People in the UK have witnessed fluctuating weather of late.
Many are wondering when summer officially starts so they know when to expect more of those welcome warmer temperatures.
Here’s what you need to know.
When is the first day of summer?
There are two dates that people refer to when talking about the first day of summer.
What you go with depends on whether you use the astronomical or meteorological calculation of the seasons.
The meteorological date is the simpler of the two.
The Met Office states that the meteorological seasons consist of splitting the seasons into four periods which are made up of three months.
"These seasons are split to coincide with our Gregorian calendar, making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics,” the forecaster explains.
Put simply, every year summer begins on 1 June and lasts until 31 August under this measurement, with autumn then starting on 1 September.
So, in these terms, summer has officially begun in Britain.
Meanwhile, the astronomical season is a bit more complicated and the start of summer depends on the date of the summer solstice.
What is the summer solstice?
Solstice comes from the Latin word sōlstitium and is made up of sōl which means "sun," and sistere which means "to stand still”.
The summer solstice is a certain point during the year when the Earth’s tilt on its axis on the northern hemisphere becomes most inclined towards the sun, with the sun reaching its highest possible position.
On the day the summer solstice occurs, we receive the most hours of daylight and the shortest night due to the position of the Earth in relation to the sun.
For many, it marks the end of spring and the start of summer which will then end with the autumn equinox.
The summer solstice has also become linked with Stonehenge and paganism in the UK - yet other cultures across the globe have celebrated the event for centuries.
Meanwhile, the winter solstice tends to happen around 21 December each year in this country, and marks the shortest day of the year and the start of winter for many.
When is the summer solstice 2021?
This year, the summer solstice will occur on Monday 21 June 2021 - a day later than its most common date.
That represents, for many, summer’s official start date.
However, this changes every year as the event can actually happen during any date between 20 and 22 of June.
The astronomical summer then lasts until the autumn equinox, which falls on 22 September this year.
During the autumn and spring equinoxes, the tilt of Earth’s axis is perpendicular to the sun’s rays.
This means the sun shines directly on the equator, resulting in it lighting up the Northern and Southern Hemispheres equally.
While this brings the start of spring in the Southern Hemisphere, it marks the beginning of autumn in the Northern Hemisphere.
The word equinox comes from the Latin words aequi, meaning “equal” and nox, meaning “night”, becoming “equal night”.