observation and are most frequently applied to
temperature. Assume, for example, that the extreme
highest temperature ever recorded at a particular station
was 106°F and the lowest recorded was -15°F. These
values are called the absolute maximum and absolute
minimum, respectively.
EXTREME
The term extreme is applied to the highest and
lowest values for a particular meteorological element
occurring over a period of time. This period of time is
usually a matter of months, seasons, or years. The term
may be used for a calendar day only, for which it is
particularly applicable to temperature. For example, the
highest and lowest temperature readings for a particular
day are considered the temperature extremes for that
day. At times the term is applied to the average of the
highest and lowest temperatures as mean monthly or
mean annual extremes.
RANGE
Range is the difference between the highest and
lowest values and reflects the extreme variations of
these values. This statistic is not recommended for
precise work, since it has a high variability. Range is
related to the extreme values of record and can be useful
in determining the extreme range for the records
available. For example, if the highest temperature
recorded yesterday was 76°F and the lowest was 41°F,
then the range for the day was 35°F.
FREQUENCY
Frequency is defined as the number of times a
certain value occurs within a specified period of time.
When a large number of various values need to be
presented, a condensed presentation of data may be
obtained by means of a frequency distribution.
MODE
Mode is defined as the value occurring with the
greatest frequency or the value about which the most
cases occur.
MEDIAN
The median is the value at the midpoint in an array.
In determining the median, all values are arranged in
order of size. Rough estimates of the median may be
obtained by taking the middle value of an ordered
series; or, if there are two middle values, they may be
averaged to obtain the median. The position of the
median may be found by the following formula:
Median =n +1
2
where n is the number of items.
The median is not widely used in climatological
computations. However, some sources recommend the
use of the median instead of the mean or average for
some climatic elements to present more representative
pictures of distribution and probability. A longer period
of record might be required to formulate an accurate
median.
DEGREE-DAY
A degree-day is the number of degrees the mean
daily temperature is above or below a standard
temperature base. The base temperature is usually
65°F; however, any temperature, Celsius or Fahrenheit,
can be used as a base. There is one degree-day for each
degree (°C or °F) of departure above or below the
standard.
Degree-days are accumulated over a season. At any
point in the season, the total can be used as an index of
past temperature effect upon some quantity, such as
plant growth, fuel consumption, power output, etc. This
concept was first used in connection with plant growth,
which showed a relationship to cumulative temperature
above a standard of 41°F. Degree-days are frequently
applied to fuel and power consumption in the form of
heating degree-days and cooling degree-days.
AVERAGE AND STANDARD DEVIATIONS
In the analysis of climatological data, it may be
desirable to compute the deviation of all items from a
central point. This can be obtained from a computation
of either the mean (or average) deviation or the standard
deviation. These are termed measures of dispersion and
are used to determine whether the average is truly
representative or to determine the extent to which data
vary from the average.
Average Deviation
Average deviation is obtained by computing the
arithmetic average of the deviations from an average of
the data. First we obtain an average of the data, then the
deviations of the individual items from this average are
determined, and finally the arithmetic average of these
6-4
