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Why is it Winter

Subject: Science

Description: What is Winter, Why is it Winter, Why doesn't it Snow at Christmas?

Posted by David Caldwell on 20/10/13 at 14:24

Why is it winter

At the time I am writing this the days are getting shorter, the evenings darker and the weather colder.  Have you ever wondered why this is?  The main culprit is the angle of the earth’s tilt.

The earth journeys around the sun with its axis at a tilt of 23.439 degrees and it takes an astronomical year to complete the trip.  Because of the tilt at certain times of the year one hemisphere gets more daylight than the other.  This is why it is possible to have summer in the northern hemisphere and winter in the southern.

The earth’s orbit around the sun passes through four astronomical waypoints which mark our seasons, we have the Winter Solstice at around the 21st of December, the Spring Equinox at around the 20th of March, the Summer Solstice at around the 21st June and the Autumn Equinox at around the 21st October.  The equinoxs mark equality of daylight while the solstices mark extremes of shortness and length of daylight hours.

Why is winter colder?

At winter the when the sun is lower, light hits the earth at more of an angle.  Imagine pointing a laser straight at the ground, you should see a small dot of light concentrated in a confined space, now point it at the ground at an angle and the light should diffuse over a wider area. The laser still has the same energy but it is spread out, such is the winter sun which has less ability to warm the earth’s surface.  During summer when the sun is at a higher angle light is more concentrated on the surface and the earth warms.

Why is it colder in January than December if the shortest day is in December?

It’s all because of energy.   The earth is warmed by the sun and absorbs its energy as temperature, at winter it loses this energy but the cooling takes a while.  It gets cold because the earth is losing more heat than it receives

If you straighten the tilt of the earth to make it represent the world as we perceive it, summer is when the sun is highest in the sky and winter is when it is at its lowest.

Does the Southern Hemisphere Have Hotter Summers?

It may seem counter intuitive but the northern hemisphere has hotter summers than the south, the reason for this is that the northern hemisphere has more land which heats and cools faster than water.  The southern hemisphere in contrast contains more sea which absorbs the heat and warms slower.

Why doesn’t it snow at Christmas?

Over 300 years ago people used the Julian Calendar (the Russian Orthodox church still does), which celebrates Christmas not on the 25th December but on the 7th of January.  January is a much colder month than December and the probability of snow much greater, the traditional white Christmas might be a memory of these times.  The little ice age occurred from around 1250 to 1850 which experienced a significant level of cooling which meant that winters were much colder in the past, so despite the occasional cold spell we are enjoying much milder winters than our ancestors.

Does distance from the Sun matter?

The earth orbits the sun in an elliptical orbit and is nearest the sun (perihelion), at around the 3rd of January and furthest from the sun (aphelion), at the 4th July.

Many people believe that the seasons are caused by the distance from the sun but at perihelion in January there is only 7% increase in solar radiation, it is also the coldest month for most in the northern hemisphere. 

Northern Hemisphere Bias

While I am writing this I am aware of the Northern Hemisphere bias which makes me wonder if the predominant civilisations came from the Southern half of our planet, maps and astronomical terminology would be reversed.

Keywords: Winter, Summer, Seasons, Solstice, Equinox , Sun, Winter, Hemisphere, Northern, Earth