Throughout the course of the year, most places on Earth goes through four noticeable seasons: summer, autumn (fall), winter and spring, each lasting for about 3 months. The seasons experienced by the northern and southern hemisphere always differ by six months – when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and so on.
Seasons are a direct consequence of the Earth’s tilted rotation axis, which makes an angle of about 23.5 degrees to a line drawn perpendicular to the plane of the ecliptic. The direction of the Earth’s axis stays nearly fixed throughout one orbit, so that at different parts of the orbit one hemisphere ‘leans’ towards the Sun (summer), while the other ‘leans’ away (winter). Six months later, the Earth is leaning in the opposite direction.
The Earth’s tilt causes the Southern Hemisphere (SH) to lean towards the Sun during SH summer. Meanwhile, it is winter in the Northern Hemisphere (NH) which leans away from the Sun. Six months later, the situation is reversed.
For locations north or south of the equator, the main feature accompanying each season is a change in temperature caused by the varying amount of sunlight that falls on each hemisphere of the Earth throughout its annual orbit. The hemisphere tilted towards the Sun will experience longer hours of sunlight, and more direct sunlight.
left: Winter in the Northern Hemisphere – the amount of sunlight falling on the Northern Hemisphere is much less than than… right: …the amount of sunlight falling on the Southern Hemisphere during Southern Hemisphere summer.
As the Sun is higher in the sky during summer, the sunlight reaching the surface is more concentrated. In winter, the Sun is lower in the sky, and sunlight is spread out over a larger area. During spring and autumn, both hemispheres receive about the same amount of sunlight.
At the equator, the temperature variation is much smaller throughout the year, and it is common to consider just two seasons: dry and wet (or monsoon). For observers right at the north pole and the south pole, there are only two seasons – an almost six-month long winter night followed by an almost six-month long summer day! Within the Arctic circle and the Antarctic Circle (latitudes 66.5 degrees north and south), there will be at least one polar day (24 hours of continuous daylight, sometime called the ‘midnight sun’) and one polar night (24 continuous hours of darkness).
The date of the start of the seasons is often chosen to start on the dates of the solstices (summer and winter) and equinoxes (autumn and spring). Alternatively, the start of a new season may be associated with the first day of the month (December, March, June and September) in which a solstice or equinox occurs.
The Earth’s changing distance from the Sun due to the Earth’s elliptical orbit is sometimes thought to cause the seasons. This is incorrect! The Earth’s distance from the Sun varies by about 3% from closest (perihelion distance = 147.09 million km) to furthest approach (aphelion distance = 152.10 million km). This small change in distance cannot account for the temperature differences between summer and winter, and cannot explain how it can be winter in one hemisphere and summer in the other hemisphere.
Above the equator, winter officially begins in December. But in many areas, January is when it really takes hold. Atmospheric scientist Deanna Hence explains the weather and climate factors that combine to produce wintry conditions at the turn of the year.
How does the Earth’s orbit influence our daylight and temperatures?
As the Earth orbits the sun, it spins around an axis – picture a stick going through the Earth, from the North Pole to the South Pole. During the 24 hours that it takes for the Earth to rotate once around its axis, every point on its surface faces toward the Sun for part of the time and away from it for part of the time. This is what causes daily changes in sunlight and temperature.
There are two other important factors: First, the Earth is round, although it’s not a perfect sphere. Second, its axis is tilted about 23.5 degrees relative to its path around the Sun. As a result, light falls directly on its equator but strikes the North and South poles at angles.
When one of the poles points more toward the Sun than the other pole, that half of the planet gets more sunlight than the other half, and it’s summer in that hemisphere. When that pole tilts away from the Sun, that half of the Earth gets less sunlight and it’s winter there.
Seasonal changes are the most dramatic at the poles, where the changes in light are most extreme. During the summer, a pole receives 24 hours of sunlight and the Sun never sets. In the winter, the Sun never rises at all.
At the equator, which gets consistent direct sunlight, there’s very little change in day length or temperature year-round. People who live in high and middle latitudes, closer to the poles, can have very different ideas about seasons from those who live in the tropics.
As the Earth orbits the Sun, sunlight strikes the surface at varying angles because of the planet’s tilt. This creates seasons.There’s an old saying, “As the days lengthen, the cold strengthens.” Why does it often get colder in January even though we’re gaining daylight?
It depends on where you are in the world and where your air is coming from.
Earth’s surface constantly absorbs energy from the Sun and stores it as heat. It also emits heat back into space. Whether the surface is warming or cooling depends on the balance between how much solar radiation the planet is absorbing and how much it is radiating away.
But Earth’s surface isn’t uniform. Land typically heats up and cools off much faster than water. Water requires more energy to raise and lower its temperature, so it warms and cools more slowly. Because of this difference, water is a better heat reservoir than land – especially big bodies of water, like oceans. That’s why we tend to see bigger swings between warm and cold inland than in coastal areas.
The farther north you live, the longer it takes for the amount and intensity of daylight to start significantly increasing in midwinter, since your location is tilting away from the Sun. In the meantime, those areas that are getting little sunlight keep radiating heat out to space. As long as they receive less sunlight than the heat they emit, they will keep getting colder. This is especially true over land, which loses heat much more easily than water.
As the Earth rotates, air circulates around it in the atmosphere. If air moving into your area comes largely from places like the Arctic that don’t get much sun in winter, you may be on the receiving end of bitterly cold air for a long time. That happens in the Great Plains and Midwest when cold air swoops down from Canada.
But if your air comes across a body of water that keeps a more even temperature through the year, these swings can be significantly evened out. Seattle is downwind from an ocean, which is why it is many degrees warmer than Boston in the winter even though it’s farther north than Boston.
How quickly do we lose daylight before the solstice and gain it back afterward?
This depends strongly on your location. The closer you are to one of the poles, the faster the rate of change in daylight is. That’s why Alaska can go from having hardly any daylight in the winter to hardly any darkness in the summer.
Even for a particular location, the change is not constant through the year. The rate of change in daylight is slowest at the solstices – December in winter, June in summer – and fastest at the equinoxes, in mid-March and mid-September. This change occurs as the area on Earth receiving direct sunlight swings from 23.5 N latitude – about as far north of the equator as Miami – to 23.5 S latitude, about as far south of the equator as Asunción, Paraguay.
This satellite view captures the four changes of seasons. On the equinoxes, March 20 and Sept. 20, the line between night and day is a straight north-south line, and the sun appears to sit directly above the equator. Earth’s axis is tilted away from the Sun at the December solstice and toward the Sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.What’s happening on the opposite side of the planet right now?
In terms of daylight, folks on the other side of the planet are seeing the exact opposite of what we’re seeing. Right now, they’re at the peak of their summer and are enjoying the largest amounts of daylight that they’re going to get for the year. I do research on Argentinian hailstorms and Indian Ocean tropical cyclones, and both of those warm-weather storm seasons are well into their peaks right now.
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But there’s a key difference: The Southern Hemisphere has a lot less land and a lot more water than the Northern Hemisphere. Thanks to the influence of the southern oceans, land masses in the Southern Hemisphere tend to have fewer very extreme temperatures than land in the Northern Hemisphere does.
So even though a spot on the opposite side of the planet from your location may receive exactly as much sunlight now as your area does in summer, the weather there may be different from the summer conditions you are used to. But it still can be fun to imagine a warm summer breeze on the far side of the Earth – especially in a snowy January.