A New Kind Of Rain

By Paul Homewood

Verity Jones, over at Digging in the Clay, reminds me of an interview with Lord Smith, the politician formerly known as Chris Smith, in the Sunday Telegraph.

According to Smith, a former Environment Secretary and now head of the Environment Agency

Last year taught us that weather patterns are getting more extreme,” says Lord Smith. “If you’d said to me a decade ago that we’d have a year in which the first three months would be facing a serious prospect of very severe drought, but we’d then have nine months of the wettest period since records began, I’d have just said, ‘No, that sort of extreme weather does not happen here in Britain.’ Increasingly, it does.

The weather is highly unpredictable and presents new challenges, he says, adding: “We are experiencing a new kind of rain.”

It may sound like an excuse from a railway company, but Lord Smith insists that it is true. “Instead of rain sweeping in a curtain across the country, we are getting convective rain, which sits in one place and just dumps itself in a deluge over a long period of time. From the point of view of filling up the rivers and the drains, that is quite severe.”

According to Wikipedia,

Convection occurs when the Earth’s surface,mainly in the equatorial region, within a conditionally unstable, or moist atmosphere, becomes heated more than its surroundings, leading to significant evaporation . Convective rain, or showery precipitation, occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective.

You will note that there is an immediate disconnect – Smith claims the rain falls “over a long period of time”, not the “relatively short time” defined in Wikipedia. There is, of course, a second problem. Summer temperatures last year in the UK were well below normal, so convection should have been much reduced.

http://www.metoffice.gov.uk/climate/uk/actualmonthly/

Nevertheless, if Smith is right, his claims should be borne out by the monthly rainfall statistics for June to August, as logically that is when the convective effect should be at its greatest. It is also the summer months that have seen rainfall trends on the increase in recent years.

http://www.metoffice.gov.uk/climate/uk/actualmonthly/

So let’s take a look at the England & Wales Rainfall Series, that is maintained by the Met Office and goes back to 1766. The following graphs show the monthly rainfall totals for each of the three months.

The rankings for 2012 were :-

June – 1st (out of 247)

July – 33rd

August – 80th

So the following points stand out.

• Even though the wettest June occurred last year, June 1860 and 1768 were almost as wet.
• Although wetter than average, July and August 2012 were by no means exceptional months, when placed in the historical context.
• In none of the months is there any indication that rainfall in recent years has been unusually high, or is exhibiting any particular trend.
• The summer, as a whole, was the wettest since 1912. However, this has occurred largely because all three months were wetter than normal, with no really dry interludes in between. This simply reflects the inherent variability of English weather, the coincidence of events and the workings of the jet stream, rather than any deep climatic changes.

It is not surprising that Smith attempts to connect last year’s rainfall with climate change, particularly when he is responsible for the UK’s flood defences and the problems experienced last year. However, if there was any basis to his claims, the monthly charts would show evidence of it. They don’t.

Footnote

I thought it worthwhile to repost the Met Office’s summary for June 2012.

The weather was dominated by low pressure over or close to the UK, with associated weather fronts. These brought rather cool days, some very large rainfall totals and also some strong winds early in the month. There was an almost complete absence of warm, settled spells.

The UK mean temperature was 0.7 °C below the 1981–2010 average and it was the coolest June since 1991. Daily maximum temperatures were well below normal, particularly in many central and eastern areas, with few warm days.

http://www.metoffice.gov.uk/climate/uk/2012/june.html

Not exactly tropical!