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 AVERAGE, MEAN, NORMAL

METEOROLOGIST JEFF HABY

An average is found by summing a set of values and dividing by the number of values. For example, the values of 92, 88, 67, 98 and 90 result in an average of 87. A common experience that people have with an average deal with grades. If each grade counts the same, then the average (on homework for example) can be calculated by calculating an average.

The term “mean” means the same as average. When statistics are performed it is more common to hear the term mean than average.

When taking a sample of a population of values measuring the same thing, the average or mean is a reliable indicator of the typical value when 30 or more values are included. A sample size of smaller than 30 increases the chance for significant variation from the population mean of the value. In statistical data in meteorology for meteorological measurements, 30 or more values are used to determine a mean. For example, the mean high temperature for October 10th will include the high temperature on at least 30 October 10’s in the past. The value produced from the last 30 years of weather measurements is termed “Normal” in meteorology and climatology. The term normal will commonly be used for a typical high temperature for a certain date, low temperature for a temperature date, typical monthly precipitation and typical yearly precipitation. Normal is an average of measurements from the last 30 years. The weather will typically not be normal. Often the temperature will be above or below a typical value. The same is true for precipitation. The normal value though helps give a sense of the climate of a location.