Atmospheric Moisture - Stability, Clouds, Precipitation, Lifting

Buoyancy

The tendency of any object to rise or sink in a fluid under the influence of gravity

If object is less dense than surrounding fluid, the object will…

float/rise

If object is denser than surrounding fluid, the object will…

sink

If object is same density as fluid, the object will…

neither rise nor sink

Air seeks equilibrium level, meaning…

It moves up or down until it reaches an altitude at which the surrounding air is of equal density

Stable Air

• Air that resists uplift

  • Occurs when cold air is beneath warm air (temperature inversion)

• Non-buoyant, unless forced to rise

• No adiabatic cooling, cloud formation, or precipitation unless forced uplift

  • Clouds = flat (stratiform) but can be thin or thick

  • Precipitation = drizzly or light steady rain

Unstable air

• Buoyant (air rises without outside force) OR it continues to rise after an external force has ended

• occurs when a mass of air is warmer than surrounding air

• Unstable air rises until it reaches its equilibrium level (the altitude where surrounding air has similar temp and density)

• Adiabatic cooling, cloud formation

  • Clouds = puffy and have vertical development (cumuliform)

  • Precipitation = showery or downpour

Conditional instability

If stable air is forced to rise above the lifting condensation level, the release of latent heat during condensation may warm the air to make it unstable

What is the dry adiabatic rate of rising cool air

10°C per 1000 meters

What are the characteristics of rising air if the environmental lapse rate of surrounding air is less than the dry adiabatic rate of the rising rise?

The rising air is cooler than the surrounding air & more stable

What are the characteristics of rising air if the environmental lapse rate of surrounding air is greater than the dry adiabatic rate of the rising rise?

The rising air is warmer than the surrounding air & more unstable

Which clouds define what air type (unstable, stable, immobile)

• Unstable air = distinct updrafts, vertical air currents & vertical clouds

  • Cumulus (puffy) clouds

  • Cumulonimbus (thunderstorm) cloud

• Stable air = horizontal clouds (stratiform)

• Immobile air = cloudless sky

Clouds

Collections of tiny droplets of liquid water or tiny crystals of ice

What are classifying clouds based on?

Form and altitude

What are the 3 cloud forms? Describe them

1. Cirriform clouds

• Thin & high

• Composed of ice crystals, not water droplets

2. Stratiform clouds

• Grayish sheets that cover most of the sky

• Rarely broken up into individual units

3. Cumuliform clouds

• Massive & rounded & flat base

• Limited horizontal extent, but billows upward

What are the 3 main subtypes (10 of them) of cloud forms?

1. Cirrus

2. Stratus

3. Cumulus

Clouds that produce precipitation have “nimb” in their name (ex: cumulonimbus)

10 cloud types divided into four families based on ___? What are the four families?

• based on altitude

1. High clouds

2. Middle clouds

3. Low clouds

4. Clouds of vertical development

High clouds

• Height: above 6km

• Characteristics: Thin, white, composed of ice crystals (bc small amount of water vapor + low temp)

• Types: Cirrus, cirrocumulus, cirrostratus

• Bring storms

Middle clouds

• Height: Between 2-6km

• Characteristics: Puffy, stratiform or cumuliform, composed of liquid water

• Types: Altocumulus, altostratus

• Changing weather

Low clouds

• Height: Below 2km

• Characteristics: Generally overcast, sometimes individual clouds, widespread

• Types: Stratus, stratocumulus, nimbostratus

• Somber skies & drizzly rain

Clouds of vertical development

• Height: Low base → 60,000 ft

• Characteristics: Restricted horizontal spread

• Types: Cumulus clouds (fair weather) & cumulonimbus clouds (thunderstorm)

• Indicate active vertical movements in the air

Fog + how it forms

• a cloud on the ground (but develops differently than cloud)

• forms when air at Earth’s surface cools to below its dew point temperature OR when enough water vapor is added to the air to saturate it

  • On the other hand, clouds develop as a result of adiabatic cooling in rising air

4 types of fog

1. Radiation fog

2. Advection fog

3. Upslope fog (orographic fog)

4. Evaporation fog (steam fog)

Radiation fog

• When the ground radiates away heat (usually at night)

