Summer: Why It Happens, When It Starts and How Heat Affects the Body

Why summer happens, and why Earth isn’t actually closer to the Sun

Summer is not caused by Earth moving closer to the Sun: it is caused by the tilt of Earth’s axis, an angle of about 23.5 degrees relative to the plane of its orbit. As the planet makes its yearly circuit around the Sun, that tilt means each hemisphere receives sunlight more or less directly depending on the time of year: when a hemisphere is tilted toward the Sun, its rays arrive close to perpendicular and concentrate more energy over the same patch of ground, producing summer; when the same hemisphere tilts away from the Sun, the opposite happens, and it’s winter.

The fact that undercuts the most common assumption is, fittingly, an orbital one. According to NASA, Earth traces a slightly elliptical path around the Sun: at its closest point, perihelion, it sits about 91.4 million miles from the star, while at its farthest point, aphelion, the distance climbs to about 94.5 million miles. The Northern Hemisphere passes through aphelion during its own summer, and through perihelion during its own winter: if heat depended on distance from the Sun, those months should bring the opposite result. The gap between the two distances, roughly 3 million miles, is still small next to the average Earth-Sun distance, and it is not what drives the seasons. Cross-checking several astronomical sources places the 2026 aphelion on July 6, at 17:30 UTC, at a distance of 152,087,774 kilometers from the Sun.

So where does that 23.5-degree tilt come from? Here the science offers a hypothesis rather than a settled fact: one of the most widely accepted explanations, as reconstructed by NASA, points to the early Solar System, when a large celestial body is thought to have struck Earth while it was still young, knocking its axis out of its original alignment — the same event often credited with forming the Moon. For that same orbital reason, the two hemispheres always experience opposite seasons at the same time: when the North Pole is tilted toward the Sun, it’s summer in the Northern Hemisphere and, at that very moment, winter in the Southern Hemisphere.

The summer solstice, in both hemispheres

The astronomical marker of summer is the solstice. At the two moments of the year called equinoxes, Earth’s axis is tilted neither toward nor away from the Sun, and daylight and darkness are spread almost evenly across every latitude; in the months between one equinox and the next, the tilt toward the Sun instead builds to a maximum, which is the solstice itself. According to NOAA, the summer solstice marks, for the hemisphere in question, the day with the most hours of daylight and the shortest night of the year: in 2022, for instance, the Northern Hemisphere’s summer solstice fell on June 21 at 5:14 a.m. (U.S. East Coast time); the exact date shifts slightly from year to year because the civil calendar and the solar year don’t line up perfectly.

An even sharper effect shows up at the poles. On the days around the Northern Hemisphere’s summer solstice, the North Pole gets 24 hours of continuous daylight while the South Pole stays in darkness; the situation flips at the December solstice, when it’s the Southern Hemisphere’s turn to have summer. By that same logic, reversed, the Southern Hemisphere’s summer solstice falls around December 21-22, while the Northern Hemisphere is deep in winter: it’s the same phenomenon described in the article on winter, simply viewed from the opposite side of the planet.

Practical example: at the North Pole, on the days around the Northern Hemisphere’s summer solstice, the Sun stays visible for a full twenty-four hours, while at that same moment the South Pole sits in darkness. Six months later, around the December solstice, the situation reverses exactly, and it’s the South Pole that stays lit around the clock, with the North Pole in darkness.

Not everyone measures summer the same way. According to the Met Office, meteorologists prefer a fixed three-month block, June, July and August in the Northern Hemisphere (December, January and February in the Southern), because it makes climate data easier to compare from year to year; it’s the definition behind official climate statistics, such as monthly temperature rankings. Astronomers, by contrast, measure summer starting from the solstice, around June 21 in the Northern Hemisphere, through the autumn equinox, which falls roughly between September 22 and 23, a window that, as noted above, drifts slightly year to year.

Thunderstorms, heat waves and the urban heat island effect

Summer heat drives a handful of characteristic weather phenomena. According to the Met Office, summer is the most thunder-prone season, because the heat and moisture that build up at ground level during the longer days create ideal conditions for storms to form: it’s one of many expressions of the water cycle, the same evaporation-and-condensation mechanism that governs rainfall throughout the year.

