From the:National Climatic Data Center Climate normals are a useful way to describe the average weather of a location. Several statistical methods are used to compute climatic normals. These methods include: Measures of central tendency (such as the mean or median); Measures of dispersion or how the values are extended across the mean (such as the standard deviation or inter-quartile range); and measures of frequency or probability of occurrence.
To the lay person, the term "climate normal" is most closely associated with the mean or average. In this context, a "climatic normal" is simply the arithmetic average of the values over a 30-year period (generally, three consecutive decades). A person unfamiliar with climate and climate normals may perceive the normal to be the climate that one would expect to happen. However, it is important to note that the climate normal (computed as an arithmetic mean or an average over many years), may or may not, be what one would "expect" to happen. This is especially true with precipitation in dry climates, such as the desert in the U.S. Southwest and temperatures at continental locations which frequently experience large swings from cold to warm temperatures.
For example, when considering precipitation in the Olympic Peninsula of Washington, the climate normal or average is what you would expect - rain almost every day. On the other hand, in the desert of Arizona where rains are very infrequent, the normal or average rainfall is not what you would expect on any given day - it probably wouldn't rain at all.
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