seasonal lag

Heat of the Moment? Why the Summer Solstice Isn't the Hottest Day

Due to a phenomenon known as seasonal lag, the hottest day of the year doesn’t coincide with the summer solstice

NBC Universal, Inc.

Welcome to summer!

The arrival of the summer solstice brings us our longest day - meaning the most amount of the daylight of the year. A whopping 15 hours, 17 minutes and 3 seconds of daylight, to be exact.

It may be the longest day of the year, but it’s not our warmest. Our warmest days don’t arrive until mid-to-late July or early August.

That’s in part because Earth is tilted on its axis at 23.5 degrees. On June 21, the Earth is tilted toward the sun in the Northern hemisphere, thus marking the start of astronomical summer. It also marks the start of a warming trend.

From spring to summer, our average high temperature rises by 13 degrees.

This is due to a phenomenon known as seasonal lag - the hottest day of the year doesn’t coincide with the summer solstice.

Instead, the delay, or offset is weeks to months. This lag is due to the heat capacity of Earth, or "the amount of heat needed to raise an object’s temperature." Average temperatures rise until the sun angle lowers in the sky.

In short, it takes time (and sun) to heat up the Earth’s ground and oceans.

This is evident in monthly average temperatures in Boston.

NBC10 Boston
  • May Average Temperature 58° (Mid-May length of daylight 14 hours, 46 minutes)
  • June Average Temperature 68° (Mid-June length of daylight 15 hours, 17 minutes)
  • July Average Temperature 74° (Mid-July length of daylight 14 hours, 57 minutes)
  • August Average Temperature 72° (Mid-August length of daylight 13 hours, 54 minutes)
  • September Average Temperature 65° (Mid-August length of daylight 12 hours, 24 minutes)
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