The science of seasonal lag
By Jared Shelton News-Press NOW meteorologist
Meteorological summer may have arrived several weeks ago, but the “official” or astronomical start to the season has just begun — starting with the summer solstice.
Opposite to its winter counterpart (winter solstice), the summer solstice is the longest day of the year here in the northern hemisphere, when the sun shines most directly onto Earth’s surface for those north of the equator. A direct result of Earth’s tilted axis, the annual flux in duration and intensity of ultimately sunlight drives the cyclical changes in weather we call seasons.
While solstices and equinoxes occur around roughly the same time each year, their exact date can vary by a few days. Like this year’s spring equinox, the 2024 summer solstice was the earliest in over a century, albeit by less than a day, taking place on June 20. Regardless of exactly when the summer solstice takes place from year to year, peak changes in average temperature always arrive several weeks later, and in some cases over a month. For example, the peak average high temperature in St. Joseph is normally around mid-July, about one month past the summer solstice.
The mismatch between Earth’s maxima in solar radiation and annual peak in temperature is called seasonal lag. A phenomenon that also applies to winter, as Earth’s lowest temperatures annually occur well after the minima in solar radiation have passed with the shortest day of the year, also known as the winter solstice.
The concept of seasonal lag may seem counterintuitive, as intensity and duration of sunlight should be directly correlated to higher or lower temperatures. It turns out, the distorted relationship between these two variables comes from a compound that makes life on Earth possible, H2O.
Water, which covers about 71% of Earth’s surface, tends to moderate extreme temperatures. For example, coastal areas typically have a lower range in annual temperatures than inland regions. This unique property of water is due to its high specific heat capacity, meaning it takes lots of thermal energy to heat a specified volume of water compared to the same volume of another substance, like air or land. Consequently, water also retains heat for lengthy periods, cooling less rapidly than air or land. The vast amounts of water covering Earth’s surface combined with our moisture-rich atmosphere, essentially slows the transfer of radiant heat from the sun on a large scale, resulting in the seasonal lag between annual solstices and changes in temperature.
