SUMMER What is it? Meteorological versus Astronomical Calendars

SUMMER
What is it?

Meteorological versus Astronomical Calendars

When I got the television weather forecast the other day, the weatherman said that ‘now that summer is here, we could expect warmer weather and more sunshine’. Yes, we had  very cool  weather this spring and more rain than I can remember. Officially though, summer had not arrived yet. So why did the weatherman say that? It certainly didn’t feel like summer?

So when do we say summer is here?  Mostly we do it with calendars that we make up to record time. The rotation of the Earth around the sun has been the basis for the astronomical calendar. We have two solstices and two equinoxes. Those natural phenomena have been the basis of the astronomical seasons for thousands of years. The tilt of the Earth and the alignment of the sun over the equator tell us when the solstices and equinoxes are. The equinoxes mark the time when the sun passes directly over the equator. That means that in the Northern Hemisphere, the summer solstice falls around June 21. The winter solstice is taking place around December 22. The spring equinox is on or around March 21 and the autumn equinox is on or around September 22. For the Southern Hemisphere the dates are the same, only the seasons are reversed.

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Let’s have look how the astronomical calendar is for this year:

Spring 2021
Start (March Equinox)
Start 20 5:37 am
Duration 92 days, 17 hrs, 54 mins

Summer 2021
Start (June Solstice)
Jun 20 11:32 pm
Duration 93 days, 15 hrs, 49 mins

Autumn 2021
Start (September Equinox)
Sep 22 :21 pm
Duration 89 days, 20 hrs, 38 mins

Winter 2021-2022
Start (December Solstice)
Dec 2110:59 am
Duration 88 days, 23 hrs, 34 mins

You can see from the above that the durations of each season are different. As Earth actually needs 365.24 days to travel around the sun, an extra day has to be added every fourth year, which we do in February - the so-called leap year. As Earth’s orbit around the sun is more oval than round, the length of the seasons vary and could be between 89 and 93 days!  This variation makes it of course difficult for scientists who compare seasons and  climate change. They were looking for a new method, which led to considering the meteorological calendar.

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Despite the regularity of the earth’s rotation around the sun (we have yet to develop a good calendar to meter it over time without adjustments like adding a day every four years), in reality every country’s experience of a year varies with latitude and geography. We have to know when to plant, to harvest, to hunt migratory game, to prepare for floods and frosts. So whether an astronomical or a meteorological calendar would serve a people’s needs best is an open question.

So which calendar serves a nation best is fluid. Some use both, while others rely more on one than the other.

The meteorological calendar also has four seasons, each is three months long, and they are based on annual temperatures. The meteorological seasons begin on the first day of the months that include the equinoxes and solstices. In the Northern Hemisphere, for example:

• spring runs from March 1 to May 31;
• summer runs from June 1 to August 31;
• fall (autumn) runs from September 1 to November 30; and
• winter runs from December 1 to February 28

(It will be reversed for the southern hemisphere.)

Let’s start with winter. We consider December, January and February as the coldest months, i.e.- winter. So the meteorological winter is these calendar months. The warmest months are June, July and August, i.e.- summer. In the Northern Hemisphere, March, April and May transition to the warmer season, i.e.- spring. The fall starts with September and ends with November, again a transitional season. These meteorological seasons will be the beginnings and ends of these calendar months.

Meteorologists started such a calendar, which makes it easier to observe and predict any weather changes, because it is more accurate than the astronomical calendar as to predicting local weather. The length of the meteorological seasons is also more consistent, ranging from 90 days for winter of a non-leap year to 92 days for spring and summer. It has become easier to calculate seasonal statistics on a monthly basis, which are very useful for agriculture, commerce and a variety of other purposes.

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Because of the geography of the earth (ocean currents like the Gulf Stream off the North American eastern seaboard- and many others, pump huge amounts of warmth around the globe, all weather is local. The orbit of the earth around the sun is less predictive of annual weather than a meteorological calendar might predict! So a different model (meteorological) might suit us better in our everyday weather needs. 

Different countries adopt different methods. In Australia, spring begins on the meteorological 1st day of September (again it is in the southern hemisphere). In Ireland, February 1st begins spring on St Brigid’s Day. But in the Scandinavian countries this method does fall apart. Their marks are more tied to temperatures than calendar dates, as might be expected with their varied geography.

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For millennia, our human activities- hunting, planting, tribal ceremonies, rituals of passage, etc., have been tied to the astronomical calendar. We’ve built monuments of stone like the Mayan temples and Stonehenge, to ensure that our cultures stay in sync with the weather that controls our food, by tracking the passage of the seasons by marking the equinoxes and solstices.  Perhaps because- unlike our ancient cultures, we’ve been able to develop math and statistical methods that allow us to make better seasonal weather predictions based on meteorological seasons, it is time to use this new calendar more.

I am wondering which of the two will be used more in the future.