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EI2GYB > ASTRO 06.01.23 14:41l 56 Lines 4651 Bytes #999 (0) @ WW
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Subj: How Earth's tilt creates short, cold January days
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How Earth's tilt creates short, cold January days
The winter solstice is past, but bundle up - January is when winter really arrives in many parts of the Northern Hemisphere.
By Deanna Hence, The Conversation | Published: Thursday, January 5, 2023
How does Earth's orbit influence our daylight and temperatures?
As Earth orbits the Sun, it spins around an axis - picture a stick going through Earth, from the North Pole to the South Pole. During the 24 hours that it takes for 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, 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 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 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.
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