Weather or climate change: what is happening?

Rare hazel dormouse is threatened by frosts hitting hazel trees that have bloomed early, killing its

Rare hazel dormouse is threatened by frosts hitting hazel trees that have bloomed early, killing its fruit. Image: SzymonBartosz/Getty Images/iStockphoto - Credit: Getty Images/iStockphoto

What are predictable deviations in weather versus climate change and what is the impact on Hertfordshire wildlife?

Marbled white butterflies. The lifecycle of caterpillars and other insects are triggered by temperat

Marbled white butterflies. The lifecycle of caterpillars and other insects are triggered by temperature, meaining they can emerge before birds hatch to feed on them. Image: 1255k/Getty Images/iStockphoto - Credit: Getty Images/iStockphoto

As part of its Nature’s Calendar programme, the Woodland Trust has been collating records of certain natural events, such as the first flowering of plants, insect emergence and bird migration. The science is called phenology, and thousands of volunteers have been sending in their observations since early this century. Now amounting to nearly three million entries, this is possibly the largest biological record of its kind in the UK. It is helping researchers to explore the effects on wildlife of weather and climate, including climate warming.

In 2019, the latest analysis available, some plant species flowered three to four weeks earlier than they did in the baseline year of 2001. Purple lilac for instance was 20 days earlier and blackthorn 27 days). In 2018, despite the Beast from the East bringing freezing weather in late February and early March, average flowering was still over a week earlier than in 2001.

This warming shift can severely affect species such as birds, whose nesting is triggered by day length, not temperature, Chicks may hatch too late to feed on caterpillars which are appearing earlier in the year, as insect emergence is dependent on temperature. Earlier flowering in hazel may mean frost damage is reducing the production of hazelnuts, in turn impacting the rare hazel dormice which relies on them for survival.

My last garden apple crop was a disappointment. Until recent years from August onwards we enjoyed the fruits of several trees selected for their early ripening apples. We have also stored plenty of our later ripening variety, Kidd’s Orange Red, which in good years kept the household supplied with eating apples until well into the new year. In 2020 and 2019 only our old cooking apple tree had reasonable numbers of apples – its blossom appears very late.

In 2019 blackthorn flowered 27 days earlier than the baseline year of 2001. Image: Tom Meaker/Getty

In 2019 blackthorn flowered 27 days earlier than the baseline year of 2001. Image: Tom Meaker/Getty Images/iStockphoto - Credit: Getty Images/iStockphoto

Think back to the height of lockdown last spring. The wonderful weather kept many people going through that time, helping them to enjoy getting outside if they could. Met Office figures show the season (measured from 1 March to 31 May) broke the record for sunshine in the UK, for which data have been kept since 1929. The country saw 626 hours of sunshine, exceeding the previous record by 70 hours.

In April and early May we enjoyed temperatures up to the mid-20s. Then in mid-May thermometers plummeted as cold northerly winds swept Britain. In my garden on the night of the 11th we recorded -2°C, and even lower two nights later. Tradition in Europe says we can blame this phenomenon on the ‘ice saints’ - St Mamertus, St Pancras and St Servatius - whose feast days fall on 11-13 May. When the garden was turned white twice in three days I feared the worst, and immediate damage was obvious to plants like tall bearded irises, just coming to flower. Fortunately later in the month the warm weather re-established itself.

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In 2019, when this cold period struck also struck, The Times reported it had occurred 60 times in the previous 100 years. A cold snap in mid-May is feared in wine-growing countries as it occurs just as the vines are flowering, and can wipe out the year’s crop. Tree fruit blossom is also very vulnerable to frost, resulting in few fruits or none at all. I live in the bottom of a dry valley in the Chilterns, so cold air tends to flow downhill and gather in our garden.

