Archives For April

2016-05-16_10-57-06

How can there possibly be a link between a modestly cool month in Reigate and the earliest start to the melt-season in Greenland, the devastating wild fires in Canada and the seventh hottest-ever global month in succession?

April summary weather statistics for Reigate

  • Average Temp 8.2C
  • Tmax 17.7C
  • Tmin 0.1C
  • precipitation 43.4mm (local Reigate) SE PPT 55mm
  • sunshine 140.4 hours
  • Max wind gust 30mph
  • average wind bearing 199 degrees

Reigate, like the UK as a whole, had a cooler than average April at 8.2C. The town even experienced some unusual snow showers on 26 April in a cool northerly air flow.

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The cool month for the UK is in stark contrast to the bulk of the planet which experienced a much much warmer month than average, at over 1.1C warmer than any previously measured April.

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Astonishingly, this is the seventh month straight that has brought record breaking global temperature anomalies.  This continuing succession of warm months globally should be of concern to everyone.  More on this below.

Back to the UK… The Central England Temperature came out at 7.5C,  0.4C below average, and the UK mean was even lower at 6.5C, 0.9C below the long term average.

Rainfall was about average in Reigate with around 40mm of rainfall.  The MetOffice SE figure came out at 55mm.

April was sunnier than usual with a total of 140 hours of sunshine.

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This continues the trend of drier and sunnier Aprils in the UK in recent years.

The first half of April was unsettled with most of the rain falling associated with low pressure systems and fronts. The second half of April saw an unusual cool period as northern blocking over the Arctic sent cool northerly winds south with attendant sunshine and showers.

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Globally April was the warmest ever April on record.  An anomaly of 1.1C sent the Paris target of keeping global temperatures below 1.5C into grave doubt as this is the 7th month in succession to yield much higher temperatures than ever. This is now being dubbed a “Climate Emergency” because of the sudden and rapid increase in global temperature to levels not expected to occur so soon.

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The UK / NW Europe was about the only part of the planet, with NE Canada, to record below average temperatures.

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The cool spot over the UK  was due to northern blocking (high pressure) over the Arctic. As pressure rose over the Arctic, cold air pushed out into mid-latitudes.

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It is a matter of chance where high pressure and low pressure set up that determines where cold polar air penetrates in these northern blocking scenarios.  This time the pattern sent the cold air to the UK and N Europe.  The Northern Hemisphere as a whole saw anomalously low snow cover as a result of incredibly high temperatures elsewhere.

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Arctic Amplification, where the northern latitudes experience highest rates of warming, is well documented and of increasing concern to climate change.  It is acting as both a response and a further driving force behind rapid climate change.

Temperatures rocketed over the Arctic this cold season with temperature departures over 3C widely across the Polar regions.  The Greenland ice sheet experienced one of the earliest starts to the ice melt season on record.

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Arctic Sea cover also recorded another record low maximum winter extent.

“On March 24, Arctic sea ice extent peaked at 5.607 million square miles (14.52 million square kilometers), a new record low winter maximum extent in the satellite record that started in 1979. It is slightly smaller than the previous record low maximum extent of 5.612 million square miles (14.54 million square kilometers) that occurred last year. The 13 smallest maximum extents on the satellite record have happened in the last 13 years.” NASA 

This is both a response and a further catastrophe for climate change.  As snow and ice melt in the Polar regions there are connections with further warming as darker sea and land surfaces heat up more readily.

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This Polar warming itself is connected with a weaker jetstream as latitudinal temperature gradients in the atmosphere decline.  It is temperature gradient, especially in Mid-Latitudes, that generates the driving force behind the jetstream.  A weaker jetstream is said to cause more blocked atmospheric conditions as it meanders with greater amplitude in a meridional pattern that locks in swoops of northerly and southerly winds. More extreme weather is caused as these pressure patterns persist for longer.  Sweeps of warmer air penetrate into the Arctic, melting more ice over Greenland and, for mid-latitudes, cooler dry Polar air leaks out causing damaging late frosts and wild fires.

