When the north pole is tilted towards the sun, the northern hemisphere has fewer daylight hours

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.

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.

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.

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SkyTellers Seasons activities for young children  
See also: Middle school Seasons activities and resources  

About Our Seasons

What causes our seasons?
We have seasons because Earth's axis – the imaginary line that goes through the Earth and around which the Earth spins — is tilted. It's tilted about 23.5 degrees relative to our plane of orbit (the ecliptic) around the Sun. As we orbit our Sun, our axis always points to the same fixed location in space. Our northern axis points almost directly toward Polaris, the North Star.

This picture shows Earth from its side as it orbits our Sun. The axis is tilted and points to the North Star no matter where Earth is in its orbit. Because of this, the distribution of the Sun's rays changes. In June, in the northern hemisphere summer, the Sun's rays reach the north pole and beyond, enveloping the Arctic circle. In December, in the northern hemisphere winter, the north pole is tilted away from the incoming sunshine.

The “fixed” tilt means that, during our orbit around our Sun each year, different parts of Earth receive sunlight for different lengths of time. It also means that the angle at which sunlight strikes different parts of Earth's surface changes through the year. Sunlight striking the surface at an angle is “spread” across a wider area compared to sunlight striking perpendicular to Earth's surface. Areas that receive more scattered sunlight receive less energy from our Sun. All of these factors combine to give Earth its annual cycle of seasons!

For part of our orbit the northern half of Earth is tilted toward the Sun. This is summer in the northern hemisphere; there are longer periods of daylight, the Sun is higher in the sky, and the Sun's rays strike the surface more directly, giving us warmer temperatures. The north pole is in constant daylight!

When the northern half of Earth is tilted toward the Sun, the southern hemisphere is tilted away. People in the southern hemisphere experience the shorter day lengths and colder temperatures of winter.

During winter in the northern hemisphere, our northern axis continues to point to the North Star, but, because we have moved in our orbit around the Sun, our northern hemisphere now points away from our Sun. The north pole is completely dark and other places in the northern hemisphere experience the shorter day lengths and colder temperatures of winter as the Sun traces a lower arc across the southern sky and the Sun's rays strike the surface at a lower angle. When it is winter in the northern half of Earth, the southern hemisphere, tilted toward our Sun, has summer.

During fall and spring, some locations on Earth experience similar, milder, conditions. Earth has moved to a position in its orbit where its axis is more or less perpendicular to the incoming rays of the Sun. The durations of daylight and darkness are more equally distributed across all latitudes of the globe.

What doesn't cause the seasons?
The seasons are not caused by how far Earth is from our Sun. Earth's orbit around our Sun has a slightly elliptical path (very slight!), and the Sun is not exactly in the center of the ellipse. This means that, during the year, Earth is sometimes farther from our Sun, and sometimes closer — but the difference is small (not so for some other planets!). Earth is closest to our Sun in January (perihelion) and the farthest away in July (Earth is 147.5 million kilometers from the Sun when it reaches aphelion). If distance were the most important factor, the entire Earth would have summer in January when we are closest to our Sun and winter in July when we are farthest away!

What are solstices and equinoxes?
Solstices occur when Earth's axis is pointed directly toward our Sun. This happens twice a year during Earth's orbit. Near June 21 the north pole is tilted 23.5 degrees toward our Sun and the northern hemisphere experiences summer solstice, the longest day of the northern hemisphere year. On that same day, the southern hemisphere is tilted 23.5 degrees away from our Sun and the southern regions of Earth experience the shortest day of the year — the winter solstice.

The second solstice occurs on December 21 or 22 when the north pole is tilting 23.5 degrees away from our Sun and the south pole is inclined toward it. This is the shortest day of the year in the northern hemisphere — the northern hemisphere winter solstice.

Twice each year, during the equinoxes (“equal nights”), Earth's axis is not pointed toward our Sun, but is perpendicular to the incoming rays. During the equinoxes every location on our Earth (except the extreme poles) experiences 12 hours of daylight and 12 hours of darkness. The vernal or spring equinox occurs in the northern hemisphere on March 21 or 22 (the fall equinox of the southern hemisphere). September 22 or 23 marks the northern hemisphere autumnal or fall equinox.

National Maritime Museum

As Earth orbits our Sun, the position of its axis relative to the Sun changes. This results in a change in the observed height of our Sun above the horizon. For any given location on Earth, our Sun is observed to trace a higher path above the horizon in the summer, and a lower path in the winter. During spring and fall, it traces an intermediate path. This means that our Sun takes a greater amount of time tocross the sky in the summer and a shorter amount of time in the winter. This effect is greater as you move toward the poles; people living near the equator experience only small changes in daylight during the year. The change is more extreme toward the poles.

During the northern hemisphere summer solstice, Earth is tilted such that the Sun's rays strike perpendicular to the surface at the Tropic of Cancer (23.5 degrees north latitude, corresponding to the tilt of Earth's axis). At (solar) noon, our Sun is directly overhead in this location (and at a decreasing height above the horizon north and south of the Tropic of Cancer). At locations north, our Sun will be at its highest position above the horizon and will take the greatest amount of time to cross the sky. All northern locations have more than 12 hours of daylight. All locations south experience less than 12 hours of daylight. Locations above the Arctic Circle (north of 66.5 degrees latitude; 90 degrees minus the tilt of Earth's axis) receive 24 hours of sunlight. Locations below the Antarctic Circle (66.5 degrees south latitude) experience 24 hours of darkness.

During the northern hemisphere winter solstice, the Sun's incoming rays are perpendicular to the Tropic of Capricorn at 23.5 degrees south latitude. The Sun's path is the lowest above the horizon in locations north of the equator, and these regions experience the shortest day of the year. Between the winter and summer solstices, daylight increases as Earth continues its orbit around our Sun.

During the equinoxes, sunlight strikes perpendicular to the surface at Earth's equator. All locations on Earth, regardless of latitude, experience 12 hours of daylight and 12 hours of darkness. The spring equinox marks the change from 24 hours of darkness to 24 hours of daylight at Earth's poles . In these extreme locations, our Sun moves above the horizon at the spring equinox and does not go below the horizon until the fall equinox.

Do other planets have seasons?
Yes! Other planets in our solar system experience seasons for the same reason Earth does; their axis of rotation is tilted. However, some planets — like Mars and Pluto — have elliptical orbits that result in more extreme variations in distance from the Sun as they revolve around it. This, combined with the axial tilt, causes greater seasonal variation.

Uranus has an extreme tilt of 82 degrees. It takes Uranus almost 84 Earth years to complete its nearly circular path around the Sun. The tilt means that the pole of each hemisphere is exposed almost directly to the Sun's rays during the summer solstice, and the opposite hemisphere is in constant darkness. Given Uranus' long period of orbit, this translates into a 20-year winter or summer!

*Summer solstice refers to the time the north pole of a planet is tilted toward the Sun.
Based on data from 1990.