Northern Ireland

Summer solstice: Longest day does not mean sunniest

Cloudy skies at Aughlisnafin Image copyright Raymond Ringland
Image caption Cloudy skies like this at Aughlisnafin in County Down are always a possibility on the longest day

The summer solstice is the longest day of the year, and in the northern hemisphere it almost always falls on 21 June.

However, the exact time of mid-summer changes every year.

This year, it falls at 17:38 BST on Sunday.

Due to the earth's tilt, summer solstice in the northern hemisphere is the point at which the sun reaches its highest point in the sky, and shines directly on the Tropic of Cancer which is 23.5 degrees north of the equator.

Image copyright Louis Auden
Image caption Dromantine lake in County Down will continue to warm up because of longer hours of solar radiation

It is also the day of the year that has the most hours of daylight, and potentially the most hours of sunshine, cloud permitting.

Astronomically, summer means the days lengthen as far as 21 June, then gradually get shorter again.

But while the summer solstice is the longest day of the year, on average it is not the warmest.

This is due to a seasonal lag, as the longer hours of solar radiation continue to warm the land and the oceans for a couple of months into July and August.

Image copyright Met office
Image caption Average temperatures recorded by month at Newforge in south Belfast
Image copyright Met Office
Image caption Belfast sea temperatures, based on satellite readings taken by the National Oceanic and Atmospheric Administration

Highest average temperatures in Northern Ireland occur from late July into early August, with the highest sea temperatures also coming in August as a build-up of heat energy continues.

Even though the days are technically getting shorter after the summer solstice, it's a slow process at first and the hours of daylight are still quite long.

This means that for meteorological purposes, the summer months are considered to be the months of June, July and August.

Now we can only hope that we get some long sunny days this summer to go along with those longer hours of daylight!

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