Lesson 15Adiabatic Processes

Hess, McKnight’s Physical Geography, 10 ed.

138 – 139 pp.

Adiabatic ProcessesRecall prior to spring break that

as the temperature of an air parcel decreases, its relative humidity increases.

Once a parcel has cooled to the dew point, it becomes saturated and condensation occurs.

Adiabatic cooling is one of the most common ways clouds form.

Adiabatic CoolingClouds are the result of

condensationThis condensation occurs when

an air parcel’s temperature reaches the dew point

Air parcels are free to flow horizontally or vertically through the atmosphere◦Most commonly, clouds are formed

with vertical movement, namely rising air

Adiabatic Cooling, cont.As an air parcel rises, it becomes

less-pressurizedWith less pressure exerted on the

parcel, it therefore expandsThis expansion causes cooling and increases RH

◦Adiabatic cooling

Adiabatic HeatingConversely, as a parcel of air

descends it compresses due to higher pressure

This is a warming process, therefore the air warms

Dry Adiabatic RateConsider an air parcel that is

unsaturated◦RH < 100%

If this air parcel rises, it will cool at what is known as the dry adiabatic lapse rate (DALR)◦10°C per 1000 meters◦5°F per 1000 feet

Dry Adiabatic Rate, cont.As this air parcel rises its relative

humidity increases (because it’s expanding)

Eventually the air temperature cools and equals the dew point temperature◦ At this point the RH=100%

The elevation that this occurs at is known as the lifted condensation level (LCL)◦ At this point, condensation occurs and cloud

forms

Moist Adiabatic RateAt this point, the air parcel is

saturated and the RH=100%If the parcel continues to rise, it

will cool even furtherHowever, because it is saturated

(i.e. 100% RH) it will cool slower (MALR)◦6°C per 1000 meters◦3.3°F per 1000 feet

This is the result of latent heat which is released once condensation begins

Heat Transfer TheoryEvaporation is a cooling process

because liquid water is converted into a gas, which takes heat energy from the environment.

Final Thoughts…Dry Adiabatic Lapse Rate (DALR):

◦10°C per 1000 meters◦10°F per 1000 feet

Moist Adiabatic Lapse Rate (MALR):◦6°C per 1000 meters◦3.3°F per 1000 feet

Example 1

Lesson 16Stability

Hess, McKnight’s Physical Geography, 10 ed.

143– 147 pp.

Stable vs. UnstableThe atmosphere and individual air

parcels can be characterized as either stable or unstable.

Stability occurs when vertical motion is suppressed.◦ No clouds form (unless they are forced, i.e.

orographic lift)Instability (unstable air) occurs when

air tends to rise on its ownThe temperature of an air parcel

relative to the air which surrounds it determines stability

Lapse RatesWe know there are two types of

adiabatic lapse rates◦Dry adiabatic lapse rate (DALR)◦Moist adiabatic lapse rate (MALR)

There is a third lapse rate, known as the environmental lapse rate (ELR)

Environmental Lapse Rate (ELR)The ELR is basically the actual air

temperature at any height in the atmosphere

It is also known as a temperature profile, vertical temperature gradient, or normal lapse rate

The average value:◦6.5°C per 1000 meters◦3.6°F per 1000 feet

Environmental Lapse Rate (ELR)

Types of Stability/InstabilityThere are three types:

◦Absolute stability◦Absolute instability◦Conditional instability

Let’s look at examples of each of these…

Absolute Stability

Absolute Instability

Conditional Instability

Final Thoughtshttp://www.youtube.com/watch?v

=Ox33TFnfWbEPlease omit pages 85, 86, 91,

and 92 from your homework.