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.
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.
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:
- The days get shorter in winter (when the northern hemisphere is
tilted away from the sun.
- 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.
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.