Why do we have seasons?

Deanna Hence is an assistant professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. This story was originally shown on Conversation.

Above the equator winter officially begins in December. But in many areas, January is the time when it really happens. Atmospheric scientist Deanna Hence explains the weather and climate factors that together create winter conditions at the turn of the year.

How does Earth’s orbit affect our daylight and temperatures?

As the Earth orbits the Sun, it revolves around its axis – imagine a rod passing through the Earth, from the North to the South Pole. During the 24 hours it takes for the Earth to rotate around its axis, each point on its surface is part of the time facing the Sun and part of the time away from it. This is what causes daily changes in sunlight and temperature.

There are two other important factors: First, the Earth is round, although it is not a perfect sphere. Second, its axis is tilted about 23.5 degrees relative to its orbit around the Sun. As a result, light falls directly on its equator, but strikes the north and south poles at angles.

When one pole is directed more toward the Sun than the other pole, that half of the planet receives more sunlight than the other half, and it is summer in that hemisphere. When that pole is tilted away from the Sun, that half of the Earth gets less sunlight and it’s winter there.

Seasonal changes are most dramatic at the poles, where light changes are most extreme. During the summer, the pillar receives 24 hours of sunlight and the sun never sets. In winter, the sun does not rise at all.

At the equator, which constantly receives direct sunlight, there is very little change in the length of the day or the temperature throughout the year. People who live in high and medium latitudes, closer to the poles, can have very different ideas about the seasons than those who live in the tropics.

An old saying goes, “As the days go by, the cold gets worse.” Why does it often get colder in January even though we get daylight?

It depends on where you are in the world and where your air is coming from.

The Earth’s surface constantly absorbs the Sun’s energy and stores it as heat. It also emits heat back into space. Whether a surface heats or cools depends on the balance between the amount of solar radiation the planet absorbs and how much it radiates.

But the surface of the Earth is not uniform. Soil usually heats up and cools down much faster than water. Water needs more energy to raise and lower the temperature, so it heats up and cools down more slowly. Because of this difference, water is a better reservoir of heat than land – especially large bodies of water, such as the ocean. That is why we tend to see greater changes between warm and cold inland than in coastal areas.

The further north you live, the longer it takes for the amount and intensity of daylight to begin to increase significantly in the middle of winter, as your location leans away from the sun. Meanwhile, those areas that receive little sunlight continue to radiate heat into space. As long as they receive less sunlight than the heat they emit, it will get colder. This is especially true of land, which loses heat much more easily than water.

As the Earth rotates, air orbits it in the atmosphere. If the air entering your area mostly comes from places like the Arctic that don’t get much sun in the winter, you may be receiving very cold air for a long time. This happens in the Great Plains and the Midwest when cold air comes from Canada.

But if your air encounters a body of water that maintains a more uniform temperature throughout the year, these swings can be significantly equalized. Seattle is downwind from the ocean, making it many degrees warmer than Boston in the winter, even though it’s further north than Boston.

How fast do we lose daylight before the solstice and get it back after?

It largely depends on your location. The closer you are to one of the poles, the faster the speed of daylight change. That’s why Alaska can go from almost no light in winter to almost no darkness in summer.

Even for a particular place, the change is not permanent throughout the year. The rate of change in daylight is slowest at the time of the solstice – December in winter, June in summer – and fastest in the equinoxes, mid-March and mid-September. This change occurs when an area on Earth that receives direct sunlight ranges from 23.5 N latitude – about as north of the equator as Miami – to 23.5 S latitude, about as south of the equator as Asunción, Paraguay.

What is currently happening on the opposite side of the planet?

As for daylight, people on the other side of the planet see just the opposite of what we see. Right now they are at the peak of summer and enjoying the most daylight they will get in the year. I am researching Argentine hail storms and tropical cyclones in the Indian Ocean, and both warm weather storms are at their peak right now.

But there is a key difference: the southern hemisphere has much less land and much more water than the northern hemisphere. Due to the influence of the southern oceans, land masses in the southern hemisphere tend to have fewer extreme temperatures than land in the northern hemisphere.

So while a place on the opposite side of the planet from your location can now receive exactly as much sunlight as your area in summer, the weather there may be different from the summer conditions you are used to. But it can still be fun to imagine a warm summer breeze on the opposite side of the Earth – especially in a snowy January.

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