tension.B.They are far enough apart that the density is not actually greaterC.Upward movement of air keeps them aloftD.It is too warm at ground level

5. Why does upward movement of air create clouds (and precipitation)?

A.Air cools as it risesB.Relative humidity decreases as air coolsC.Cooling of saturated air results in condensation of water vaporD.A, and CE.A, B, and C

3. What “lifting mechanism” occurs when rising air remains warmer (less dense) than the surrounding air?

A.ConvectionB.Orographic LiftC.ConvergenceD.Weather Fronts

4. Air converging towards an area of _______________ is forced upward, often creating clouds and precipitation.

A.Warm temperatureB.Low pressureC.High pressureD.Cold temperature

Precipitation Processes: Growth of Cloud DropsPrecipitation Processes: Growth of Cloud Drops

•A raindrop is 100 times larger than a cloud droplet.•A cloud droplet is 100 times larger than a condensation nucleus.•1,000,000 cloud droplets = 1 raindrop

Two Important Precipitation Processes

• Collision and Coalescence (warm cloud) Process– Strictly liquid cloud droplets

• Ice-Crystal (Bergeron) Process– ice crystals and liquid cloud droplets

both must co-exist at temperature below freezing

Collision & Coalescence ProcessCollision & Coalescence Process

Warm Cloud Raindrop Formation ProcessesWarm Cloud Raindrop Formation Processes

•Collision and coalescence operates in warm clouds to produce rain, and is Collision and coalescence operates in warm clouds to produce rain, and is affected by the:affected by the:•1) clouds liquid water content (LWC)1) clouds liquid water content (LWC)•2) droplet sizes 2) droplet sizes •3) cloud thickness 3) cloud thickness •4) updrafts4) updrafts•5) drop electrical charges.5) drop electrical charges.

Ice Crystal (Bergeron) ProcessIce Crystal (Bergeron) Process““important precipitation process in the middle and high latitudesimportant precipitation process in the middle and high latitudes””

•Cloud droplets are still composed Cloud droplets are still composed of liquid water below the freezing of liquid water below the freezing level (level (supercooledsupercooled). ).

•The pure water in a small amount The pure water in a small amount has a lower freezing temperature. has a lower freezing temperature. (small cloud droplets)(small cloud droplets)

•Cold clouds may drop below -40° C Cold clouds may drop below -40° C before small droplets freeze into ice before small droplets freeze into ice embryos that can serve as embryos that can serve as condensation nuclei (condensation nuclei (ice nucleiice nuclei).).

COLD CLOUDS

WARM CLOUDS

more water droplets than ice crystals

Molecules from Water to IceMolecules from Water to Ice

Ice crystals have lower saturation vapor pressures than liquid droplets, creating a gradient of high to low water molecules from liquid to ice that encourages ice growth.

Ice Particle ChangesIce Particle Changes

Accretion or Riming

Graupel

Aggregation

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

•Falling icy matter is called graupel (snow pellet), and aggregation describes the joining of two ice Falling icy matter is called graupel (snow pellet), and aggregation describes the joining of two ice crystals into snowflakes.crystals into snowflakes.

•If the snowflake melts while falling, it continues down as a raindrop. If the snowflake melts while falling, it continues down as a raindrop.

•Much of the rain, even in the summer, in the middle and northern latitude falls as snow initially.Much of the rain, even in the summer, in the middle and northern latitude falls as snow initially.

Precipitation Types

• Rain

• Snow

• Sleet and Freezing Rain

• Snow Grains and Snow Pellets

• Hail

Rain

• A falling drop must have a diameter 0.5 mm to be considered as rain.

• Uniform drops with diameter < 0.5 mm are called drizzle.

• Showers are considered intermittent precipitation, usually short in duration but often heavy.

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.

Snow

• Can fall as much as 300 m (1000 ft) below the freezing level before melting completely.

• In a drier environment, snowflakes can reach the ground even if the surface layer temperature is above freezing due to evaporative cooling effect.

• It is never “too cold” to snow.

Average Annual Snowfall

Sleet & Freezing RainSleet & Freezing Rain

Surface

1000m

2000m

3000m

Surface

1000m

2000m

3000m

Surface

1000m

2000m

3000m

Surface

1000m

2000m

3000mSNOW

(Below Freezing)

SNOW

(Below Freezing)

SNOW

(Below Freezing)

SNOW

(Below Freezing)

RAIN SNOW SLEET FREEZING RAIN

For each case, determine whether the air will be above or below freezing at each level in order that precipitation of the selected type will occur. There may be more than 1 possible answer. Try this based on how each precipitation type was described in class, rather than using your book.

* * * * * ** * * * * * * * * * * * * * * * * *

Hailstones & DamageHailstones & Damage

Radar RainfallRadar Rainfall

•Doppler radar uses microwave transmission and reception to Doppler radar uses microwave transmission and reception to measure backscatter intensity, or reflectivity for large geographic measure backscatter intensity, or reflectivity for large geographic areas.areas.

•This signal is converted into maps of precipitation intensity, This signal is converted into maps of precipitation intensity, while phase shift data provide information on storm movement.while phase shift data provide information on storm movement.