Why is winter so cold?

Why is winter so cold?

Investigate the reasons for the seasons...

We've had some snow on the hills already this year, even though it's only November!

Everyone loves building snowmen, sledging and throwing snowballs at unsuspecting passer-bys, but take a moment this year to wonder WHY we have the seasons at all.

What are seasons?

Here in the UK we are in the northern hemisphere of the Earth, a little bit more than halfway between the equator and the North Pole.  That means that we have our winter at the end of the year, between the end of November and the start of March.

aa Earth UK

Planet Earth (c) NASA

After winter comes spring, when all the bare trees start to grow their leaves again, animals come out of hibernation and the weather starts to get warmer (even if it does tend to get wetter as well).

Summer follows spring and in Britain we enjoy a few days of warm sunshine (if we're lucky) before autumn blows in, whips all the leaves off the trees and sends us running for our warm clothes again.

But while we are having our winter, snuggling up in front of the telly and waiting for Christmas, people living in the southern hemisphere are getting out their flip-flops and surfboards and heading to the beach. So how does that work?

What causes the seasons?

By far the most important thing causing the seasons is the fact that the Earth is not spinning upright like a top.  In fact, our planet tips over to one side by about 23.5 degrees.

http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/GEO_Globe.jpg/416px-GEO_Globe.jpg

Tilted globe (c) Christian Fischer (CC BY-SA 2.5)

But why would the Earth tilting affect the seasons and weather where we live?

Well, because the sun shines towards the equator, the tilt of the planet affects the way that the sun shines on our particular part of the world meaning that:

  1. The days get shorter in winter (when the northern hemisphere is tilted away from the sun.
  2. The sun is lower the sky during winter, which changes the angle of the light hitting us and how much heat energy we get from it.

For some more details about the way the tilt of the Earth affects the sunlight we receive have a look at the National Geographic activity here and at the picture below.

earth tilt

So why is winter cold in the UK?

In the end, the type of weather and temperatures we get during the different seasons is controlled by the average temperature during each day.

As the temperature is the result of the amount of heat energy we receive from the sun. The shorter days and the reduced height that the sun reaches during winter means that in the UK we are warmed up a lot less in the winter than in the summer.

The same is true of all the different countries in the northern hemisphere.  In fact, most countries that are as far north as the UK get much colder winters than we do.  If you travelled east from the UK without moving north or south you would get t places like Ufa in Russia where the average temperature in January is between -7°C and 19.5°C.

Maybe we should be asking why it is so warm during winter in the UK, but that's a topic for another time.

Take a look at the video below from the BBC which travels from Cichen Itza in Mexico to Cornwall in the UK to reveal the Earth's tilt and the way in which it affects the seasons.

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