While Thursday marks the longest day of the year in Northern Hemisphere, this will not necessarily be the hottest.
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| The summer solstice in the Northern Hemisphere is accompanied by the winter solstice in the Southern Hemisphere [File: Kin Cheung/AP Photo] |
Millions of people across the Northern Hemisphere, including in the United States and India, are enduring heatwaves even before the summer solstice, which marks the first day of summer.
Although daylight periods will shorten and nights will lengthen after the solstice, high temperatures are expected to persist. On Wednesday, climate activists spray-painted the United Kingdom’s Stonehenge monument to demand a phase-out of fossil fuels, which are blamed for the prolonged heatwaves and extreme weather conditions globally.
Revellers gather at Stonehenge, one of the world’s most famous prehistoric sites, to celebrate the summer solstice. The monument is believed to have been constructed with a specific alignment to the solstice sunrise.
What is the significance of the 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 simultaneously.
The Northern Hemisphere’s summer solstice typically occurs between June 20 and 22. This year’s solstice is the earliest in 228 years, last occurring on June 20, 1796. The most recent June 22 solstice was in 1975; since then and until last year, it had occurred on June 21.
The date of the solstice shifts because the Earth's orbit around the sun does not exactly match our calendar year. Although the orbital cycle and Earth's tilt on its axis remain relatively constant over short periods, gradual natural variations cause slight changes in the solstice date every few decades.
Why is it the longest day of the year?
During the summer solstice, the Northern Hemisphere experiences the longest period of sunlight in a single day, making it the longest day of the year.
This phenomenon occurs because of Earth's position in its orbit around the sun and the tilt of the North Pole toward the sun during the solstice. The sun's rays strike the Northern Hemisphere at their most direct angle, resulting in the maximum amount of daylight. Regions further from the equator will experience longer daylight hours than those closer to it. For example, areas of India near the equator will have nearly 14 hours of daylight.
In the Arctic Circle, there will be a polar day, also known as the midnight sun, where the sun remains visible for 24 hours.
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.
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