Above the equator, winter officially begins in December. But in many areas, January is when it sticks. Atmospheric researcher Deanna therefore explains the weather and climate factors that combine to produce wintry conditions at the turn of the year.
How the Earth’s orbit affects temperature and daylight
When the Earth orbits the Sun, it spins around an axis – imagine a stick going through the Earth, from the North Pole to the South Pole. During the 24 hours it takes for the Earth to rotate once on its axis, every point on its surface faces the Sun part of the time and away from it 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 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 the equator but hits 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 most dramatic at the poles, where light changes are most extreme. In summer, a pole receives 24 hours of sunlight, and the sun never sets. In winter, the sun never rises at all.
At the equator, which receives consistent direct sunlight, there is very little change in day length or temperature throughout the year. People who live in high and middle latitudes, closer to the poles, may have very different perceptions of seasons than those who live in the tropics.
There is an old saying: “As the days grow longer, the cold grows stronger.” Why does it get colder in January even though we get daylight?
It depends on where you are in the world and where the air is coming from.
The Earth’s surface constantly absorbs energy from the sun and stores it as heat. It also emits heat back into space. Whether the surface warms or cools depends on the balance between how much solar radiation the planet absorbs and how much it radiates away.
But the earth’s surface is not smooth. Land usually warms and cools much faster than water. Water requires more energy to raise and lower its temperature, so it heats and cools more slowly. Because of this difference, water is a better heat reservoir than land – especially large bodies of water, such as oceans. That is why we tend to see greater fluctuations between hot 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 mid-winter since your position tilts away from the sun. Meanwhile, the areas that receive little sunlight continue to radiate heat into space. As long as they receive less sunlight than the heat they emit, they will keep getting colder. This applies especially 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 largely comes from places like the Arctic that don’t get much sun in the winter, you could be on the receiving end of bitterly cold air for a long time. It happens in the Great Plains and Midwest when cold air flows down from Canada.
But if your air comes over a body of water that maintains a more consistent temperature throughout the year, these fluctuations can be smoothed out significantly. Seattle is upwind from an ocean, which is why it is many degrees warmer than Boston in the winter, even though it is further north than Boston.
When do we start losing (and regaining) daylight?
This greatly depends on where you are located. The closer you are to one of the poles, the faster the rate of change in daylight. Therefore, 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 throughout the year. The rate of change in daylight is slowest at the solstice – December in winter, June in summer – and fastest at the equinoxes, in mid-March and mid-September. This change occurs when the area of Earth that receives direct sunlight swings from 23.5 N latitude—about as far north of the equator as Miami—to 23.5 N latitude, about as far south of the equator as Asunción, Paraguay.
This satellite view captures the four seasons. On the equinoxes, March 20 and September 20, the line between night and day is a straight north-south line, and the sun appears to sit directly above the equator. The Earth’s axis is tilted away from the sun at the December solstice and towards the sun at the June solstice, scattering more and less light in each hemisphere. At the equinox, the inclination is at right angles to the sun, and the light is spread evenly.
What is happening on the opposite side of the planet right now?
When it comes to daylight, people on the other side of the planet see the opposite of what we see. Right now they are at the peak of summer and enjoying the largest amounts of daylight they will get for the year. I’m researching Argentine hail storms and Indian Ocean tropical cyclones, and both of those warm weather storm seasons are well into their peak right now.
But there is one key difference: the Southern Hemisphere has much less land and much more water than the Northern Hemisphere. Thanks 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 even though a place on the opposite side of the planet from your location may receive exactly the same amount of sunlight now as your area does in the 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 other side of the world — especially on a snowy January day.
This article was originally published on The conversation of Deanna Therefore on University of Illinois at Urbana-Champaign. Read the original article here.