weather and climate ch.3

Stable air parcel

Resists upward displacement or when disturbed tends to return to starting place

Stable air tends to ___ movement

Resist

Unstable air tends to ___ and ___ movement

Rise, encourage

Warm air has ___ density m

Unstable air parcel

Continues to rise until it reaches an altitude where it can no longer rise

Parcel

What does atmospheric stability depend on?

Density of the air

Moist air is

Less dense than dry air

Adiabatic process

As an air parcel rises it expands and cools, as it sinks it compresses and heats up

*no heat exchange with the surrounding environment

What causes an air parcel to rise?

Convection (intense surface heating), convergence, frontal, orographic uplift

Convergent lift

Results when air flows toward an area of low pressure

Convectional lift

When air is stimulated by local surface heating and rises

Orographic uplift

Occurs when air is forced over a barrier like a mountain range

Windward side

Side facing the wind that gets the rain

Leeward side

Side facing away from wind, very dry rain-shadowed area

Rain shadow

Leeward side of the mountain that rceives anomalously low precipitation because of its motion relative to the mountains

Frontal lifting

Occurs as air is displaced upward along the leading edges of contrasting airmasses

Lapse rate

Rate at which atmospheric temperature decreases with altitude

Environmental lapse rate (ELR)

Actual lapse rate at a particular time and place

Dry adiabatic lapse rate (DALR)

Rate at which the temperature of a dry (unsaturated) air parcel changes as it rises or sinks

As a parcel rises…

Volume expands and temperature falls

Moist adiabatic lapse rate (MLAR)

Rate at which temperature of a moist (saturated) air parcel changes as it rises or sinks

Why is the MALR slower than the DALR?

Latent heat release; condensation releases heat and partially offsets cooling. This varies because of the amounts of condensation.

More condensation=

More Latent heat release (LHR) and smaller MALR

Lifting condensation level (LCL)

Altitude at which a dry air parcel cools to its dew point temperature, achieve saturation, and undergo condensation

Relationship of the DALR and MALR to the ELR At any gien time and place determines what?

The stability of the atmosphere over an area.

Temperature relationships in the lower atmosphere produce three conditions

Unstable, conditionally unstable, and stable

Absolute stability (DALR>ELR)

•Air parcel has a LOWER temperature (COLDER) than the surrounding environment

•Air parcel COOLS FASTER than surrounding environment

•Air parcel tends to resist lifting unless forced

•If air continues to be forced, layered clouds form

•If forcing stops, air parcel SINKS

Absolutely unstable (DALR<ELR)

•ELR is GREATER than the DALR

•Air parcel COOLS SLOWER than surrounding environment

•AIR PARCEL WARMER THAN ENVIRONMENT

•Air continues to rise without forcing,

•Clouds with vertical development form

•NEEDED FOR THUNDERSTORMS

Conditionally unstable (DAR>ELR>MAR)

•LOWER PART OF THE ATMOSPHERE IS STABLE

•UPPER PART OF THE ATMOSPHERE IS UNSTABLE

•Air parcel goes from being COOLER than environment to WARMER

•Clouds form when air is unstable

•ELR in between DALR & MALR

•Atmosphere is neither unstable nor stable

•If an air parcel is unsaturated, it will resist upward movement unless forced

•If it becomes saturated at the LCL, it will then be unstable and rise

Global winds

Trade winds, westerlies, Rossby waves

Synoptic scale winds

Monsoon, chinook, mid-latitude cyclones

Mesoscale winds

Thunderstorms, land-sea breezes, mountain/valley breezes.

Microscale winds

Turbulent eddies, dust devils

Air mass

A distinctive body of air with specific moisture and temperature characteristics. Greater than 1000km

Air masses are characterized by

Relatively horizontal homogenous temp and moisture

Atmosphere gains water from

Evaporation off the surface

Source regions are defined by

Surface moisture and temperature characteristics

Ideal source region must

Be extensive and physically uniform, also have a tendency tp stagnate over a region

Source regions typically form

In association with anticyclones and high pressure systems

Classify air masses by

Surface type (continent or ocean (moisture)) and Latitude (temperature)

Continental polar (cP)

Cold stable air, clear skies, high pressure, anticyclonic winds. Only in Northern Hemisphere, often in winter and cold-weather.

Maritime polar (mP)

Cool moist unstable air, cyclonic wind flows. Sit over oceans at high latitudes.

Maritime tropical (mT)

Warm humid, stable to unstable air. Form over tropical oceans