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Define the terms used to express climatic elements and the methods used to derive these terms.


Mean (average)









Average and standard deviations


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.

Learning Objective: Define the terms used to express climatic elements and the methods used to derive these terms.


The mean is the most commonly used climato-logical 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 75F and the minimum temperature is 57F; the mean temperature for the day is 66F.

Unfortunately, the term mean has been used in many climatological records without clarifica-tion 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.


In climatology, the term normal is applied to the average value over a period of time which serves as a standard with which values (occur-ring on a date or during a specified time) maybe compared. These periods of time may be a par-ticular 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 80F over a specified period of time, the NORMAL tempera-ture for your station on 10 June is 80F. If the temperature on 10 June this year was only 76F; then, the temperature for that day is 4F below normal.


In climatology, the term absolute is usually ap-plied to the extreme highest and lowest values for any given meteorological element recorded at the place of observation and is most frequently ap-plied to temperature. Assume, for example, that the extreme highest temperature ever recorded at a particular station was 106F and the lowest recorded was 15F. These values are called the absolute maximum and absolute minimum, respectively.


The term extreme is applied to the highest and lowest values for a particular meteorological ele-ment 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 is the difference between the highest and lowest values and reflects the extreme varia-tions 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 re-cords available. For example, if the highest temperature recorded yesterday was 76F and the lowest was 41F, then the range for the day was 35F.


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 maybe obtained by means of a frequency distribution.


Mode is defined as the value occurring with the greatest frequency or the value about which the most cases occur.


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 maybe found by the following formula:

where n is the number of items. The median is not widely used in climato-logical computations. However, some sources recommend the use of the median instead of the mean or average for some climatic element to present more representative pictures of distribu-tion and probability. A longer period of record might be required to formulate an accurate median.


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 65F; however, any temperature, Celsius or Fahrenheit, can be used as a base. There is one degree-day for each degree (C or F) of depar-ture 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 41F. Degree-days are frequently applied to fuel and power consumption in the form of heating degree-days and cooling degree-days.

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