• Air closest to ground cools as heat flows from it to the cool ground → fog condenses in cooled air at dew point

Advection fog

• When warm, moist air moves horizontally over a cold surface (ex: snow or ocean)

• Commonly comes from air moving from sea → land

Upslope fog (orographic fog)

Created by adiabatic cooling, when humid air climbs a topographic slope

Evaporation fog (steam fog)

• When water vapor is added to cold air that is already near saturation

• When cold air flows over warm body of water

Areas of heavy, radiation, and minimal fog in America

• Heavy fog = coastal

• Radiation fog = western-mountain & Appalachian

• Minimal fog = Southwest, Mexico, Great Plains

Dew

• Nighttime radiation cools objects → adjacent air is cooled by conduction

• If air is cooled enough to reach saturation, droplets collect on cold surface of object

Precipitation

droplets of liquid or solid

What are the 2 mechanisms to produce precipitation

• Collision / coalescence

  • Produced in warm clouds (cloud temp > 0℃)

  • Tiny droplets collide and merge to form larger droplets

  • Not all collisions result in coalescence (combining)

  • Location: tropics, some middle latitudes

• Ice-crystal formation

  • Ice crystals grow by absorbing water vapor → falls when heavy enough as snowflakes or melt or raindrops

  • Occurs in cool/cold clouds (clouds that are high enough to have temperatures below freezing point)

What are the forms of precipitation? (6)

1. Rain

2. Snow

3. Sleet

4. Glaze

5. Hail

6. Virga

Rain

• Drops of liquid water (most common & widespread)

• Occurs when condensation & precipitation in rising air has temperature above freezing

• OR occurs when ice crystals melt as it falls through warmer air

• Showers: brief period of time, large drops

• Drizzle: some time, small drops

Snow

• Solid precipitation in form of ice crystals, small pellets, or flakes

• Forms when water vapor is converted to ice through deposition (no liquid stage)

Sleet

• Small raindrops that freeze during fall and reach ground as small pellets of ice

• Mixture of rain and snow

Glaze

Freezing rain – rain that turns to ice when it collides with a solid object

Hail

• Small pellets or larger lumps of ice

• Formed in strongly unstable cumulonimbus clouds → result of instability and strong updrafts & downdrafts

  • Lower part of a cloud must be warm, upper part must be cold

Virga

• Streaks of rain that disappear before hitting the ground

• Occurs when the relative humidity of air below a cloud is low → rain evaporates before reaching ground

What are the 4 types of atmospheric lifting?

1. Convective lifting

2. Orographic lifting

3. Frontal lifting

4. Convergent lifting

Convective lifting

• Unequal heating of surface areas causes spontaneous lifting

  • As warm air rises, it expands and cools adiabatically to the dew point temperature

• Convective precipitation: showery, large raindrops, short duration

• Location & Season: warm parts of the world & warm seasons

Orographic lifting

• When wind encounters a topographic barrier, the air is forced to travel upslope

  • Triggers instability

  • Can produce orographic precipitation if rising air is cooled to dew point

• Rain shadows: the downward slope; the area as far as drying influence extends

  • Air rises to mountain top → descends over mountain → adiabatic cooling replaced by adiabatic warming → condensation and precipitation stop (dry area)

• Location & Season: any latitude, any season, any time of day

Frontal lifting

• When warmer air mass rises over cooler air mass

• When unlike air masses meet, they DO NOT MIX. They form a front (a zone of discontinuity)

• Results in frontal precipitation as warm air rises & cools adiabatically to dew point

• Location: Midlatitudes – meeting ground for cold polar air and warm tropical air

  • Less significant in highest and lowest latitudes where air masses tend to be like one another

Convergent Lifting

• When air converges → general uplift (less common)

• Enhances instability, produces showery convergent precipitation

• Associated with cyclonic storm systems (tropical disturbances as hurricanes and easterly waves)

• Location: Intertropical Convergence Zone (ITCZ)

Regions of high annual precipitation

1. ITCZ and Trade-Wind Uplift region

2. Tropical Monsoon regions

3. Coastal areas in Westerlies

Regions of low annual precipitation

1. Areas of subtropical highs

2. Interiors of continents + rain shadows

3. High latitude regions