Lecture #9: 
Latent and Sensible Heating;  Atmospheric Stability
Wednesday, 7 February 2001

Text Reading for Lecture #9
Atmospheric Moisture (Read pages 105-122)
Atmospheric Stability (Read pages 160-175)

HEAT CAPACITY - The ratio of the amount of thermal
energy absorbed (via heating) to the corresponding
rise in temperature. 

Water has a very high heat capacity -- sun shining
on a swimming pool versus the sidewalk nearby.
Which one heats faster?  Put another way, which
one requires more energy to change temperature
by the same amount?

This concept is critical in the formation of
thunderstorms because of the uneven heating
of the ground based upon soil moisture,
vegetation, etc. 

LATENT HEAT(ING) - The amount of thermal
energy needed to change a substance from one
state (or phase) to another. 

But why LATENT?  As a drop of water evaporates,
molecules leave the surface and thus leave fewer
behind in the liquid.  Thus, the energy of the drop
decreases (fewer molecules left to bang around
inside the drop).  Thus, evaporation is a cooling
process (e.g., skin cooling on a warm day).  The
energy for the evaporation comes from the
environment (air).  Because the cooling/heating
do not occur until the phase change takes place, the
heat is called LATENT -- it's present in stored
form and is not realized until the phase change
occurs.

When the phase change occurs, the physical
warming that occurs is manifest as SENSIBLE
HEATING (not sensible heat, as the book
incorrectly states). 

Latent heating is a necessary ingredient for
thunderstorms.  Without it, air would cool much
faster as it rises.  We'll look at this more closely
now in the context of ATMOSPHERIC
STABILITY.

Reading:  Pages 160-175

Recall our earlier qualitative discussion about
atmospheric stability, and how it involves
comparing the temperature of an air parcel
moving vertically with the change of temperature
in the surrounding environment. 

We now dig a bit deeper to quantify stability and
provide a mechanism for understanding
thunderstorm type.  A quantity known as the Bulk
Richardson Number measures the amount of
instability present in the atmosphere, and tells us
something about how the winds change with
height.  Both are important for determining storm
type, and the BRN is a single number that is used
by forecasters to determine storm type:

Key concept:  Adiabatic process.   When a parcel of
air rises or sinks, it cools or warms at a specific
rate known as the adiabatic lapse rate (a lapse
rate is a decrease of something with height).  The term
"adiabatic" means no exchange of energy between
the parcel and its surroundings (an idealized
assumption that holds true to a very good degree
in the real world).

An unsaturated air parcel cools at a rate of about
10 C for every 1000 m of altitude (warms at the
same rate if descending).  This is called the DRY
ADIABATIC LAPSE RATE (more accurately,
the unsaturated adiabatic lapse rate).

A saturated parcel cools less rapidly with height
because the release of latent heat creates sensible
warming that counteracts the adiabatic cooling.
Thus, a saturated parcel cools at about 6 C for
every 1000 feet of altitude.  This is the MOIST
ADIABATIC LAPSE RATE (also called the
saturated adiabatic lapse rate).

So now we know how unsaturated and saturated
parcels cool with height, and if we know how
the atmosphere's temperature changes with height,
we can assess the stability.

Recall that the vertical temperature profile is
determined by a balloon-borne instrument called
a radiosonde.  The temperature trace with height
is a "sounding," and the rate at which the
temperature changes with height is called the
ENVIRONMENTAL LAPSE RATE (ELR).

We can graphically determine atmospheric
stability by plotting the ELR and parcel lapse
rates.  We don't even have to worry about the
numbers!!

In the above diagram, the slope of the line
indicates the steepness of the lapse rate -- or
how rapidly the temperature changes with
height.  A nearly vertical line means that the
temperature changes slowly with height, while
the closer the line becomes to horizontal, the
faster the cooling with height. 

For the case shown, the parcel cools more
rapidly than the environment, so the parcel's
tempearture is always colder at each altitude.
Thus, the parcel won't continue to rise on its
own, and the atmosphere is said to be STABLE.

How does the situation change if the parcel is
saturated?

The parcel, even though cooling less rapidly with
height, still is colder than the environment at each
level -- and thus the atmosphere is again stable.

Comparing the two cases:

Here, saturation doesn't make a difference, and thus
the atmosphere is said to be ABSOLUTELY
STABLE if a rising parcel, either saturated or
unsaturated, is always colder (more dense) than
its environment.  In such cases, the ELR is less
than both the dry and moist adiabatic lapse
rates.

The only way clouds form in an absolutely stable
environment is through FORCED LIFTING, say
by a mountain, front, etc.  Usually such clouds are
stratiform in appearance and are called stratus:

Let's now consider the opposite extreme, that is,
the rising parcel is everywhere WARMER than
the environment.  Such an environment is said
to be ABSOLUTELY UNSTABLE, because
whether saturated or not, the rising parcel always
will be warmer than its environment.

In such cases, the ELR is greater than both the
dry and moist adiabatic lapse rates.   This
condition is common in the springtime and is
associated with severe storms.  Below, we'll
look at thermodynamic diagrams, which allow
one to assess stability quickly.

In most cases, especially during the thunderstorm
season, the atmosphere is CONDITIONALLY
UNSTABLE, the condition being whether the
rising air is saturated or unsaturated.