Tuesday, December 17, 2019

Reflecting on a record-breaking 2019

Back in July, the UK surpassed its maximum temperature record during an exceptional period of heat. Persistent low pressure sat across eastern Europe and Russia, alongside another area of low pressure that had stagnated just to the west of Europe (occasionally bringing unsettled and cloudy weather to parts of Spain and the UK), trapping an area of high pressure in between. 
MSLP analysis chart - valid 06 UTC 25th July 2019.

This allowed anticyclonic conditions to set in across much of central and western Europe, bringing clear skies, plenty of sunshine by day and light winds too. This weather combination ensured that by the 22nd July (Wednesday) temperatures were beginning to rise into the high 30's, with the peak heat concentrated across parts of France, Belgium and the border region between Germany and the Netherlands. Indeed, by the end of Wednesday (24th), the Belgium national met service (RMI - Royal Meteorological Institute) confirmed that the temperature in Kleine-Brogel had peaked at 39.9°C during the day. Meanwhile KNMI released a statement, stating that the official maximum temperature record for the Netherlands had been surpassed in several locations on the 24th, with the highest temperature (39.3°C) recorded in Eindhoven.

As for the UK, a breezy and rather cloudy start to Monday 22nd July gave way to increasing sunshine and lighter winds by Tuesday as an approaching cold front swung up towards the northwest of the UK. This allowed a southeasterly feed of air to develop through the day across much of the country.  Even at this point, numerous AWS (Automatic Weather Stations) on WOW recorded in excess of 35°C, although an official UK maximum temperature of 33.7°C was recorded at Northolt.

Intense overnight thundery downpours spread their way across England and Wales, clearing to brighter skies for a time on the 24th, although a band of cloud arrived from the west during the afternoon capping temperatures somewhat in across central and western parts. In the east, however, back garden AWS sites suggested 30°C+ temperatures across parts of East Anglia, Cambridgeshire and London. In fact, the official maximum was recorded at Writtle (34.3°C).

Visible satellite image taken on 25th July
By Thursday (25th), low pressure had retreated westwards a little and was now just to the west of Northern Ireland, allowing a more widespread stream of hot air from the continent to feed across the UK. Official Met Office observation sites across parts of Wales and Scotland recorded temperatures in excess of 30°C, whilst numerous official sites across England saw temperatures rise into the mid 30's.

With a maximum UK temperature record of 38.5°C previously recorded in Kent (2003), unsurprisingly, there was a lot of interest from a range of press outlets regarding the maximum temperature on the 25th July. 

Multiple WOW locations recorded temperatures in the high 30's, with a few recording in excess of 40°C by 3pm. Clearly many journalists were watching WOW with some interest at the time as some journalists even quoted temperature readings from individual weather stations. Likewise, Met Office Meteorologists were keeping an eye on WOW alongside official Met Office network temperature data.
UK WOW observations recorded 15:00-15:59 UTC 25th July 2019.

Whilst WOW data was not treated as official UK observational data, it did provide a steer as to where the highest temperatures could be found in real time (in three areas; Cambridgeshire, London and Kent). Temperature readings from WOW also filled in data sparse gaps in official Met Office networks (often in towns and cities), providing an indication of just how widespread the heat was.

WOW observations near Cambridge recorded 15:00-15:59 UTC 25th July 2019.
However, as the Met Office has a number of official networks, that range from official automatic observation sites (that report in as near to real time as possible) to Voluntary Climate Network sites, it wasn't possible for the Met Office to release the highest temperature from the 25th July on the same day. It was not until the next morning that the UK learnt just how high the temperature had risen on the 25th. As it turns out the highest temperature was actually recorded at Cambridge Botanic Gardens - a Voluntary Climate Network site, manually read once every 24 hours by volunteers. Consequently, this particular site did not report in on the hour or the day but delivered a dramatic pause overnight until the value was released into WOW at 0900 UTC on the 26th July.

A provisional maximum temperature of 38.7°C was released by the Met Office thereafter. Meanwhile, in the background, a rigorous series of processes were undertaken by Met Office staff in order to verify this figure. These processes included:
  •          A site inspection and sensor inspection
  •          Review of the temperature trace
  •          Discussion with the onsite climate observer to ensure the correct processes were followed on the day (i.e. the Stevenson Screen was not opened other than at the prescribed time)
The result was a verified record at Cambridge Botanic gardens of 38.7°C.

It is worth noting that there were some serious impacts of the exceptional temperatures seen in late July. For example, in the UK, the rail network was severely affected across south-east England with train cancellations and main lines closed out of London due to concerns with rail buckling. Meanwhile, damage occurred to overhead electric wires as they sagged in the heat, and trackside vegetation caught fire in several locations. In addition, the exceptionally hot weather made conditions difficult, particularly for the frail and elderly. Further afield, as temperatures rose into the 40's across parts of France, Belgium, Germany, Luxembourg and the Netherlands, a number of fatalities were reported.