Seasonal lag

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Seasonal lag" – news · newspapers · books · scholar · JSTOR (May 2012) (Learn how and when to remove this message)

Seasonal lag is the phenomenon whereby the date of maximum average air temperature at a geographical location on a planet is delayed until some time after the date of maximum daylight (i.e. the summer solstice). This also applies to the minimum temperature being delayed until some time after the date of minimum insolation. Cultural seasons are often aligned with annual temperature cycles, especially in the agrarian context. Peak agricultural growth often depends on both insolation levels and soil/air temperature. Rainfall patterns are also tied to temperature cycles, with warmer air able to hold more water vapor than cold air.

In most Northern Hemisphere regions, the month of February is usually colder than the month of November despite February having significantly later sunsets and more daylight overall. Conversely, the month of August is usually hotter than the month of May despite August having later sunrises, increasingly earlier sunsets, and less daylight overall. The fall equinox (September 21-22) is typically much warmer than the spring equinox (March 21-22), despite the two receiving identical amounts of sunlight. In all cases, the change in average air temperature lags behind the more consistent change in daylight patterns – delaying the perceived start of the next season for a month or so.

An analogous temperature lag phenomenon occurs in diurnal temperature variation, where maximum daily temperature occurs several hours after noon (maximum insolation), and minimum temperature usually occurs just before dawn -- well past midnight. Both effects are manifestations of the general physical phenomenon of thermal inertia.