By the time the last snowflakes have settled today, this may be the worst snowfall in southern England since the savage winter of 1962-63.

About 1ft of snow is expected to be lying on the ground in London this morning and there is no sign of it melting soon, which means that the snow showers expected later in the week will pile it even higher. Yet it was only last week that Britain was bathed in balmy, mild winds. What went wrong?

In short, the wind switched from west to east. Westerlies swept warm air off the Atlantic, where the tail end of the Gulf Stream pumps the heat equivalent to a million power stations over Britain, usually keeping us several degrees warmer than equivalent latitudes in Canada.

But over the weekend the winds shifted around to blow from the east, the source of our coldest winter weather. As the old saying goes: “When the wind is in the east, ’tis neither good for man nor beast.”

Even the forecasters were taken aback by the size of the snowfalls during Sunday night and yesterday. London and the Home Counties were hit hard because the cold air from the continent was funnelled into the Thames Estuary, blowing into the city and the Thames Valley and dumping huge amounts of snow as it went. The depression that delivered it began in France and worked its way along the East Coast after hitting the capital.

As for the outlook this week, the only words for it are pretty dismal.

That same depression looks set to pirouette around Britain and pay the South East another visit on Thursday, this time dragging down cold air from the Arctic. The Met Office has issued an alert for a chance of snowfalls across a large swath of central and northern Britain tomorrow night and on Thursday morning, although the fall will probably not be as great as that at the start of the week.

Even though daytime temperatures will ease slightly, there is no thaw in prospect as sub-zero temperatures at night keep the snow on the ground intact. The Met Office offers one glimmer of hope – a possible return to our milder, wetter westerly winds next week. However, with so much snow on the ground any thaw accompanied by rainfall could set off flooding.

So why has this winter suddenly turned so nasty? The blame lies in the stratosphere, 6 to 30 miles (10km to 50km) high, where the winds usually blow around the globe from west to east.

Forecasters spotted something strange two weeks ago, when the temperature of the stratosphere over the Arctic suddenly shot up by about 50C, an extraordinary event. This sounded alarm bells because it signalled that the winds had done a somersault, blowing from east to west.

The raised temperature was explained by the hugely increased volume of air in the stratosphere.

Although this easterly wind began at the top of the stratosphere, 30 miles high, it worked its way down over the following fortnight into the bottom layer of the Earth’s atmosphere, the troposphere, where all our weather happens, and sent easterly winds racing across into Britain.

“It’s all about which way the winds blow,” Adam Scaife, at the Met Office’s Hadley Climate Centre, said. “When the stratosphere really puts its foot down, the troposphere responds by turning easterly, and so cold air from Siberia blows over us.”

The bad news is that it can take several weeks for the stratosphere to return to normal. The last time this happened was in the winter of 2005-06, when Europe froze, buildings collapsed under the weight of snow and the cold lasted well into March. Britain escaped that time, but the stratosphere reversal is particularly strong on this occasion and there can be no escape.

The outlook for the rest of February is not good either. It takes several weeks for the stratosphere to relax and return to its usual westerly direction.

“We expect the cold to last the whole of February, on average, with dry or average rainfall,” Dr Scaife said.

But before you think that all of this has put paid to any notion of an early spring, remember that this is by no means a terrible winter yet.

For that we have to go back to 1991, season of the notorious “wrong kind of snow” when British Rail claimed that its locomotives could not work because the snow was too powdery to be removed from the tracks.

Worse still was the winter of 1962-63 – the coldest for more than 200 years – when root crops were dug out of the ground with pneumatic drills. It was so cold then that the sea froze, icebreakers were brought in to keep some ports open, a car was driven across the Thames at Windsor and 49 people died outdoors of the cold, many trapped in vehicles during blizzards. In fact, such was the severity of the weather that there was talk that a new ice age might be on the way.

But perhaps the most apocalyptic of all winters in modern history came in 1947, when snows were so deep that buses were lost in drifts. Coal was rationed, causing widespread power cuts, food was rationed more severely than during the Second World War, and the country was left bankrupt. The snows eventually thawed in March but unleashed some of the worst floods of the century.