Q&A: Why is it so cold across Europe?
- 3 February 2012
- From the section Europe
Temperatures have hit new lows across Europe, where a week-long cold snap has killed scores. Meanwhile, forecasters have warned that the freezing weather will continue through the weekend.
Colin Seddon from the BBC Weather Centre takes a closer look at what is causing the phenomenon.
Why is it so cold across Europe at the moment?
For the answer, we need to look east, towards the vast plains of Russia.
Currently there is an intense area of high pressure over much of Europe, extending from Siberia in the east to the UK in the west.
This high pressure means clear, cloud-free days and, crucially, cloud-free nights. In the summer this same set up brought a heatwave to much of eastern Europe. But when it happens at this time of year, a heatwave could not be further from the truth.
What is the difference?
In the summer, the northern hemisphere is tilted towards the Sun. Summer days are longer than summer nights and the angle of the Sun in the sky means that a high proportion of the Sun's energy falls directly on the Earth's surface.
In the winter, the opposite is true. Nights are longer than days (more and more so the further north you go) and the tilt of the Earth's axis means that the northern hemisphere is furthest away from the Sun.
The result of this is that the Sun's rays deliver more of a "glancing blow" to the Earth's surface in the northern hemisphere, resulting in a less powerful heating effect.
Less heating for a shorter time means that we would expect the temperature to be lower - and that's what we see; winter is colder than summer.
So why so cold now?
The reason for this lies in the intensity of the high pressure. High pressure means that air from aloft is being pushed downwards.
As air descends, it dries out. This results in little or no cloud. By night, cloud acts as a blanket, stopping heat that is in the ground radiating away into the sky.
So at the moment, we have short days, low heating of the ground, long nights and nothing to stop the heat from the ground escaping.
If these conditions last for a long time, it just gets colder and colder. That is where we are at the moment - the intensity of the high means no cloud, no cloud means colder and colder temperatures.
Why is the rest of Europe so cold then?
In the northern hemisphere, air flows clockwise around a high-pressure system.
The current set-up, with high pressure over Russia, means that our air is coming from the east, bringing low temperatures with it.
The path that this air takes is crucial to whether it is warmed up or cooled down on its journey.
If the air passes over cold land, it may see little change of temperature. A path over warmer land may result in gentle warming. The biggest change of all comes when the air travels over a large area of (comparatively) warm sea.
So this is why different parts of Europe are seeing such a wide spread of temperatures, from lows of -11C (12F) in the UK to -29C (-20F) in Poland - it is all to do with the journey.