Phys Geo 10/18: Air parcels, Pressure, Adiabatic Processes, Condensation, Air Masses

The dry adiabatic rate (DAR) is the rate at which

"dry" air cools by expansion (if ascending) or heats by compression (if descending). The term dry is used when air is less than saturated (relative humidity is less than 100%). The DAR is 10 C0/1000 m (5.5 F0/1000 ft).

DAR Lapse Rate =

10.0 C ° /1000m.

• MAR Lapse Rate

= 6 C ° /1000m.

how to make a cloud

Adiabatic cooling by vertical lift • Air parcel cools to the dew‐point temperature. • Air parcel becomes saturated. • Condensation occurs.

Lifting Condensation Level =

Altitude at which dewpoint temperature of parcel is reached and point at which a cloud forms decreases at the DAR until the lifting condensation level is reached when it reaches this, it will change the lapse rate from the DAR to the MAR so it will cool less quickly bc the energy of condensation will be added back into the parcel

At the same time, the dew point (i.e. condensation temperature) itself will decrease (typically at a rate of 2° C per 1000m) from an intial value of 10° C

DP(x) = 10 C ° - (2°C/1000m) x

cP

N hemi only cold, dry, stable, high pressure

Given an unsaturated air parcel at 2000m that is 20°C, the equation for the decrease in temperature (due to dry adiabatic processes) with altitude above 2000m (x) in meters is:

T(x or Z as in prev) = 20 C ° - (10 C°/1000m) (x or Z)

Temperature at Altitude Z =

Temperature at surface ‐ Lapse Rate * Z Z=height above the ground/change in altitude

in order for condensation to occur at the dew pt, you need to have

a certain amount of foreign particles larger than 10−4mm, called cloud condensation nuclei (CCN) and to which water molecules are attracted, must be present. Continental air masses average 10 billion cloud‐condensation nuclei per cubic meter. • Natural sources of CCN: meteoric dust, windblown clay and silt, volcanic material, smoke from forest fires, sea salt, pollen • Anthropogenic CCN sources: combusting products (sulfur and nitrogen compounds). • The concentration of Anthropogenic CCN is much higher than that of natural sources of CCN. • Cloud seeding: artificial introduction of nuclei

a parcel of air is

a volume (on the order of 300 m, or 1000 ft, in diameter) that is homogenous in temperature and humidity. The temperature of the volume of air determines the density of the air parcel. Warm air has a lower density in a given volume of air; cold air has a higher density.

environmental lapse rate

applies to the air around the parcel

adiabatic ascending process,

as the air parcel rises, it will expand and the temperature inside the air parcel will decrease

Normal lapse rate:

average decrease in temperature with increasing altitude, a value of 6.4°C/1000m.

mP

cold, humid, unstable all yr

The reason that the MAR is much less than DAR, is that

condensation releases energy and heats surrounding air

specific humidity

determines the temp at which condensation occurs the lower the specific humidity, the lower the temp can be before condensation occurs if you are close to the max spec humidity, as the parcel cools, the temp of condensation is going to occur sooner (at a higher temp)

perturbation causes

difference in albedo orographic lifting (hills)

in order for rain droplets to fall, they have to overcome the air resistance, which can happen in 2 ways:

droplets adding together (drop collision) or ice crystals

set air parcel temp equation and dew point equation equal to each other

for hw slide Atmospheric Temperatures and Stability

diabatic

heat transfered into a parcel from air by a source, maybe the sun

stability depends on the temp of the parcel

if it's warmer it will tend to ascend, if it's colder it will tend to descend An ascending (rising) parcel of air cools by expansion, responding to the reduced air pressure at higher altitudes. A descending (falling) parcel heats by compression. Temperature changes in ascending and descending air parcels are adiabatic, meaning they occur as a result of expansion or compression, without any significant heat exchange between the surrounding environment and the vertically moving parcel of air.

An air parcel is unstable if

it continues to rise until it reaches an altitude where the surrounding air has a density and temperature similar to its own.

An air parcel is stable if

it resists displacement upward, or when disturbed, tends to return to its starting place.

as the unsaturated air parcel rises and its pressure decreases,

its dew point also decreases (at the dew point lapse rate) as a result of decreasing air pressure.

adiabatic

looking at differences in volume and pressure due to mechanical forces

air masses are categorized by

moisture content: m=maritime, wetter c=continental, dryer latitude: A=arctic (cold) P=polar (a bit less cold) T=tropical (warm) E=equatorial (hot) AA=antarctic (cold)

ELR

observed rate of cooling outside the air parcel

Stability

refers to the tendency of an air parcel (a body of air with a specific temperature and humidity) either to remain in place or to change vertical position by ascending or descending.

air mass

regional volume of air homo in humidity, stability, cloud coverage can extend through half of the troposphere each area of earth's surface imparts its characteristics to surrounding air, creating regional air masses air masses move in response to pressure gradients and upper level wind patterns, and their characteristics change to reflect underlying regions (air mass modification) interactions of air masses producefronts and weather patterns

meteorologist

scientific study of the atm

weather

short term condition of the atm

when ELR is less than MAR

stable air parcels occur Lower Environmental Lapse Rate = slower decrease in temperature as a function of altitude = air around the air parcel cools less quickly than the air parcel itself. This removes the contrast between air parcel and environment and stops the upward motion of the parcel.

medit. climate

summer wet

weather elements

temp air pressure humidity wind speed and direction seasonal factors: sun angle, ect

• Environmental lapse rate is

the actual lapse rate at a particular time and place. It can vary by several degrees per thousand meters.

• In an adiabatic descending process,

the air parcel will be compressed and the temperature inside the air parcel will increase.

The moist adiabatic rate (MAR) is

the average rate at which moist (saturated) air cools by expansion on ascent or warms by compression on descent. The average MAR is 6 C0/ 1000 m (3.3 F0/1000 ft). This is roughly 4 C0 (2 F°) less than the dry rate. The MAR, however, varies with moisture content and temperature and can range from 4 to 10 C0 per 1000 m (2 to 5.5 F0 per 1000 ft).

• If the relative humidity of the parcel is less than 100%

the cooling will occur at the dry adiabatic rate which is a constant ‐10 C°/1000m

Unstable air parcels occur when

the environmental lapse rate is greater than the dry adiabatic lapse rate Greater Environmental Lapse Rate = greater decrease in temperature as a function of altitude = air around the air parcel cools more quickly than the air parcel itself. This sustains the contrast between air parcel and environment and continues the upward motion of the parcel.

Conditional unstable parcels occur when

the environmental lapse rate is greater than the moist adiabatic lapse rate (the air around the parcel is cooling faster than the parcel) and less than the dry adiabatic lapse rate, e.g., the environmental lapse rate = 7°C/1000m. an air parcel will end up being warmer than the air around it, so the air parcel will continue to rise

cA

very cold, very dry, stable

mT

warm, humid, stable to conditionally unstable comes up to the SW of north amer

mT

warm, humid, unstable comes up to SE of north amer

To find where condensation occurs, (which is where parcel temp and dew pt temp are equal and condensation occurs)

we need to account for both the dry adiabatic cooling of the parcel, and the decrease in the dew point temperature from the initial dew point.

to the E of the continental divide

weather comes from the gulf coast

to the W of the continental divide

weather comes from the pacific ocean