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Isotherm
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Map of the Arctic regions centered on the North Pole, showing surrounding landmasses including northern Russia, Alaska (U.S.), Canada, Greenland (Denmark), Iceland, Norway, Sweden, and Finland. A bold red line marks the climatological limit of the Arctic, defined by the July 50-degree Fahrenheit isotherm, enclosing the Arctic Ocean and adjacent seas. Major islands, seas, and notable Arctic settlements are labeled.
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Map of the Arctic regions centered on the North Pole, showing surrounding landmasses including northern Russia, Alaska (U.S.), Canada, Greenland (Denmark), Iceland, Norway, Sweden, and Finland. A bold red line marks the climatological limit of the Arctic, defined by the July 50-degree Fahrenheit isotherm, enclosing the Arctic Ocean and adjacent seas. Major islands, seas, and notable Arctic settlements are labeled.
Arctic map The average location of the 50 °F July isotherm in the Northern Hemisphere—the climatological limit of the Arctic.

Isotherm

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Isotherm, line drawn on a map or chart joining points with the same temperature. Isotherms are commonly used in meteorology to show the distribution of temperature at the Earth’s surface or on a chart indicating constant level or constant pressure. They are also used to show the time variation of temperature with height in the atmosphere or with depth in soil or water; the characteristics of heat flow into soil, for example, are easily visualized from a graph showing temperature as a function of depth and time. A convenient way to compare thermal climates is to plot the temperature for each location as a function of time of day (vertical axis) and time of year (horizontal axis).

This article was most recently revised and updated by Richard Pallardy.