Chapter 5 - Cloud Development & Precipitation

How rising air is caused

Air has to be warmer than the environment

Importance of clouds

Serve as predictors for processes happening in the atmosphere

Adiabatic Process

Unsaturated rising air expands and cools while unsaturated sinking air compresses and warms

Dry-adiabatic lapse rate

Dry parcel temperature will cool -10C per km upwards

Reaching the dewpoint

Reaching saturation

Moist adiabatic lapse rate

Moist parcel will cool -6C per km upwards

What parcels release more of as they go up

Latent heat

Environmental lapse rate

Rate that ambient air decreases with altitude at a specific time

Unstable parcel

If it is forced to rise and is warmer than environment, it will continue to rise

Stable parcel

If it is forced to rise and is cooler than environment, it will sink back down

Unstable parcel temperature

Warmer than air temperature

Stable parcel temperature

Cooler than air temperature

Absolutely stable atmosphere

Parcel is cooling faster than the environment

Environment T > parcel T

Conditionally unstable atmosphere

Dry parcel cooling faster than environment, moist parcel cooling slower than environment

Absolutely unstable atmosphere

Parcel cooling slower than environment (stays warmer)

Environment T < parcel T

Saturated parcels

Rises in conditionally unstable atmosphere

Unsaturated parcels

Sinks back to their original position in conditionally unstable atmosphere

Why cumulus clouds grow very tall

Only saturated parcels continue to rise

Hot / warmer air below cold air is unstable

Changes stability in atmosphere

Warming at low levels / cooling aloft

Changes stability in atmosphere

Mixing

Changes stability in atmosphere

Causes air to rise and sink

Cold dense air moving down (cold front)

Convection

Causes air to rise and sink

Upslope fog (topography)

Causes air to rise and sink

Low pressure

Causes air to rise and sink

Mechanisms for cloud development

Surface heating + convection, uplift along topography, widespread ascent, uplift along weather fronts

Lifting Condensation Level

Level at which condensation occurs - represents the cloud base

Lifting = cooling

How parcels of air become saturated

Why lower dewpoint often means higher cloud base

Air rises and cools

Level of Free Convection (LFC)

Level at which air will continue to rise without forcing

Equilibrium level

Level above LFC at which air will stop rising

Orographic uplift

Forced lifting along a topographic barrier

Orographic clouds

Formed through orographic uplift when lifting produces cooling and air is humid

Rain shadow

Precipitation is noticeably low and the air is often drier

Lenticular cloud

Lens shaped wave clouds

Rotor cloud

Visible rising swirling air

Airflow over a mountain

If saturation reached, heat is released in condensation

Mountain wave clouds

Flying saucer-like clouds because waves set up airflow

Cloud condensation nuclei

Need hygroscopic cloud condensation nuclei (CCN)

Large drops fall [faster/slower] than smaller drops

Faster

Collision

Larger drops fall faster than smaller ones and collide into each other

Coalescence

Larger and smaller drops more likely to stick together

When a droplet falls

Droplets run into more droplets and get bigger, producing rain

Liquid Water Content of a Cloud

Most important factor in producing raindrops

Condition in which collision-coalescence only works

Cloud is warmer than freezing

-40C

Temperature needed for “spontaneous freezing” for ice crystals

Supercooled water

Water in liquid form at temperatures below 0C

Ice crystals grow at the expense of…

Supercooled water

Molecules more easily exit

Water

Saturation vapor pressure over water

Greater than vapor pressure over ice

The Bergeron Process

It takes more molecules to saturate the air above water than above ice - ice crystals grow at the expense of liquid cloud droplets

Tor Bergeron

One of the leaders of “Bergen school” of meteorology, discovered ice crystals grow to form precipitation from different saturation vapor pressures

Accretion

Ice particle increases in size by colliding with supercooled water droplets

Aggregation

Ice crystals colliding with other ice crystals and producing snowflakes

Warm stratus

Weak collision-coalescence

Warm cumulus

Collision-coalescence process more efficient and can produce warm rain

Cold layered clouds (nimbo and altostratus)

Less liquid water available - ice crystals grow more by aggregation

Cold convective clouds

Starts quickly through Bergeron process or collision-coalescence; typically starts as ice

Cloud seeing

Inject cloud with small particles that act as condensation or ice nuclei to trigger precipitation process

Cannot initiate a thunderstorm

Virga

Rain that does not reach the surface; drops evaporate in low humidity

Layered clouds with less intense updrafts

Rain falls steadily and uniformly

Snow

Temperature and water vapor content determine type of snowflake that forms - need ice nuclei

Graupel

Opaque white grains that are brittle and crunchy

Sleet

Snowflakes melt as they fall then freeze again before reaching ground

Freezing rain

Snowflakes melt as they fall but refreeze on contact when they hit cold surfaces

Rime

Supercooled liquid water freezing on something

Accretion

Ice running into supercooled liquid water

Hail

Forms in deep cumulonimbus clouds

Aircraft icing

Precipitation hazard; increased weight as ice forms on the body of the plane

Floods

Precipitation hazards; waters rise rapidly with little / no warning

Rain gauge

Used as a water measurement

Remote rain sensing

Using radio detection and ranging