Heavy rainfall patterns over last thirty years

The Climate Change Summit is being held next week in New York and the World Meteorological Organization recently made this statement “It’s not too late to prevent dangerous climate change and preserve the planet for future generations, but it may soon be”.

After the wettest September day I have recorded since 1984, over the nighttime of 18th September (40.6mm), it is interesting to analyse the data on heavy daily rainfall over this extended period.

There is a rising trend for the incidence of daily rainfall in excess of 10mm. In the 1980s it averaged around 22 days per year. The average has now risen to approximately 29 days a year with the greatest occurrence of 45 days in 2012.

Investigating higher daily rainfall totals reinforces the suggestion that our climate is changing. Taking the last thirty years in 10-year blocks the incidence of heavy daily rainfall of 30mm and above was 6 occasions between 1985 and 1994, 9 in the intermediate ten years and 14 in the last ten years, with three months still to go in 2014.

Researching the occasions when the daily rainfall is equal to or exceeds 40mm highlights the change. There have been 6 occasions when I have recorded daily rainfall above 40mm and they have all occurred since October 2000. The record was set on December 23rd 2013 when 46.4mm was measured.

The average local temperature has been rising over this 30-year period. In the 1980’s to mid 1990’s the average was 9.1C, but the rising trend since then brings the current average annual temperature to 9.7C. With warmer air holding greater quantities of moisture it is not surprising that the incidence of heavy, localised daily rainfall is rising.

However, scientists have struggled to explain the so-called pause that began in global warming in 1999, despite ever increasing levels of CO2 in the atmosphere.

The latest theory says that a naturally occurring 30-year cycle in the Atlantic Ocean is behind the slowdown. The researchers say this slow-moving current could continue to divert heat into the deep seas for another decade.

Record rainfall for a September day

The total rainfall for the last twenty-four hours was 40.6mm which fell during the many hours of thunder activity last night. This amount is a record and exceeds the previous high for September of 35.6mm, set exactly 30 years ago today – 19th September 1984.

Fifteen days without rain

The continuous period without rain now totals 15 days. There are 9 occurrences of long continuous dry spells in the last three years. A lengthy period of 22 days was noted in August 1995 and April 2011 but the record stands at 24 days, which occurred in April 1997.

Longest continuous dry spell for two years

There have now been 13 continuous days without rain, which is the longest dry spell since May 2012. There were three occasions in 2014 with 12 continuously dry days but July 2013 had 19 dry days and April 2007 brought 21 continuously dry days and a total rainfall for the month of just 7.6mm.