Summer Solstice: Always The Longest Day In Middle Latitudes; Rarely The Hottest

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Solstice and Equinox. Courtesy of NOAA

This is a visual representation of the winter and summer solstice, and the vernal and autumnal equinox. Image courtesy of NOAA

The sun marches north for about six months, then, on the summer solstice, does an about-face at 23-1/2 degrees latitude and heads south.

The sun will be farthest north this year on June 21.

You would think that north of 23-1/2 degrees, June 21 would be the hottest day of the year. But it almost never is.

Seasonal Lag Causes Maximum Temperatures To Occur In July Or August In Most Places

In the winter, with the sun low in the sky, temperatures plummet. As spring and then summer come, and the sun climbs higher, the ground warms, but to overcome the effect of the winter, the sun continues to heat the atmosphere until a balance is achieved between incoming and outgoing radiation.

The date of that balance depends on proximity to the ocean.

Oceans distribute the heat of the sun quickly and efficiently, and ocean surface temperatures respond only slowly to increased sunlight. In many places, the ocean reaches its maximum temperature in September. The land holds the sun’s heat at the surface and distributes it quickly to the atmosphere.

 

Places far from the sea reach their yearly maximum temperature earlier than those near the water. In the desert, where the temperature is dependent almost entirely on daily heating, the maximum may occur very close the solstice.

Here are some samples of the dates of maximum temperature:

  • Atlanta, July 28
  • Boston, July 20
  • London, August 1
  • Moscow, July 20
  • San Francisco, June 26
  • Phoenix, July 5
  • St. Louis, July 14
  • Honolulu, Sept. 2
  • Mumbai, India, May 16

Local Influences Can Cause Exceptions To The Maximum Temperature Rule

The entry of Mumbai in the chart above may look like an error — but it is not. India experiences a monsoon, which brings offshore (land to sea) winds in the winter and onshore (can you guess?) winds in the summer. Mumbai, on the coast, is one of the first places to receive the wind shift, and this normally occurs in May.

Summer Is Longer Than Winter

Though it may not have seemed so in a practical sense to inhabitants of the eastern United States this year, technically summer is longer than winter by both meteorological and astronomical measures.

Meteorological summer is June, July, and August: 92 days. Winter is December, January, and February: 90 days (91 on leap years). This is entirely due to the shortness of February. So you might think that the astronomical seasons would be equal, since they do not depend on the number of days in any month, but go strictly by the sun’s position in the sky.

But there’s a catch — and it means astronomical summer is actually longer relative to winter than its calendar counterpart.

The Orbit Of The Earth Around The Sun Is Not Round

As the earth revolves around the sun, it comes closest in its elliptical path on January 3; it is farthest away on July 4.

According to Kepler’s second law of orbital motion, a planet sweeps out equal areas in equal amounts of time. The earth speeds up at perihelion (closest approach to the sun) and whizzes through winter in 89 days, while it languishes 94 days for summer.

Effect Of The Earth’s Eccentric Orbit On The Weather

The earth is about three million miles closer to the sun in January than in July. It would seem that this would make the seasons more extreme in the southern hemisphere and less in the northern — but this is not the case.

Because of the distribution of land masses (two-thirds of earth’s land surface is in the northern hemisphere) and the contribution of local effects, the seasons actually vary quite a bit more in the northern hemisphere.

What Good Is The Solstice?

The French, ever resourceful, decided in 1982 that the summer solstice would be a good time to celebrate with music. Their idea, Fete de la Musique, has spread to over 500 cities in over 100 countries.

The solstice is obviously a good day to have a rockin’ good time; so let’s paaahty!

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