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Weather Wise: Seasonal Lag

Seasonal Lag
Posted at 5:06 PM, Jul 31, 2023

It's the hottest time of the year, the end of July and the beginning of August, here in the Northern Hemisphere, even though we've lost nearly an hour of daylight since the Summer Solstice on June 21.

Seasonal Lag is the phenomenon where the date of maximum average air temperature is delayed until some time long after the date of maximum solar intensity. Maximum solar intensity in Montana and the Northern Hemisphere occurs on the Summer Solstice usually June 21. However, the hottest time of year is the last week of July into the first week of August. This also applies to the minimum temperature or coldest time of year, several weeks after the shortest day of the year on December 21.

Seasonal Lag on Earth is largely caused by the presence of large amounts of water and the high latent heat. Latent energy or heat of transformation is energy released or absorbed between a body and its surroundings. The sun's strongest rays in the northern hemisphere occur at the solstice, but it takes several weeks for the air and oceans to change and maximize temperature.

Due to Seasonal Lag, in the Northern Hemisphere, the Autumnal Equinox in late September is considerably warmer than the vernal equinox in late March despite the fact that both days have almost equal amounts of daylight and darkness.

There are some exceptions to this rule because of localized weather. For example, Tucson's hottest weather usually occurs around the summer solstice with the sun's strongest rays because monsoon moisture that develops in July and August will cool that area down.