The phenomenon that matters most for health, though, is the heat wave. The World Health Organization defines it as a period when heat builds up locally over a sequence of unusually hot days and nights, with an onset and end that are often not sharply defined, but a minimum duration of two or three days and a measurable impact on human activity. The Met Office ties the persistence of heat waves to areas of high pressure, anticyclones, that block air circulation for several days in a row and let heat accumulate instead of dispersing.

Practical example: a single day of intense heat isn’t enough to make a heat wave, because persistence is what counts. The two most severe heat waves of recent decades make the point well, the 2003 European one, which ran from June through August, and the 2010 Russian one, which lasted 44 consecutive days: both had a far greater impact on mortality than any single hot day could produce on its own.

In Italy, according to ISPRA, these phenomena are tracked using climate indices such as the WSDI, the Warm Spell Duration Index, and the trend over the last fifty years has been a structural, not occasional, increase in both the number of summer days and the frequency of tropical nights and the length of heat waves, a trend that fits into the same underlying mechanism described in the article on the greenhouse effect. The year 2023, with roughly 29 more heat wave days than the 1991-2020 average, ranked as the third-hottest year in Italy’s historical record; 2025 saw the average Italian sea temperature come in 1.18°C above that same baseline, the second-hottest year for Italy’s seas since 1982, with average temperatures around 20°C and peaks above 26°C in July and August.

In cities, heat compounds with another effect: the urban heat island. According to ISPRA, temperatures in urban centers can run up to 3°C higher than in the surrounding countryside, a gap that intensifies episodes of extreme heat exactly where most of the population lives. The mechanisms and risk thresholds of heat waves are covered in more depth in the dedicated article.

Effects on people: health and tourism

Intense heat puts the cardiovascular system under stress. According to the WHO, heat stress can raise heart rate and left-ventricular contractility while reducing central blood volume, ventricular filling pressures and cerebral perfusion. Symptoms from heat exposure range from swelling and cramps to heat exhaustion and, in the most severe cases, heat stroke, which the WHO describes as involving fainting and hot, dry skin, because the body loses the ability to regulate high temperatures; heat can also worsen existing chronic conditions, including cardiovascular disease, asthma, diabetes and mental health disorders, and it raises the risk of acute kidney injury and accidents.

The numbers help put the scale of the problem in context. Still according to the WHO, an estimated 489,000 heat-related deaths occurred worldwide each year between 2000 and 2019, 45% of them in Asia and 36% in Europe; in the summer of 2022 alone, an estimated 61,672 excess deaths in Europe were attributed to heat, while the European heat wave of June-August 2003 is thought to have caused around 70,000 deaths and the 2010 Russian one, lasting 44 days, around 56,000. The trend is rising: comparing 2000-2004 with 2017-2021, heat-related mortality among people over 65 increased by 85%, and the WHO expects the frequency, duration and intensity of heat waves to keep increasing throughout the twenty-first century because of climate change.

The most vulnerable populations, according to the WHO, include infants and young children, older adults, people with disabilities, people already under medical care, those taking medication or using alcohol, outdoor workers, and people living in poverty or informal settlements, in both urban and rural areas; even heat waves of low or moderate intensity can affect these groups.

A curiosity collected by the Met Office ties heat to the animal world too: you can roughly estimate the temperature in degrees Celsius by counting a cricket’s chirps over 25 seconds, dividing by 3 and adding 4. It’s an anecdotal method, not a scientifically validated measuring tool, but it shows how the activity of some insects tracks air temperature.

Summer heat is also an economic season. According to Eurostat, in the 2018 summer season, the latest detailed reading published in this data series and not a figure for the current year, nights spent in hotels and similar establishments across the European Union rose by 7.5 million, or +0.8% compared with the previous year, with two-thirds of hotel stays concentrated in five countries, Spain, Italy, Germany, France and the United Kingdom; August recorded the highest occupancy rates in most member states, while in a few, including Bulgaria, Denmark, Estonia, Cyprus, Finland and Sweden, the peak shifted to July.