Clouds, Stability, Adiabatic Processes, Precipitation

What is a cloud? Dense mass suspended water droplets, supercooled water droplets and/or ice particles on condensation / ice nuclei of solid matter (sea salt, soil particles, organic matter, pollutants, aerosols)

Clouds form When a parcel of air rises and cools to the dew point temperature. Then the water vapor in the air parcel condenses into water droplets or deposits into ice crystals

Why would a parcel of air rise?

If surrounding air is more dense / colder than the parcel, parcel will rise. warm Colder / more dense surrounding air THAT IS CALLED UNSTABLE

A parcel will rise until it gets to an altitude at which surrounding air is at same density / temperature

If surrounding air is less dense / warmer than the parcel, parcel will sink; or will not rise. Warmer / less dense air STABLE; NO CLOUDS cold

Therefore, stability is determined by temperature difference between parcel and surrounding air.

Temperature of surroundings decreases with altitude at AVERAGE environmental lapse rate: (3.5°F / 1000 ft; 6.4°C / 1000 m)

What about parcel? As parcel rises, it expands due to lower atmospheric pressure of surrounding air; as parcel expands, its temperature drops.

As a parcel of air descends, it contracts due to greater atmospheric pressure of surrounding air; as parcel contracts, temperature rises. .

QUIZ 5 1. What is this? Fog B. a cloud C. Frost D. dew

3. What is this? Fog B. a cloud C. Frost D. dew 4. What is this? Fog B. a cloud C. Frost D. dew

5. For the previous 4 questions, which of the following was required? A legal ID B. saturation C. Low relative humidity D. extreme heat 6. What do you call a dense mass of water droplets, supercooled water droplets and/or ice crystals in the troposphere? A cloud B. the frozen three C. Dunno D. aerosols

Yesterday’s heavy fog On radar

On surface analysis

On visible satellite image

On thermal IR

Water vapor

Long range forecast A vigorous upper-level trough moving from the Rockies to the central U.S. on Wed is expected to quickly deepen and spin up a significant low pressure system. The surface low is expected to move from the Central High Plains Wed morning to the Midwest on Thu and the Great Lakes by Fri, before quickly moving northeast into Quebec by Fri night/Sat. Confidence in the forecast for this system has improved significantly over the past 24-48 hours.

These changes in temperature: NOT due to heat exchange with surroundings, due to expanding and contracting ONLY!. called ADIABATIC (change in temperature with no gain or loss of heat)

Top & bottom elevations of clouds: Bottom: lifting condensation level: temperature of rising air reaches dew point Top: Cloud rises until cloud temperature reaches temperature of surroundings (becomes stable). Cloud thickness depends on thickness of unstable layer.

Lifting condensation level

General cloud forms: Stratiform: layer Cumuliform : globular layer of air forced to rise over denser layers Cumuliform : globular parcels of air forced to rise above denser surrounding air

Altitude classes: High Cloud (cirro-): ice crystal clouds > 7000 m (23,000 ft.) CIRRUS, CIRROSTRATUS, CIRROCUMULUS Middle Cloud (alto-) 2000 – 7000 m (6500 - 23,000 ft.) ALTOSTRATUS, ALTOCUMULUS Low Cloud (stratus) <2000 m (6500 ft) STRATUS, NIMBOSTRATUS, STRATOCUMULUS Vertically developed clouds (CUMULONIMBUS)

Mechanisms causing air parcels to rise: Spontaneous: Convergence Convection Forced : Orographic Uplift Frontal Lifting

GOES

Cold front

Warm front

PRECIPITATION (rain , snow, sleet, freezing rain, hail) Large parcel of air rises and cools Adiabatic cooling Cools to dew point Cloud droplets grow large enough to fall

Why do cloud droplets need to grow?

Processes by which cloud droplets/crystals grow : A. Bergeron Findeisen Process Mid- and High latitudes: “Cold Clouds” (tops colder than 0°C) Saturation vapor pressure is greater over water than ice It takes more water vapor to saturate air over water than over ice ice crystals grow at the expense of water droplets

Bergeron Findeisen process

Collision/Coalescence process (tops warmer than -15°C) Low latitudes: “Warm Clouds” (tops warmer than -15°C) Collision and coalescence: some big drops form; they fall faster than small drops and collect small drops

Precipitation Types

rain Precipitation as liquid water Height Melting level 0°C Temperature

virga

snow Snowflake is several aggregated crystals. Height 0°C Temperature

graupel “riming”: shell forms around crystal as it passes through tiny suspended water drops: forms “graupel” (ice pellets)

Sleet Frozen raindrops; raindrops that freeze as they descend. Height Melting level 0°C Temperature

Sleet hurts! and is loud.

Freezing rain Supercooled droplets hit a surface which is below freezing Height Melting level 0°C Temperature

Very destructive; Causes tree limbs and power lines to come down