greater than at low temperatures. Also, since moist air
is less dense than dry air at constant temperature, a
parcel of air has a greater specific humidity at
saturation if the pressure is low than when the pressure
is high.
Mixing Ratio
The mixing ratio is defined as the ratio of the mass
of water vapor to the mass of dry air and is expressed in
grams per gram or in grams per kilogram. It differs
from specific humidity only in that it is related to the
mass of dry air instead of to the total dry air plus water
vapor. It is very nearly equal numerically to specific
humidity, but it is always slightly greater. The mixing
ratio has the same characteristic properties as the
specific humidity. It is conservative (values do not
change) for atmospheric processes involving a change
in temperature. It is non conservative for changes
involving a gain or loss of water vapor.
Previously it was learned that air at any given
temperature can hold only a certain amount of water
vapor before it is saturated. The total amount of vapor
that air can hold at any given temperature, by weight
relationship, is referred to as the saturation mixing
ratio. It is useful to note that the following relationship
exists between mixing ratio and relative humidity.
Relative humidity is equal to the mixing ratio divided
by the saturation mixing ratio, multiplied by 100. If any
two of the three components in this relationship are
known, the third may be determined by simple
mathematics.
Dew Point
The dew point is the temperature that air must be
cooled, at constant pressure and constant water vapor
content, in order for saturation to occur. The dew point
is a conservative and very useful element. When
atmospheric pressure stays constant, the dew point
reflects increases and decreases in moisture in the air. It
also shows at a glance, under the same conditions, how
much cooling of the air is required to condense
moisture from the air.
REVIEW QUESTIONS
Q1-16. Name the three states in which moisture in the
atmosphere may be found.
Q1-17.
What is the primary source of atmospheric
moisture?
Q1-18.
What is the difference between relative
humidity and absolute humidity?
Q1-19.
What is the definition of mixing ratio?
Q1-20.
What
information
does
the
dew
point
temperature provide to meteorologists?
SUMMARY
In
this
chapter,
we
introduced
the
basic
fundamentals of meteorology. It is important to have a
basic knowledge of systems of measurement, how the
earth and sun relate to each other, and how pressure,
temperature and moisture are measured and calculated.
An understanding of the basic fundamentals is
necessary before proceeding on to the next chapter.
1-21
