Cold front brings change overnight Plus November Review

Monday was another gloomy day under thick, low cloud without any sunshine that saw the thermometer rise to 9.7C, just below average for November. There was a little light rain late afternoon and again at 21.00 as the very narrow cold front passed through bringing 0.3mm of rainfall. The monthly rainfall total rose to 71.2mm

As the cold front passed through the temperature slowly fell away to a minimum of 2.1C at 07.27 on Tuesday with evidence of a ground frost.

Tuesday after dawn brought variable, high cloud.

November 2020 Review.

A mild and moist month

The new month arrived with unsettled weather from the tail end of Ex hurricane Zeta crossing the country. The wind gusted to 38mph on the 1st with 5.9mm of rainfall and then 8.5mm on the 2nd. During the first two days of November the warmest day of the month was set producing a maximum of 15.5C, which was 5.5C above average and the warmest night of the month when the thermometer did not sink below 11.8C being 8.1C above average.

On the 3rd a huge buckle occurred in the jet stream allowing the influence of an intense high pressure system in the North Atlantic to bring fine, sunny and dry weather for five days. After so many dull, wet and sunless days it was a pleasure to have several hours of sunshine with light winds, a maximum gust of just 9mph on the 4th. However, the clear skies at night allowed the thermometer to fall low with ground frosts on two nights, the coldest being -1.7C in the early hours of the 4th. Fog also formed in the early hours of the 5th and 6th limiting visibility to 200m.

The second week saw a succession of depressions advance from the Atlantic that combined with high pressure over Germany brought a flow of mild, moist air from Iberia that meant several days with above average maxima. The wettest day of the month occurred on the 20th with daily rainfall of 26.1mm making it the wettest day since 3rd October

On the 10th a sub tropical storm developed in mid-Atlantic that broke the record for the number of storms in 2020. Storm Theta was the 29th such storm to form, however, it drifted north-eastwards towards Madeira rather than the usual course towards the Caribbean. The 30th storm, that far exceeded the record set in 2005 for Atlantic storms, was Storm Iota that formed on the 14th, which wreaked havoc over Central America for several days with sustained winds of 155mph.

The middle of the month saw south-westerly winds persist with modest rainfall amounts but as the 20th approached high pressure built up and was in residence for several days bringing much calmer weather but allowing a light air frost under clear skies that saw the thermometer drop to -0.4C on the 23rd.

The global mean sea level has risen about 21–24 centimetres since 1880, according to the National Oceanic and Atmospheric Administration, with about a third of that coming in just the last two and a half decades. They state “As global temperatures rise, melting glaciers and ice sheets have combined with the thermal expansion of seawater to increase sea levels at an alarming rate”. Marlborough is far from the coast so will not experience any flooding from this phenomenon but low lying coastal areas could be devastated in the future as sea levels are forecast to continue rising. Oceans absorb about 90% of the excess heat trapped by greenhouse gases, which contributes to sea level rise. Currently a flotilla of autonomous floats, that have covered the seas since the early 2000’s, bob down as low as 2,000 meters every day to capture data at a depth. Research is under way to utilise these floats to accurately ascertain sea-level rise. Sound travels faster in water when it is warmer therefore differences in speed of sound can reveal the change in water temperature.

In addition, the Sentinel-6A satellite was launched on the 21st to also monitor sea levels that will provide “unprecedented accuracy”. It carries a radar altimeter, which measures the time it takes for radar pulses to travel to Earth’s surface and back again. A sister satellite will be launched in another five years to support these measurements.

The anticyclone eased away on the 24th allowing southerly winds to bring mild, moist air from Iberia over the country again with light rain showers and above average temperatures. At that point in the month we had only experienced one day with a below average maximum temperature.

The month finished with five days of below average temperatures with the coldest day of the month on the 27th when the thermometer struggled to reach 5.4C making it the coldest day since 5th March. That night a hard frost set in when the minimum fell away to -3.7C being 7.4C below average and the coldest night in 2020 equalling that recorded on 21st January.

November has been a relatively warm month with the mean temperature 1.2C above average making it the warmest since 2015. Analysing the data the mean daytime and nighttime extremes were 1.5C and 1.0C above average.

It has seemed a damp month with numerous days of rain and recently morning fog. A large proportion of the monthly rainfall, totalling 71.2mm, fell on the 14th when 26.1mm was recorded. The monthly total was 77% of the 36-year average or 21mm below.

Fog was observed during the early hours of eight days and an air frost recorded on seven mornings.

Autumn 2020 was a mild season with the mean temperature 0.3C above average and rainfall of 292mm being 51mm above average.