PRECIPITATION
Precipitation is the second most important climatic
element. In most studies, precipitation is defined as
water reaching Earth’s surface by falling either in a
liquid or a solid state. The most significant forms are
rain and snow. Precipitation has a wide range of
variability over the Earth’s surface. Because of this
variability, a longer series of observations is generally
required to establish a mean or an average. Two stations
may have the same amount of annual precipitation, but
it could occur in different months or on different days
during these months, or the intensity could vary.
Therefore, it often becomes necessary to include such
factors as average number of days with precipitation
and average amount per day. Precipitation is expressed
in most studies in the United States in inches, but
throughout the rest of the world, millimeters are
normally used.
Since precipitation amounts are directly associated
with amount and type of clouds, cloud cover must also
be considered with precipitation. Cloud climatology
also
includes
such
phenomena
as
fog
and
thunderstorms.
WIND
Wind is the climatic element that transports heat
and moisture into a region. The climate of an area is
often determined by the properties of temperature and
moisture that are found upstream of that region.
Climatologists are mostly interested in wind with
regard to its direction, speed, and gustiness. Wind is
therefore usually discussed in terms of prevailing
direction, average speeds, and maximum gusts. Some
climatological studies use resultant wind, which is the
vectorial average of all wind directions and speeds for a
given level, at a specific place, and for a given period.
REVIEW QUESTIONS
Q6-4. What is the most important climatic element?
Q6-5. Which climatic element transports heat and
moisture into a region?
EXPRESSION OF CLIMATIC
ELEMENTS
LEARNING OBJECTIVE: Define the terms
used to express climatic elements and the
methods used to derive these terms.
Climatic elements are observed over long periods
of time; therefore, specific terms must be used to
express these elements so they have definite meaning.
This lesson defines the most commonly used terms and
discusses how they are used to express climatic
elements.
MEAN (AVERAGE)
The
mean
is
the
most
commonly
used
climatological parameter. The term mean normally
refers to a mathematical averaging obtained by adding
the values of all factors or cases and then dividing by
the number of items. For example, the average daily
temperature
would
be
the
sum
of
the
hourly
temperatures divided by 24.
Other methods are used for computing various
meteorological elements. For example, the mean
temperature for 1 day has been devised by simply
adding the maximum and minimum values for that day
and dividing by 2. Assume the maximum temperature
for a certain day is 75°F and the minimum temperature
is 57°F. The mean temperature for the day is 66°F.
Unfortunately, the term mean has been used in
many climatological records without clarification as to
how it was computed. In most cases, the difference in
results obtained is slight. In analyzing weather data, the
terms
average
and
mean
are
often
used
interchangeably.
NORMAL
In climatology, the term normal is applied to the
average value over a period of time, which serves as a
standard with which values (occurring on a date or
during a specified time) may be compared. These
periods of time may be a particular month or other
portion of the year. They may refer to a season or to a
year as a whole. The normal is usually determined over
a 20- or 30-year period.
For example, if the average temperature for your
station on 10 June has been 80°F over a specified period
of time, the normal temperature for your station on 10
June is 80°F. If the temperature on 10 June this year was
only 76°F, then the temperature for that day is 4°F
below normal.
ABSOLUTE
In climatology, the term absolute is usually applied
to the extreme highest and lowest values for any given
meteorological element recorded at the place of
6-3
