One might wonder what possessed nice young Mary Shelley to write as beastly, gruesome and depraved a tale as Frankenstein.

The answer, it seems, lies in the weather - or at least in the effect on the weather of the eruption of a volcano many thousands of miles away from the Shelleys and their friends. But let us start at the beginning.

Mary Wollstonecraft Shelley was born 200 years ago today - on August 30th, 1797. She was of good stock, as they say, being the only daughter of William Godwin, a well-respected philosopher of the time, and Mary Wollstonecraft, his wife.

At the tender age of 17, however, young Mary went a little wild: she eloped with her future husband Percy, and spent the next eight years wandering around Europe in the company of the poet and his disreputable acquaintances.

In the summer of 1816 she found herself in Switzerland, in the Villa Diodati on the shores of Lake Geneva, with Shelley, the picaresque Lord Byron, and Dr John Polidori, Byron's friend and personal physician.

That summer was a dreary one throughout the whole of Europe. The Continent, and indeed the world, was recovering from the upheaval which had begun with the revolution in France a quarter of a century before, and had ended with Napoleon's defeat at Waterloo the previous year.

But it is also a year remembered in the annals of meteorology in Europe and North America as "the year without a summer".

The explanation was simple. Three major volcanoes had erupted in the recent past: Soufriere on St Vincent's Island went up in 1812; Mayon in the Philippines followed suit in 1814; and the greatest of all was the eruption of Tambora on the island of Sumbawa in the East Indies in April, 1815.

These volcanoes had blasted millions of tons of solid matter into the atmosphere, which in due course worked its way into the stratosphere and spread out into a worldwide veil.

The result was the opposite of the greenhouse effect that has been so topical of late; the dust blocked the incoming energy from the sun, while interfering little with the outgoing radiation. The average temperature of the Earth dropped, global wind patterns were significantly distorted, and summer temperatures throughout Europe were 2 or 3 degrees below their normal values.

The weather, right through the summer months, was dull and cold and stormy.

Mary herself described the sequence of events: "At first we spent our pleasant hours upon the lake, or wandering on its shores. But it proved a wet and uncongenial summer, and the incessant rain often confined us to the house for days on end. `We will each write a ghost story,' said Lord Byron."

And so they did! The efforts of Shelley and Byron were quickly abandoned; Polidori produced a story called The Vampyre, which was good enough for many to attribute to Byron years later when Polidori himself had drifted into obscurity - and which is still in print.

BUT Mary's tale became a classic, beginning on a dreary, wet November night with "vast mists rising from the rivers and curling in wreaths around the opposite mountains whose summits were hid in uniform clouds," when Baron Victor Frankenstein infused the spark of life into his newly-assembled monstrous being.

It was published in 1818 as Frankenstein, or the Modern Prometheus, a bizarre Gothic novel about the man-made monster that turns on its creator.

As it happened, 1816 was an eventful year for Mary. When she and Percy had eloped two years previously she was not, as yet, Mary Shelley, but rather Mary Godwin.

They had left behind Percy's pregnant wife, Harriet, but Harriet fortuitously committed suicide by drowning in that dismal June, so Percy and Mary were able to return to London to be married in December.

In the early spring of 1818, however, harried again by creditors, ill-health and social ignominy, Shelley and his wife sailed from England to make their permanent home in Italy. But their respite there was brief.

A few months later Shelley wrote: "Alas I have not hope nor health, nor peace within, nor calm around."

And then, in July 1822, he set sail across the Gulf of Genoa in his small schooner Ariel to visit Byron in Livorno; on the return journey his frail craft was overtaken by a violent summer squall, and floundered before it was possible to reduce the area of sail.

The body of Percy Bysshe Shelley, aged only 30, was found on the seashore some days later: they cremated it in situ, and his ashes were buried afterwards in Rome.

Mary Wollstonecraft Shelley published several other books, and lived on in rather straitened circumstances until 1851.

None of her other work, however, was as enduring as the story that describes the bizarre adventures of Frankenstein and his creation.