metr2603.lecture7

Lecture #7: Moisture in the Atmosphere
Friday, 2 February 2001
GROUND HOG DAY!!!

Text Reading for Lecture #7

Atmospheric Moisture (Read pages 105-122)

Unlike many substances, water exists naturally in all three states
in the atmosphere -- solid, liquid, and gas. The particular
PHASE or STATE of water determines its energy level and
thus the extent to which it plays a role in the atmosphere.

A strange characteristic of water is that it can exist in all three
phases, simultaneously!

However, the CHANGE of phase or state is the critical element
in many severe and unusual weather phenomena, and we'll
look at how much energy is released (huge thunderstorms)
or absorbed (evaporative cooling in microburst downdrafts)
when phase changes occur. The diagram below shows the
direction of energy transfer when water changes phase:

Be sure to study carefully the above diagram and understand the names
and directions of energy transfer.

About 600 calories are required to evaporate 1 gram of water at room
temperature

About 80 calories are required to melt 1 gram of ice.

About 680 calories are required to evaporate 1 gram of ice (the energy
required is that for melting plus evaporation).

The reverse processes require the same amount of energy, and all
of these processes are continually active in the atmosphere.

One of the most important conditions for air is saturation. It occurs
when the rate of evaporation and condensation are equal, or more
simply, when the air cannot hold additional vapor without converting
some of it to liquid. Knowing when and where the air becomes
saturated is critical in severe and unusual weather. Take for example
the microburst:

Additionally, high cloud bases (not much moisture near the ground)
usually means less severe thunderstorms. We'll look at this more
closely in the context of stability.

Condensation can occur in one of two principal ways:

1. By cooling the air (for example, upward motion)

2. By adding moisture to the air (for example, evaporation from a
lake or pond)

However, no matter how cold the air becomes, it will always contain
some amount of vapor. For the same number of water molecules,
it is easier to reach saturation in cold air compared to warm air.
In other words, cold air takes less vapor to reach condensation.
Said another way, warm air can "hold more water vapor" than cold air.
Quantifying the amount is very important and can be done using the
following quantities:

1. Absolute humidity

2. Mixing ratio (used most often in severe storms)

3. Specific humidity

4. Vapor pressure

5. Relative humidity

6. Dew point

7. Wet bulb

In looking at these quantities, it is valuable to use the concept of an AIR
PARCEL -- a conceptual blob of air that moves around in the atmosphere
and retains its properties.

HUMIDITY - Refers to a variety of ways for specifying the amount of
water vapor in the air.

ABSOLUTE HUMIDITY (page 109 in text) - This is simply the density
of the water vapor in an air parcel and is given by the ratio

                                  mass of water vapor
                              -----------------------------------
                             volume of air in the parcel

This is usually expressed as grams of water vapor per cubic meter of
air.

Example: If an air parcel 5 meters on a side contains 300 grams of
water vapor, then the absolute humidity is

300 grams/(5x5x5 cubic meters) = 2.4 grams/meters cubed.

IMPORTANT NOTE: The absolute humidity changes with parcel
size/volume.

SPECIFIC HUMIDITY (page 109 in text) - It is very useful to express the
amount of water vapor in a non-dimensional form, especially one that
compares the masses of water vapor and air in the parcel. The specific
humidity does this as follows:

                                  mass of water vapor
                              -----------------------------------
                             total mass of air in the parcel

Note that this ratio is non-dimensional.

Example: If an air parcel having a total mass of 100 kilograms contains
300 grams of water vapor, then the specific humidity is

300 grams/(100,000 grams of air) = 0.003 grams/gram = 3 grams/kilogram.

MIXING RATIO (page 109 in text) - Just to confuse you, meteorologists
use a quantity closely related to the specific humidity -- called the mixing
ratio. It is defined as follows:

                                  mass of water vapor
                              -----------------------------------
                             mass of dry air in the parcel

Note that this ratio also is non-dimensional, and that it emphasizes the mass of
of water vapor relative to the dry air.

IMPORTANT NOTE: The specific humidity and mixing ratio are independent
of parcel size/volume -- and thus the concept of an air parcel. Thus, these
quantities do not change as air parcels move around and can thus be used
to track the origin of air.

VAPOR PRESSURE (pages 110-112 in text)

To preface our discussion on vapor pressure, consider a basic question:
when/why does water boil?

The actual definition of the boiling point of water is the temperature when the vapor pressure of the water equals the atmospheric pressure (the pressure of the air pushing down on the surface of the water.) Think of it this way: In the liquid, the water molecules are attracted to one another by their very nature and further held together by the pressure of the atmosphere pushing down on the surface of the liquid. As they are heated they gain energy to begin to pull away from one another. When they gain enough energy, they are able to separate and to overcome the force of the atmospheric pressure and form the gas (water vapor) that makes up the bubbles you see in the boiling process. The standard temperature at which this occurs is 100 C or 212 F. As for your question, there are several parts. In the very early stages of heating, you may see some small bubbles even though the water is only warm. This is some air that is dissolved in the cold water and is being expelled by the heating. This is not boiling. As the water nears the boiling point, some areas of the container close to the heat source may be hot enough to cause some boiling in that area - produce some local bubbles. If you want to be sure that all of the water is "at the boiling point", you should wait until there is a large amont of bubbles rising to the surface. This is what people refer to as a "rolling boil."

--Dr. Jerry Franzen, Chemistry Department, Thomas More College, Crestview Hills, KY 41017 606-344-3377 franzenj@thomasmore.edu

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.

For Monday: Continue Reading Atmospheric Moisture (Read pages 105-122)