So, whilst it seems tenuous to connect these far-off events to our own rather benignly cool April, it is still important to think globally when considering how our own weather links to increasingly extreme weather elsewhere.

http://www.theguardian.com/environment/2016/may/16/april-third-month-in-row-to-break-global-temperature-records?CMP=share_btn_tw

http://www.bbc.co.uk/news/science-environment-36212145

April 2014: quick monthly weather summary from our weather station in Reigate, Surrey, UK.

Another relatively dry month with 43.6mm of rain falling on Reigate, about the same as April 2013. Please note that we are still calibrating the automatic tipping bucket rain gauge which is possibly under-reading by around 10%.  Calibration is a tricky affair and we want to get it right.  We are supplementing all data by using monthly totals using our CoCoRahs manual rain gauge which is, by default, a more accurate measure of total rainfall. Raw data is available on our data page here.

There were no dramatic warm-ups or cool-downs in temperature during the month.  April 2013 saw a fairly significant warm-up from mid-month but April 2014  flat-lined with no especially warm or cold temps.  It was noticeable that air temps never dipped below freezing.  This allowed the average monthly temp to climb higher than April 2013 overall, despite a lower Tmax.

Tmax 20.4c

Tmin 1.8c

Average temp 11.2c

Total rain 43.6mm (CoCorahs)

Sunshine 138.7 hours

A high pressure dominated the South of England for the middle of the month and reduced rainfall totals.  Later in the month April showers delivered moderate rainfall totals.  There were no significant thundery episodes.

April is the month when many trees come into leaf in the UK. Below are the school lime trees in the playground taken at either end of the month.

April is also a month of lengthening days and misty mornings with heat building at the surface triggering fluffy cumulus clouds in the afternoon.  Some of these produce enough instability to produce “April showers” in cool maritime airstreams with warming at the surface.

Away from the UK, the US experienced a significant tornado outbreak at the end of April when an active cold front swept clean across the entire country and triggered 133 identified tornadoes in the Mid-West / tornado alley and at least 40 deaths.  The Mayflower tornado (EF4) on 27 April in Mississippi tore through the state leaving a 41 mile trail of destruction and 16 deaths.  Tornado damage cost over US $1 billion. http://www.weather.com/news/tornado-central/tornado-scars-april-outbreak-20140507

Excellent article here on the US tornado outbreak April 2014.

In stark contrast, but related to extreme weather elsewhere, the drought in the SW of the USA continued through the month.

Other weather news includes a potentially mega outbreak of the El Nino this year.  The El Nino is a warming of the eastern Pacific Ocean waters.  This huge ocean warming is a major fly-wheel of global climate and the ENSO is a significant gear change that impacts weather systems globally.  The warming is natural (nothing to do with AGW) but has major impacts on weather extremes around the world and could, potentially, make 2014/2015 the warmest year ever. More detail here on El Nino from the great weather guru Gav at http://youtu.be/VJXpvv0P2pw

The map below shows the usual weather relationships expected during the course of a major El Nino event (ENSO).  Note that the UK and Europe does not experience a known / correlated weather impact: i.e. an El Nino does not have a regular impact on our weather.  However, this does not mean we will not experience some impacts from this major global weather gear change. All things considered, we are lucky in the UK to have less severe weather than many countries around the world, albeit this can be a little frustrating for weather enthusiasts who like a bit of exciting non-injurious weather occasionally!

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After a cold March, April jump-started Spring 2013. The key moment when spring started was thundery showers on 12 and 13 April: a cold front wrapped round a depression broke the Arctic block. This allowed much warmer upper air temperatures to stream in from the south and west pulled in as the jetstream meandered north of the UK. Temperatures leapt from around 10ºC to 18ºC daytime max over the 12 and 13 April. April was a dry month with only 38.6mm of rain compared to 68mm in March and 114mm in December 2012. Nearly half of the monthly total rainfall fell on the 12&13 April during thundery weather along a cold front which swept away the cold and brought in warmer and more humid tropical airmasses. April rainfall after that wet spell amounted to less than 6mm with sunny and dry weather dominating through to the start of May.  

Here is a movie of clouds seen over Reigate in April! 😉