above the equator, Winter officially begins in December. But in many areas January is taking effect. Atmospheric researcher Deanna Therefore explains the weather and climate factors that combine to create wintry conditions at the turn of the year.
How Earth’s orbit affects temperature and daylight
As the earth orbits the sun, it rotates about 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 is at times facing the sun and at times away from it. This 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 an angle.
When one of the poles faces the sun more than the other pole, that half of the planet receives more sunlight than the other half, and that hemisphere is summer. 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 the summer, a mast receives 24 hours of sunlight, and the sun never sets. In winter the sun does not rise at all.
At the equator, which receives consistent direct sunlight, there is very little change in day length or temperature throughout the year. People living at high and mid-latitudes, closer to the poles, can have very different ideas about the seasons than people living in the tropics.
There’s an old saying, “The longer the days get, the colder it gets.” Why does it 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 solar energy and stores it as heat. It also gives heat back to space. Whether the surface warms or cools depends on the balance between how much solar radiation the planet absorbs and how much it radiates.
But the surface of the earth is not uniform. Land typically warms and cools much faster than water. Water takes more energy to rise and fall in temperature, so it heats up and cools down more slowly. Because of this difference, water is a better heat sink than land — especially large bodies of water like oceans. That is why we tend to see larger fluctuations 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 increase significantly in midwinter as your location tilts away from the sun. Meanwhile, the areas that get little sunlight continue to radiate heat into space. As long as they get less sunlight than they emit heat, they get colder and colder. This is especially true over land, which loses heat much more easily than water.
As the Earth spins, air circulates in the atmosphere around it. If the air coming into your area is mostly from places like the Arctic that don’t get much sun in winter, you can get bitterly cold air for long periods of time. This is what happens in the Great Plains and Midwest when cold air pours down from Canada.
But when your air meets a body of water that maintains a more consistent temperature year-round, these fluctuations can be significantly offset. Seattle is downwind of an ocean, so it’s many degrees warmer than Boston in the winter, even though it’s farther north than Boston.
When do we start losing (and regaining) daylight?
This highly depends on your location. The closer you are to one of the poles, the faster daylight changes. Because of this, Alaska can go from little daylight in the winter to little darkness in the summer.
Even for a given location, the change is not constant throughout the year. The rate of change of daylight is slowest at the solstices — December in winter, June in summer — and fastest at the mid-March and mid-September equinoxes. This change occurs as the area on Earth that receives direct sunlight swings from latitude 23.5N — about as far north of the equator as Miami — to latitude 23.5N, about as far south of the equator as Asuncion, Paraguay.
This satellite view captures the four changing seasons. On the equinoxes of March 20 and September 20, the line between day and night is a straight north-south line, and the sun appears to be directly over the equator. The 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 into each hemisphere. At the equinoxes, the tilt is at right angles to the sun and the light is evenly distributed.
What is happening on the other side of the planet right now?
In daylight, people on the other side of the planet see the opposite of what we see. Right now they are at the peak of their summer and enjoying the greatest amounts of daylight they will get for the year. I’m researching hailstorms in Argentina and tropical cyclones in the Indian Ocean, and both seasons of warm-weather storms have just peaked.
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 experience less extreme temperatures than land in the Northern Hemisphere.
Although a place on the other side of the planet from your location now receives as much sunlight as your area does in the summer, the weather there can be different than 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 from Deanna Therefore at from the University of Illinois at Urbana-Champaign. Read the original article here.