Archives for May 2012

The very warm spell which has dominated our weather in recent days has come to an end, with rain affecting much of northern England today.

It's a wet end to a month which has seen a remarkable change from extreme cold to extreme warmth.

And it's a good example of how statistics can hide big swings in the weather.

Because statistically temperatures, sunshine and rainfall for May have turned out to be around normal.

But this masks a very cold first half of May, with some sharp frosts and heavy rain which brought an end to drought restrictions in parts of Yorkshire; and a very warm second half of May which saw temperatures of 27C (81F) across the region.

Weather Action's widely publicised forecast of the coldest, or near coldest May in 100 years in parts of the country, which appeared in The Express, has turned out in the end to be wide of the mark - despite what was a very cold first half - because of the heat which developed in the last 10 days.

The switch from one extreme to another in May follows March which was one of the warmest, driest and sunniest on record, and April, which was the wettest on record, continuing the very polarised weather we have been experiencing.

All eyes are now on June - and there's a strong indication that the first half of the month will be very unsettled, with low pressure bringing showers or longer spells of rain, some of which may be heavy at times.

More specifically for the Jubilee weekend northern Britain should have mostly dry weather, but parts of southern Britain could experience a washout on Sunday, with some of the rain continuing into Bank Holiday Monday.

Just how far north the rain gets on Sunday is very uncertain at the moment, but there is a risk of it pushing as far north as the Humber before it returns south later in the day - although the precise northern extent is open to considerable doubt at the moment.

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Research published this week which identifies thousands of sites in the Arctic where methane gas is being released into the atmosphere could have serious ramifications for global warming.

Methane is twenty times more powerful a greenhouse gas than carbon dioxide and it's estimated that there's billions of cubic metres of natural methane gas trapped underneath huge areas of perma-frost in Siberia, and under the frozen wastes of the Arctic.

The Arctic has warmed more quickly than any other area of the planet, and has led to year on year decreases in the extent and thickness of ice cover since satellite measurements began in 1979.

As the ice has melted, naturally formed methane gas which would otherwise be trapped under the ice is bubbling to the surface, according to the research in the journal Nature Geoscience.

The research is very important in that such 'feedback mechanisms' - whereby initial warming caused by carbon dioxide leads to more warming by, in this case, natural methane gas - has been widely predicted by scientists, and could lead to an acceleration of warming in the coming decades.

Simple physics shows that an approximate doubling of carbon dioxide in the atmosphere would cause an approximate 1 degrees Celsius rise in global temperatures, which in itself would be manageable.

This is a point which most scientists - on both sides of the global warming debate - agree on.

After that, most climate scientists believe that the real danger is that feedback mechanisms kick in, leading to further warming, an example of which is methane gas release.

Another example is evaporation from the oceans, releasing water vapour, itself the most powerful greenhouse gas of all, which would also lead to more warming.

Climate sceptics though argue that not all feedback mechanisms will be positive, in other words, they may not lead to further warming, and cause cooling.

An example of a negative feedback would be that evaporation from the oceans, forming increased levels of water vapour (or higher humidity), would cause more cloud development.

And a cloudier planet would reflect sunlight back out into space and cause the planet on average to cool, according to climate sceptics.

It's still early days and work is on-going into the vital area of climate feedback mechanisms.

But this research does give an ominous first glimpse of what may lie in store in the coming decades should global temperatures rise, because of possible future warming caused by man-made carbon dioxide.

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At long last warm weather is returning to much of the country for the first time since March, after weeks of cold, wet weather.

High pressure will dominate our weather all week, with temperature reaching 23C (73F) across inland parts of Yorkshire and Lincolnshire as early as tomorrow.

The chart below shows tomorrow's estimate maximum temperatures based on the American GFS computer model.

That said, even coastal areas are likely to see some warm sunshine at times too.

During Thursday a weak cold front will introduce drier air, meaning clear skies developing even along the coast, for Friday and Saturday.

The air will be cooler, but with unbroken sunshine temperatures inland will still reach 17C (63F) making it feel warm, but with a brisk easterly wind there'll be a noticeable wind chill along the coast.

Looking further ahead into the last week of May, normal service looks set to resume with unsettled weather returning.

This change will probably occur during Sunday or Monday, although the exact timing of this will be subject to uncertainty until later in the week.

