In meteorology, the term "isotherm" refers to a contour line that connects points on a weather map that have equal temperatures at a given time. Just as contour lines on topographical maps are used to show gradients of elevation, isotherms are used to show gradients of temperature. Isotherms usually are shown in five- or 10-degree intervals on maps. The temperature differences they delineate are often indicated by different colors.
The curve of an isotherm is shaped by variations in temperature across the area being mapped. Factors that affect temperature include elevation, the presence of bodies of water, ocean currents and latitude. Mountainous areas with higher elevation are usually colder than lower areas, causing isotherms around these areas to curve sharply. Isotherms also curve at ocean-land boundaries because of the mitigating effect that oceans have on extreme temperatures. Oceans have a higher specific heat than land, meaning that it takes more heat or cold to raise or lower their temperature, so mid-ocean temperatures tend to be more stable than inland temperatures.
Temperature decreases as latitude increases, so isotherms generally show a steady gradient as one moves towards the poles. The Arctic Circle is often defined using the 50 degrees Fahrenheit (10 degree Celsius) July isotherm. Although this isotherm shifts based on climate factors, it usually curves across the upper parts of Russia and Canada during the month of July, circling the North Pole.
The isotherm gradient can vary based on time of year. During the winter, there is a wider variation in solar radiation, and therefore temperature, between low and high latitudes. This means that isotherm gradient in winter months is globally “steeper” than in summer, and the isotherms on a map will appear closer together. In other words, the change in temperature across distance from the equator to the pole is greater in winter than in summer.
The 32 degrees Fahrenheit (0 degrees Celsius) isotherm is an important meteorological marker. This isotherm has great importance for weather forecasts, because it shows the temperature line at which rain will freeze into sleet or snow. By following this isotherm at different atmospheric pressures, forecasters can predict movement and accumulation of precipitation in a given area.
The process that produces cloudiness and occasional extreme weather is known as temperature advection. Temperature advection occurs in regions that are baroclinic, meaning that winds are blowing across the isotherm gradient. Cold advection occurs when cold air is transported across isotherms to warmer areas, and warm advection occurs when warm air is transported to colder areas.