MOLINE, Ill. — June 21, the summer solstice, brought more daylight time than any other day of the year, but it wasn't our hottest day of the summer. Since then, we have continued to see temperatures increase despite the amount of daylight continuing to decrease.
A month after the solstice, we experience our warmest air temperatures in July, and two months later, in the middle of August, we experience our warmest water temperatures.
This delay is known as seasonal lag. It occurs because land and water have a high heat capacity - the amount of heat that must be added to either land or water in order to change its temperature. Land has a lower heat capacity than water, so it takes much less heat and time for it to warm up.
This is the same reason why the year's shortest day occurs on Dec. 21 but the coldest air and water temperatures occur in January. It takes land and water longer to cool off just like it takes a longer time for them to heat up.