Lecture #8:  Moisture in the Atmosphere (Continued)
Latent and Sensible Heating
Monday, 5 February 2001

Text Reading for Lecture #8
Atmospheric Moisture (Read pages 105-122)

VAPOR PRESSURE - In a gas that contains
many constituent gases, the total pressure
exerted is the sum of the individual pressures
exerted by the individual constituents.

The partial pressure of water vapor in the air
is called the ACTUAL VAPOR PRESSURE
(page 110), and it is measured in the same
units as the regular pressure, e.g., millibars. 
A LOW VAPOR PRESSURE means a relative
lack of water vapor.

As noted above, the actual vapor pressure
reflects the amount of vapor in the air,
but it says nothing about how much the air
can actually hold.  Thus, the SATURATION
VAPOR PRESSURE is the pressure that
would be exerted by water molecules
if the air were saturated with vapor AT A
GIVEN TEMPERATURE.  It turns out that
the saturation vapor pressure is ONLY a
function of temperature. 

Why?  Because at higher temperatures,
the number of water vapor molecules leaving
the surface of a liquid increases, so in order
to maintain equlibrium (at saturation),
it takes more water vapor to saturate the air
-- and thus the partial pressure is higher.

When both water and ice are present in
the atmosphere simultaneously, the SVP
over water is greater than over ice.  What
conseqeuence does this have for clouds?

RELATIVE HUMIDITY - This is the most
familiar measure of moisture in the air, but
also the most misunderstood!  In words,
the RH is the ratio of the amount of
moisture actually in the air compared to
the MAXIMUM amount the air can hold
AT A GIVEN TEMPERATURE & PRESSURE:

                        water vapor present
RH =    ----------------------------------------------
            max amount of vapor the air can hold


Based on the above discussion, we can thus
write

                           vapor pressure
RH =    ----------------------------------------------
                   saturation vapor pressure

To get a percentage, multiply the above by 100%.


The term RELATIVE in relative humidity reminds
us of the fact that the humidity is specified
relative to the temperature (and pressure).  If
the amount of water in the air is a given value
but the temperature rises, then the SVP rises
and the RH drops.   This is exactly what happens
during the daytime, and is why RH drops at
night.

Can the RH exceed 100%?  Yes!  The air can
in fact exceed saturation -- a condition known as
supersaturation.  This is very important in the
context of clouds, as we'll see later.

EXAMPLE:  If the vapor pressure is 25 mb and
the temperature is 86 degrees F, what is the RH?

From the formula given above, RH=VP/SVP.  To
find the RH, given the VP, we need the SVP.  Using
the graph shown in the 2/2/01 lecture,





we simply find the SVP associated with a
temperature of 86 degrees -- about 42 mb.  
Thus, RH=(25/42) x 100% = 60%.

Q:  Why is indoor air so dry in winter?   The VP of air
inside is the same as air outside (the water vapor
content doesn't change as air comes in from the
outside), but what about the temperature?

Suppose the outside air temperature is 32 F and
the air is saturated (RH = 100%, or the VP=SVP). 
What will be the RH indoors if the temperature
is 86 F?  Bring the answer to class tomorrow.

Forecasting thunderstorms requires knowledge not
only of the air's moisture content, but also of how
the moisture might change over time!  How can RH
change with time?  Is RH important for thunderstorm
forecasting?

We know that rising air cools, and if it contains enough
moisture, it will condense to form clouds.

Numero-Uno Question:  How much cooling is
required?  In other words, how far will the air have
to be lifted?

DEW POINT - The temperature to which air has to
be cooled, with no change in air pressure or water
vapor content, to reach saturation.  (In a rising parcel
the cooling comes about via a change in pressure.)
If the dew point is below freezing, it is called the
FROST POINT.

Because pressure changes little from one location to
the next, the dew point is a good indication of how much
moisture the air contains, independent of temperature
(in this sense, it's more useful than RH).

The DIFFERENCE between the dew point and the
actual air temperature is called the dew point
depression and is an indicator of RH. 

When the dew point and air temperature are equal,
the RH = 100% and the air is saturated.

The dew point is a key parameter in thunderstorm
forecasting.  Generally, dew points of 50-55 F are
needed for severe thunderstorms, though we'll see
later that both winds and instability are critical.

Note that air need not rise to encounter lower pressure
and condense.  Can you think of an example?

As noted earlier, the PHASE CHANGE of water
substance is one of the most key aspects of severe
and unusual weather. We're now going to examine
concepts of "heat" (incorrect wording used by the
book) that are critical to clouds and storms.

READING - pages 28-30 in the text.

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).