Explainer

What is meteorological summer and why does it end on 31 August?

Credit: Adam Jones

The last week has brought cooler evenings, with some exceptionally wet summer months leaving many feeling as though the season has been cut short this year.

The Met Office has warned that wetter and windier conditions are on their way, bringing a more autumnal feel.

But according to meteorology this shouldn't be particularly surprising - as 31 August actually marks the end of summer.

However, there are others ways of measuring the seasons, which can often cause confusion over when they begin and end.

So what is meteorological summer and why does it end today?

This summer brought record temperatures and rainfall. Credit: PA Images

Meteorological summer always begins on 1 June and ends on 31 August.

According to the Met Office, meteorological seasons are split into four periods made up of three months each.

These seasons are divided using the Gregorian calendar - used by most of the world - making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics.

In meteorology, the seasons are defined as:

  • Spring - March, April, May

  • Summer - June, July, August

  • Autumn - September, October, November

  • Winter - December, January, February

Despite meteorologists using this method, when many of us refer to the end of summer we are actually talking about astronomical summer.

Credit: Dean Taylor

The astronomical calendar determines the seasons due to the 23.5 degrees of tilt of the earth's rotational axis in relation to its orbit around the sun.

Both equinoxes and solstices are related to the earth's orbit around the sun.

In the northern hemisphere, the summer solstice (20-22 June, depending on the year) marks the start of astronomical summer - the point at which the northern hemisphere is pointing directly towards the sun.

This means longer days, shorter nights and more solar radiation reaching the northern hemisphere, compared to the southern hemisphere.

Three months later, the autumn equinox in the northern hemisphere (21-23 September) takes place, as the sun is directly above the equator. This provides equal solar radiation to both hemispheres. In the southern hemisphere, they experience their spring or vernal equinox.

The winter solstice (December 20-22) occurs when the northern hemisphere points away from the sun, meaning shorter days, longer nights and colder weather.

Finally, the northern hemisphere spring equinox (March 20-22) takes place when the sun is directly above the equator again.

Since the seasons vary in length, the start date of a new season can land on different days each year. This makes yearly comparisons for statistics rather difficult - hence the need for meteorological seasons.

This year, astronomical summer began on 21 June 2021 and ends on 22 September 2021. It will begin on the same date next year but end a day later, on 23 September 2022.

Astronomical winter begins on 21 December 2021 and ends on 20 March 2022.