Atmospheric Stability
and
Cloud Formation
Recall: Saturation is when evaporation = condensation
Also remember that
the higher the air
temperature, the
more water vapor
will be present in
the air at saturation.
Relative humidity = actual water vapor in air/maximum water
vapor possible
Relative humidity depends on two factors:
 the actual amount of moisture in the atmosphere
 the temperature
 remember that temperature determines how much water
vapor can be in the air at saturation
We can express/calculate Relative Humidity in a variety of ways:
RH=[(vapor pressure)/(saturation vapor pressure)] X 100%
RH=[(mixing ratio)/(saturation mixing ratio)] X 100%
Dew Point = the temperature to which air must be cooled in
order to become saturated
Dewpoint temperature is a better "absolute" measure of moisture in the air.
Why? Because it doesn't change when the air temperature changes; it only
changes when the moisture content changes. (Assuming constant
pressure). For example:
Temperature
Dew Point
Relative
Humidity
30
10
29%
20
10
53%
10
10
100%
High dew points mean high moisture content of the air, which often
translates to muggy and uncomfortable conditions.
In general, most people consider dew points above 20 degrees C very
uncomfortable (regardless of air temperature and relative humidity).
NOTE: Dew point temperature can NEVER be greater than the actual air
temperature
When air temperature = dew point temperature, RH = 100%
One of the clues a
meteorologist uses for
forecasting tonight's low
temperature is to look at
today's dew point: if no
fronts are expected to
come through, tonight's low
temperature will not get
much below today's dew
point. WHY?
Quick summary of conditions of saturation
•
•
•
•
Air temp = dewpoint temp
Relative humidity = 100%
Mixing ratio = saturation mixing ratio
Vapor pressure = saturation vapor pressure
Once saturation is reached:
1) If more water is added, then condensation will dominate
2) If temperature is decreased, then condensation will dominate
In other words, a cloud will form (given presence of CCN, etc.)
To make a cloud we need really only 3 things:
• Moisture
• Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN)
(more detail later on this)
• A method of cooling the air to saturation
So how, exactly, do convective clouds form in the
atmosphere?
Pressure is essentially the
“weight” of the atmosphere
above you
As you go up, less atmosphere
is above you, so pressure is
less
This is why your ears “pop” as
you drive up a mountain or go
up in an airplane
-- basically air inside your ears has
retained the pressure of the lower
elevation and starts to expand
Consider a “parcel” of air at
1000 millibars (that is, at the
ground)
Assume that the parcel is not
saturated
The parcel will exert 1000
mb of pressure to counteract
the atmospheric pressure
acting on the parcel. i.e.,
parcel pressure is in
equilibrium
If we take this parcel of air at
Earth’s surface and somehow lift it
up to 500 mb:
• as we go higher in the
atmosphere, there is less
atmosphere above us
• with fewer molecules pressing on
our parcel of air, the molecules in
the parcel can move more and the
parcel can expand
• when a parcel expands, the
molecules are “doing work”
ΔU = Q – W
• when molecules do work, they
lose energy so the parcel cools
That is – RISING AIR ALWAYS
COOLS
Short aside: first law of thermodynamics
ΔU = Q – W
Change in
internal
energy
Heat added
to the
system
Work done
by the
system
When air parcels rise (or sink),
the process is labeled adiabatic.
Physically, this simply means the
parcel keeps its heat constant
(remember, heat and temperature
are not the same!!) (Q in the
equation above does not change)
Recall: temperature is a measure of kinetic energy. As kinetic energy
increases, temp will increase. As kinetic energy decreases, temp decreases.
So – we know that rising/sinking parcels are “doing work” – thus we know they
are change their internal energy.
Parcel
Temperature
Internal energy
Work
Rises
cools
decreases
done by
Sinks
warms
increases
done to
Short derivation based on the first law of
thermodynamics
(derivation posted on course web page).
PARCEL lapse rates
Don’t confuse “parcel”
lapse rates with
“environmental lapse rates”
– the two are different!
Rising air parcels will COOL.
IF UNSATURATED, their rate
of cooling is fixed: 10°C / km
(ten degrees celsius per
kilometer).
A parcel that rises 500 m (1/2
km) will cool 5C, one that
rises 1267 m will cool 12.67C.
The math is simple 
This lapse rate is called the
“dry adiabatic lapse rate”.
