Millions of people across countries in the Northern Hemisphere, including the United States and India, bake in heatwaves even before the summer solstice, which marks the first day of summer.

The daylight periods will reduce after the solstice, while nights get longer, but there will likely be little respite from the spell of high temperatures.

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Climate activists on Wednesday spray painted the United Kingdom’s Stonehenge monument to demand phase out of fossil fuel, which contributes to climate change blamed for the prolonged heatwaves and extreme weather conditions around the world.

Revellers gather at one of the world’s most famous prehistoric sites to celebrate the summer solstice. Stonehenge is believed to have been constructed with a specific alignment to the summer solstice sunrise.

So what is the significance of summer solstice, which marks the longest day of the year?

Here’s what to know.

When is the summer solstice in 2024?

The 2024 summer solstice falls on Thursday, June 20 at 20:50 GMT. People across the Northern Hemisphere will experience the solstice at the same moment.

The Northern Hemisphere’s summer solstice typically falls between June 20 to 22. This year’s solstice is the earliest in 228 years, it last occurred on June 20, 1796.

The last June 22 solstice was in 1975 – since then and until last year, it had been occurring on June 21.

Shifts in the date occur because the time the Earth takes to orbit around the sun each year does not exactly align with our calendar year.

Although the orbital cycle and Earth’s tilt on its axis remain relatively constant over short periods, gradual, natural variations create a slight change in the solstice date every few decades.

Why is it the longest day of the year?

During the summer solstice, the Northern Hemisphere will experience the longest period of sunlight in a day or the longest day of the year.

This is due to Earth’s position in its orbit around the sun and the way the North Pole is tilted closest to the sun during the summer solstice. The sun’s rays hit the Northern Hemisphere at their most direct angle, resulting in the most extended period of daylight.

Areas further away from the equator will experience longer daylight hours than those that are closer. For instance, parts of India closer to the equator will experience nearly 14 hours of daylight.

The Arctic Circle will experience a polar day, also known as midnight sun with 24 hours of sunlight.

Is it the hottest day of the year?

Despite the long hours of sunlight, it will not be the hottest day of the year.

This is due to a concept called “seasonal lag”. The Earth and its atmosphere take time to warm up. This is similar to how the hottest part of the day is usually in the afternoon, not at noon when the sun is highest in the sky.

The oceans, which cover about 70 percent of the Earth’s surface, absorb, store, and release heat slowly. The warming of the oceans contributes to the overall delay in reaching peak temperatures.

Why will it get hotter now?

The hottest days of the summer will likely occur over several weeks after the summer solstice, often in July or August in the Northern Hemisphere.

The heat from long daylight hours will have accumulated on the earth. Additionally, even though the days start to get shorter immediately after the solstice, they are still quite long and the sun remains relatively high in the sky, which means the solar energy received remains substantial.

The Earth’s surface and atmosphere are efficient at absorbing this solar energy but release heat gradually.

Due to such factors, temperatures will continue to rise until late July or August, when the energy balance tips towards more energy being lost than gained.

Aren’t there already several heatwaves?

From India to Greece and the US, heatwaves have seared several countries across the Northern Hemisphere.

A heatwave is marked by at least two or more days of unusually high temperatures compared with typical averages in a location during that period or month.

A heatwave in the northeast US, for example, is when the temperature reaches or exceeds 32.2 degrees Celsius (90 degrees Fahrenheit) for three consecutive days.

In India, a heatwave is declared when the maximum temperature reaches at least 40C (104F) in the plains, or 30C (86F) in hilly regions, according to India’s meteorological department.

In the US, heat domes – high-pressure masses of trapped hot air – are exacerbating heatwaves.

Humidity can also make certain areas feel hotter.

What about the Southern Hemisphere?

At the same moment as the summer solstice in the north, the Southern Hemisphere will experience the winter solstice.

This is because the seasons are reversed there, so while the astronomical start of summer occurs in the north, the astronomical winter kicks off in the south.

Astronomical events relate to the Earth’s position in space, whereas the meteorological start of a season is based on the calendar year and the annual temperature cycle.

The Southern Hemisphere will experience its shortest day of the year, or the one with the least duration of sunlight.