As the weeks passed last year, a few apples appeared on my early flowering trees, and the pear tree, which flowers earlier than the apples, was covered in fruit. The Kidd’s Orange Red had almost no emerging fruit, while the old cooker seemed to have plenty. Why were my different apple varieties affected differently? I can only assume that when the May frosts occurred the very early flowering varieties had already finished flowering and the old cooker had yet to. So the poor Kidd’s Orange Red fared the worst. As earlier flowering is undoubtedly here to stay, perhaps bumper crops from this variety will become much less frequent while the ice saints continue to strike.

Such periods of unusual weather, which go against the general trend at around the same time each year, are called singularities. They were first recognised by Scottish meteorologist Alexander Buchan (1829–1907). In the 1860s he was the first to draw lines joining points of equal sea-level atmospheric pressure, called isobars. These enable forecasters to depict areas of high and low pressure – anticyclones and depressions – which we see every day on weather charts and forecasts. Buchan went on to chart weather systems over Europe and later across the globe, which had a huge influence on the subsequent development of meteorology and weather forecasting.

While later collating maps of average weather conditions across Britain, Buchan noticed times of the year which tended to be either warmer or colder than the average temperature for the period. These became known as ‘Buchan spells’. He later admitted that the dates he first proposed for these spells varied from year to year and didn’t always appear every year.

Philip Eden’s Weatherwise, published in 1995, is an amalgam of many articles he wrote on weather for the Sunday Telegraph. The book recounts how in 1928 a bill was debated in parliament, aiming to fix the date of Easter on the second weekend in April. An MP pointed out that this was the time of Buchan’s second cold spell. The bill was nonetheless passed. It subsequently required ecclesiastical approval for the change, which has never happened.

Buchan spells achieved notoriety as a result of the press coverage of the parliamentary debate and by coincidence in 1929 every one of the warm and cold periods he predicted arrived on schedule. Research work on Buchan spells and other anomalous warm and cold periods was carried out in Europe in the early 20th century, and taken up again by researchers in the UK Met Office in the 1930s. Analysis of 60 years of weather data over Europe and the north Atlantic found more than 20 singularities which occurred more than 50 per cent of the time.

A singularity which appears to occur in three years out of four is the ‘June monsoon’, which usually peaks around the middle of that month. In 2020 rain and thunderstorms in the first week of June brought to an end the long dry spell which we enjoyed for most of May. The rainy weather continued for much of the month apart from a brief hot spell during the fourth week. In June in past years downpours have frequently turned the Glastonbury festival site into a mud bath, and before Wimbledon was moved to July play was frequently interrupted by rain. It happens as low pressure systems and their associated fronts head across the Atlantic bringing rain and strong winds to the British Isles. As with the ice saints, The Times reported similar conditions in 2019. The rain did at least bring some respite from preceding long dry periods in both years.

Singularities we can expect in January include a stormy period, peaking around the 8th, (86 per cent likelihood), another stormy period at the end of the month when snow is likely (84 per cent), and a settled and frosty spell in-between (84 per cent). At this time of year winds tend to be south-westerly, bringing low pressure systems with relatively mild conditions off the Atlantic, turning the weather stormy. Snowfall is likely if such storms displace a cold air mass lying over cold ground. Occasionally the pattern is different, as was the case in 1963 when there were no south-westerlies at all. Instead, almost continuous easterlies brought us snow over most of England and Wales for the whole month and the coldest January in southern Britain for 250 years.

By the end of January two of the three winter months are behind us and we might hope for signs of spring. However, there’s an old saying: ‘as the days grow longer the cold gets stronger.’ A singularity period in the second week in February is more often than not the coldest period of the year. If we get through most of February without cold or snow don’t forget that in 2018 the onset of easterly winds which brought the Beast from the East only began on 26 February, and continued into March. The lowest February temperature that year occurred on the 28th, when -11.7°C was recorded in Hampshire.

It’s time to stock up on plenty of thermal vests, and remember the Scandinavian saying: ‘There’s no such thing as bad weather, only the wrong clothing.’

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