Sinking parcels are – by
definition – unsaturated.
WHY?
Their rate of warming is fixed
at the dry adiabatic lapse rate. I.e., sinking parcels always warm at 10°C / km
What happens when the rising parcel becomes saturated?
 As an air parcel rises and cools, the relative humidity increases
 When the parcel cools to the point when the parcel temperature and the
dew point temperature are equal, RH will be 100%
 If lifting continues, the parcel will continue to cool – BUT the parcel would
be “supersaturated” (not good)
 Thus, it MUST “expel” water vapor – & condensation occurs
The difference between
wet adiabatic lapse rate
(6 C / km) and dry
adiabatic lapse rate (10
C / km) is due to latent
heat release
Notice also that MOST
rising parcels first cool at
the dry rate, then reach
saturation & cool at the
wet rate.
• Short temperature calculation exercise
Parcel air temp & dewpoint temp example
Cloud Formation
When we lift the air, where will condensation occur?
Depends on the moisture content of the air that is being lifted.
The lifting condensation level (LCL) is the altitude (usually
expressed as a pressure) at which the lifted air is cooled dry
adiabatically to saturation. Clouds will form at this level.
Typo: 6C/km
Now all we have to do is get the parcel of air lifted. We
can do that in four ways:
 Orographic Lifting
 Frontal Uplift (also known as frontal wedging)
 Convergence
 Convection
Orographic Lifting
Air is forced upward by topography
 As air is forced up the mountains (windward side) it cools, forms clouds, and
maybe precipitation
 As air goes down the mountain on the leeward side, it is compressed and warms
Therefore it is usually wetter on the windward side than on the leeward side.
Atmospheric stability
Why do some clouds stay relatively thin while others may grow to the depth
of the troposphere?
-- It has to do with how STABLE the atmosphere is at a given time and place
Why do clouds almost never grow up into the stratosphere?
Basic definition of
stability:
When air parcels are
warmer than their
environment, they are
unstable (and will seek
to rise)
When air parcels are
cooler than their
environment, they are
stable (and seek to sink)
How do we find out what the temperature of the “environment” is?
Weather
balloons are the
primary source
of data above
the ground.
They provide
valuable input
for computer
forecast models,
local data for
meteorologists
to make
forecasts and
predict storms,
and data for
research.
Twice a day, every day of the year, weather balloons are released
simultaneously from almost 900 locations worldwide, including 92
released by the National Weather Service in the US and its
territories. The balloon flights last for around 2 hours, can drift as far as
125 miles away, and rise up to over 100,000 ft. (about 20 miles) in the
atmosphere.
These balloon-borne instruments
are sent aloft just prior to 0000
UTC and 1200 UTC on each day.
During their ascent, they radio
back to the ground-based
receiving station a nearly
continuous stream of information
until the balloon bursts at
approximately 10 mb.
Now that we have data from the atmosphere, what do we do with it?
The data that are
collected from
radiosonde
instruments are
called soundings.
Also, when these
data are plotted on
special charts called
thermodynamic
diagrams, these plots
are called soundings.
Lower pressures correspond to higher altitudes. So most thermodynamic
diagrams (thermo diagrams, for short) have pressure plotted along the vertical
axis (decreasing upward on the graph), and temperature plotted along the
horizontal axis. Thus, the top of the graph corresponds to higher altitudes, and
the right side corresponds to warmer temperatures.
On this blank
graph, we can
plot the sounding
measurements
such as a
temperature and
dew point for a
variety of heights.
We get a sense
of how saturated
the environment
is at certain
pressure levels.
Dots are temperature (T), and x's are dew-point temperature (Td)
An actual sounding
has much more
information on it than
just temperature and
dew point
Most important use of
a sounding:
-- show (graphically
plot) how atmospheric
variables CHANGE
WITH HEIGHT
Air temperature (red)
Dewpoint temperature
(green)
Wind speed & direction
(black “flags” & “barbs”)
Pressure (blue), & by
analogy, height
Stability summary
• 3 types of environmental stability:
– Absolutely stable
– Absolutely unstable (very uncommon)
• Does occur in few tens of feet above parking lots &
roads in summer (mirages are often co-located)
– Conditionally unstable
• Conditional on when the rising air parcel reaches
saturation (i.e., the height of the LCL)
– If parcel can maintain its temperature WARMER than
environment, then it remains unstable