Lesson 15
Adiabatic Processes
Hess, McKnight’s Physical Geography, 10 ed.
138 – 139 pp.
Adiabatic Processes
Recall 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 Cooling
Clouds are the result of condensation
 This 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 lesspressurized
 With less pressure exerted on the parcel,
it therefore expands
 This expansion causes
cooling and increases
RH

◦ Adiabatic cooling
Adiabatic Heating
Conversely, as a parcel of air descends it
compresses due to higher pressure
 This is a warming process, therefore the
air warms

Dry Adiabatic Rate

Consider 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 Rate
At this point, the air parcel is saturated
and the RH=100%
 If the parcel continues to rise, it will cool
even further
 However, 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 Theory

Evaporation 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 16
Stability
Hess, McKnight’s Physical Geography, 10 ed.
143– 147 pp.
Stable vs. Unstable
The 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 own
 The temperature of an air parcel relative
to the air which surrounds it determines
stability

Lapse Rates

We 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/Instability

There 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 Thoughts
http://www.youtube.com/watch?v=Ox33T
FnfWbE
 Please omit pages 85, 86, 91, and 92 from
your homework.