So the message is very much enjoy the next few days of summer while they last.

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The average global temperature for April on the UAH satellite measure jumped to +0.295C above the 30 year running mean.

Significantly, April Arctic sea ice was close to the 1979-2012 average, and the highest in April for over 10 years, shown below.

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Record rainfall in April and a wet start to May has improved East Yorkshire's ground water levels to such an extent that Yorkshire Water has today announced that they are now back to normal for the time of the year.

This is a remarkable turnaround considering stocks were 20% depleted only 6 weeks ago.

With more rainfall forecast during the rest of May, pressure will mount on the Environment Agency to lift the drought order that currently affects the county.

Droughts have a history of ending far quicker than experts anticipate.

Back in the summer of 1976, experts talked about endless months of rain being needed to restore ground water levels, and that the situation may take many years to fully get back to normal.

Despite the Met Office in August 1976 forecasting a 'dry September, with no clear indications of a wet autumn anywhere', September and October were extremely wet that year, so much so, that by the end of October, soil moisture deficits were normal, and by November, most bore holes were at or above average.

In the end the famous drought of 1976 was over far quicker than anyone anticipated.

And from a forecasting point of view, history seems to be repeating itself.

The Met Office seasonal forecast issued in March, and seen by the Environment Agency at the time, stated that April, May and June would see a continuation of the dry weather, with April being the driest of the 3 months.

Moreover, the Met Office said that there was only a 10-15% chance of seeing the heavy rainfall that we experienced in April, which turned out to be the wettest on record.

One main difference with 1976 is that peak evaporation occurs in June, July and August, and rainfall, should it occur in these months, would have to be heavy for it to outweigh moisture which is lost through evaporation, and the moisture taken by plants and vegetation.

In 1976, heavy rainfall fell through autumn, a time when evaporation rates fall sharply, leaving much more of the rain to replenish stocks.

According to Anglian Water, groundwater levels in Lincolnshire have recovered, but they are starting from a much lower position than those in East Yorkshire, and so remain depleted and will take longer to get back to normal.

Nonetheless the fact that East Yorkshire's groundwater levels have now returned to normal shows that once long dry periods come to an end, the prolonged heavy rain that follows can improve the situation quicker than anyone expects, like in 1976.

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News from NASA this week that on-going weak solar activity will continue, leading to the weakest solar cycle in around 100 years, could have important implications for the UK's weather.

Research over the last few years has pointed to a link between low solar activity, and incidences of 'blocking' weather patterns.

In three out of the last four winters, the jet stream has been much weaker than normal, leaving us more exposed to colder air from the north and the east, as 'blocking' areas of high pressure become established.

In these situations, winters are not just colder than average but drier, too.

It's these weather patterns that have led the government to warn of the risk of stand-pipes in the streets next summer should winter again be dry in southern and eastern areas.

It suggests the government could be right to be worried about the risk of low winter rainfall in coming years, and the implications for water supplies in parts of the UK.

Normally our winters are dominated by a strong jet stream, bringing mild air and rain in from the west.

Winter rainfall is crucial in that evaporation is very low, and plants and trees don't take up much of the water in the ground, and this is the time when ground water levels and reservoirs normally recover.

In summer, evaporation and water taken up by plants and trees in all but the very wettest summers outweighs any rain that falls.

The theory that weak solar activity could impact our weather gained credibility when Professor Mike Lockwood of Reading University published research two years ago.

He found a correlation between weak solar activity and the occurrences of 'blocking' weather patterns, leading on average to colder & drier winters.

Professor Mike Lockwood said that weak solar activity does not guarantee colder winters, but suggests that such winters could become more frequent.

There could also be implications for weather at other times of the year.

The intense heat wave that Russia and parts of Eastern Europe experienced in summer 2010 was also caused by a similar blocking high.

Professor Lockwood told the New Scientist following this event 'there's enough evidence to suspect that jet stream behaviour is being modulated by the sun.'

And it's certainly a headache for computer models that predict our future climate based on increasing levels of man-made greenhouse gases.

They are unable to model the impact of weak solar activity, simply because the precise mechanism of how this affects climate patterns is unknown.

These climate projections suggesting that winters will become milder and wetter, with summers drier and warmer, have been of little use to the water authorities in the south and east of the UK who are trying to cope with successive dry winters.

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