VOLCANIC eruptions sending ash and gas into the atmosphere can have a massive effect on the weather.

In 1815 a huge eruption on the Indonesian island of Sumbawa caused freak weather conditions throughout the world.

Mount Tambora spewed out massive amounts of sulphur dioxide which combined with water vapour to form a sulphuric acid mist that reflected sunlight away from the earth.

That caused such a drop in temperatures that 1816 became known as ''the year with no summer''.

Crops failed due to low daytime temperatures, late frosts and abnormally high rainfall, provoking food riots, famine and disease.

In Ireland rain fell on 142 days that summer and across France the grape harvest was virtually non-existent.

In North America there was snow in June and lakes and rivers froze as far south as Pennsylvania during July and August.

It followed a smaller eruption in Iceland just over 30 years earlier that caused a thick fog of gas virtually wiping out the summer of 1783 across much of Europe and North America.

American statesman and amateur meteorologist Benjamin Franklin wrote of a ''constant fog'' over Europe and North America that year.

Forecasters say the cloud of ash drifting the across the UK at the moment will not produce anything so momentous but it could still affect the weather.

Brendan Jones, a forecaster with MeteoGroup, the weather arm of the Press Association, said: ''If you look back in history there have been some periods where the weather has been changed by big volcanic eruptions like Mount Tambora and Mount St Helens.

''They have been proved to lower temperatures. There is so much ash in the atmosphere that it reduces the amount of sunlight getting to the ground.

''If the ash remains in the atmosphere for weeks or months it can reduce temperatures slightly but we are talking about fractions of degrees.''

In fact the most noticeable result could be more spectacular sunsets as the sun's rays reflect off the ash cloud.

Mr Jones said: ''The sunset lights up the underside of the ash and you could see a glow from the ash in the sky.''

The eruption of Eyjafjallajokull, which has sent this cloud of ash into the sky, is the first in nearly 200 years and geophysicists fear it could trigger a much larger explosion of nearby Mount Katla.

Katla is described as ''enormously powerful'', and because it lies under a glacier its eruption would cause a huge glacial outburst flood and could spread its shadow over a much larger area.