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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 insolation. This also applies to the minimum temperature being delayed until some time after the date of minimum insolation.

Earth's seasonal lag is largely caused by the presence of large amounts of water, which has a high latent heat of freezing and of condensation. Its length varies between different climates, with extremes ranging from as little as 15-20 days (for polar regions in summer) to as much as 2 1/2 months (for low-latitude ocean areas). Interestingly, in many locations, it is not "seasonally symmetric", that is, the time between the winter solstice and coldest time is not the same as between the summer solstice and hottest time. In low and middle latitudes, the summer lag is longer, while in polar areas the winter lag is longer. In mid-latitude continental climates, it is approximately 20-25 days in winter and 25-35 days in summer.

Other planets have different seasonal lags. Neptune, for instance, has a year that lasts 165 Earth years and a seasonal lag of about 30 Earth years.[1] The gas giants Jupiter, Saturn, Uranus and Neptune, as well as Saturn's moon Titan, all have substantial seasonal lags corresponding to the equivalent of between two and three months in Earth terms. Mars and Venus on the other hand have negligible seasonal lag of no more than a few days, and the same would be expected of Mercury since it has no atmosphere.

• http://pds-atmospheres.nmsu.edu/education_and_outreach/encyclopedia/radiative_time_constant.htm