Clouds and Precipitation

Question 1

Three Basic Cloud Types

Answer 1

1. Stratus
2. Cumulus
3. Nimbus

Question 2

Stratus Clouds

Answer 2

wide flat layer, stable ELR, no vertical motion, vertically confined

Question 3

Cumulus

Answer 3

puffy/piles of clouds, unstable ELR, vertical development, horizontally confined

Question 4

Nimbus

Answer 4

precipitating cloud, can also be stratus or cumulus

Question 5

Cirrus Clouds

Answer 5

-high location above 19,000 ft
-made up of ice crystals
-low water content
-typically not opaque, appear wispy

Question 6

Middle (Alto) Clouds

Answer 6

-bases between 6,000 and 19,000 ft
-mainly liquid droplets

Question 7

Low Clouds

Answer 7

-below 6,000 ft
-mostly made of liquid droplets
-stratus, stratocumulus, nimbostratus

Question 8

What are cumulus cloud associated with? Fair weather? Cumulonimbus?

Answer 8

-cumulus clouds can have violent updrafts, heavy precipitation, and large temperature differences
-arise in warm, humid, and unstable air
-characteristic anvil shape
-fair weather cumulus clouds don’t precipitate
-cumulonimbus associated with thunderstorms

Question 9

Recipe for Precipitation

Answer 9

1. Unsaturated air rises and cools by adiabatic expansion, reaching saturation (you need rising air and an unstable environment)
2. Presence of cloud condensation nuclei (CCN such as aerosols) allow for condensation, leading to formation of tiny droplets
3. Droplets grow by additional condensation

Question 10

Terminal Velocity definition

Answer 10

the velocity at which a rain drop falls

Question 11

What force does a droplet experience as it accelerates downward?

Answer 11

it experiences frictional drag

Question 12

When is terminal velocity reached?

Answer 12

when the drag force balances the gravitational force and the droplet falls at a constant speed

-the bigger the radius of the droplet, the larger the terminal velocity

Question 13

How do droplets begin growing?

Answer 13

droplets begin to grow by condensation onto smaller droplets; as the droplet gets larger, condensation stops because it equals evaporation

Question 14

How do warm clouds grow?

Answer 14

-warm clouds (mainly in the tropics and during the warm season) grow from collision-coalescence

Question 15

Collision-Coalescence

Answer 15

collector droplets, which have higher terminal velocities, grow as they run into small droplets as they fall

Question 16

Three layers in cold/cool clouds

Answer 16

-lowest layer water droplets (liquid and water vapor)
-middle is a mix of ice and supercooled droplets (enter a region below 0 C but they take time to freeze)
-all three phases exist in the middle layer
-top of the cloud is glaciated (made of ice), not much growth because little water vapor at that height

Question 17

Bergeron Process

Answer 17

the process by which droplets and ice crystals grow to precipitation size in midlatitude clouds

growth of ice crystals by deposition of water vapor from supercooled droplets
-saturation vapor pressure of ice is less than that of supercooled water

Question 18

Riming

Answer 18

when ice crystals fall through a cloud and collide with supercooled droplets, the liquid droplets freeze onto the ice crystals

Question 19

Aggregation

Answer 19

the joining of two ice crystals to form a single, larger one

-occurs more easily when ice crystals have a thin coating of liquid water to make them more adhesive

Question 20

Snow crystals

Answer 20

single crystals
-shape of ice crystals depend on temperature and degree of supersaturation

Question 21

Snowflakes in warm vs cold clouds

Answer 21

snowflakes: aggregates of snow crystals
warm clouds: snowflakes mostly form by riming and form dense wet snowpack (good for snowball fight)
cold clouds: snowflakes mainly formed by aggregation and forms a less dense, powdery snowpack (good for skiing)

Question 22

Lake Effect Snow

Answer 22

strong creation of snowfall that occurs downwind of Great Lakes and other large bodies of water

-occurs when lake warms and evaporates moisture into the lower atmosphere, causing the air to become unstable – air that passes over lakes thus has the three mechanisms for precipitation: unstable air, sufficient water vapor, and uplift caused by convergence

Question 23

Where are warm clouds mainly located? Cool clouds?

Answer 23

warm clouds predominate in the tropics
cool clouds predominate in mid-latitudes

Question 24

How does rain form in cool clouds?

Answer 24

precipitation starts out as snow then melts into rain when it falls through the warmed layer
-all types of precipitation can originate from a cold cloud

Question 25

Graupel

Answer 25

rimed ice crystals that have a spongy texture and white appearance
results from in cloud processes

Question 26

Hail

Answer 26

concentric layers of ice built around graupel
growth continues as long as the ice crystal is kept in the cloud
updraft caused by unstable air must be very strong to keep hail lifted
-forms from cumulonimbus clouds

Question 27

Freezing Rain

Answer 27

snow melts, hilts cold air and becomes supercooled then freezes on the ground
occurs if the layer of below freezing temperatures is relatively shallow

Question 28

Sleet

Answer 28

raindrops that have fallen through a cold layer and frozen before they reach the ground
-requires an inversion

Question 29

Cirrostratus clouds

Answer 29

-high clouds
-reduce solar radiation reaching the surface
-often found above warm fronts
-create a halo effect around the sun or moon

Question 30

Altostratus clouds

Answer 30

-middle cloud
-scatter incoming sunlight back to space so absence of shadows
-sun or moon appears as a bright spot behind the clouds

Question 31

When do raindrops fall?

Answer 31

when they become large enough that gravity overcomes the effect of updraft

Question 32

Why do very large collector drops have a lower percentage of collisions?

Answer 32

as it falls it compresses air in its path a creates a small gust of wind that pushes smaller droplets out of its way

Question 33

Cold cloud

Answer 33

clouds that have a temp below 0C throughout and consist entirely of ice crystals, supercooled droplets, or a mixture of the two

Question 34

Two ways that hail grows

Answer 34

1. Dry growth regime: supercooled droplets freeze onto ice crystals (opaque white appearance because trapped air bubbles)
2. Wet growth regime: water remains as liquid for longer, allowing air bubbles to be released (giving it a clear appearance)