ANYONE celebrating the midsummer solstice might have been anxiously checking the weather forecast over the last week. This June is currently set to be the wettest on record, turning what should be long sunny summer days into gloomy grey ones. Unfortunately, this is consistent with increasingly variable weather year-on-year in Britain due to climate change.

Seventy-five years ago, it was with a very different aim in mind that commanders of the Allied forces were checking weather forecasts during WWII. Everything was ready for the invasion of northern France, but the actual date of D-Day had to be determined. They were waiting for the crucial combination of weather conditions to allow hundreds of thousands of soldiers to land on the beaches in flat landing craft.

Writing in 1994, Lawrence Hogben, a meteorologist involved in the planning of D-Day, wrote that the exact weather requirements (on top of a full moon and a suitable tide) were “a quiet day with not more than moderate winds and seas and not too much cloud for the airmen, to be followed by three more quiet days.” Only four days could potentially fit the bill: June 5, 6, 19 or 20. Hogben and his colleagues worked out that the odds of any one of these days meeting these weather requirements was less than 10 per cent.

In the run-up to D-Day, the five-day weather forecasts turned out to be too unreliable; the state of meteorological science at the time only allowed (at most) a two-day forecast. General Eisenhower, the American commander, had chosen June 5. But the weather in early June was unsettled, with calm spells breaking into stormy weather. On June 4, it was clear June 5 was a no-go. Eisenhower postponed the invasion by at least 24 hours. The crucial question was whether the bad weather would clear for June 6. Or would the invasion have to be postponed for two weeks until June 19 — with no guarantee of the weather being any better then?

Meteorologists led by Group Captain James Stagg predicted on the evening of June 4 that there would be a temporary break in the bad weather on June 6: conditions would be enough to select it as D-Day. Eisenhower was convinced (the story of how is told in David Haig’s play “Pressure)”, and the invasion was launched on June 6: 160,000 troops on the first day.

One of the unlikely people who arrived in northern France soon afterwards was a communist Irish scientist, JD Bernal. A famous left-wing intellectual, Bernal’s scruffiness was almost as legendary as his politics. One notorious story had military officers being suspicious of his apparent discourtesy at failing to dress smartly when visiting their research headquarters. But when they visited him at his university the following day and saw how much scruffier he was than the day before, they realised that he had indeed put on his best suit to visit them.

Bernal’s peacetime field was crystallography, but he was heavily involved in the planning of D-Day. He was adamant that science was not just a source of technical advice, but should be used in the formulation of the right questions. When asked how shallow depths could be measured accurately using a portable echo-sounder by one naval officer, Bernal asked what the reason for this request was. Although the officer refused at first, he eventually admitted that this was his idea for a method to measure the gradients of beaches secretly to determine if troops could land on them.

Bernal said that in that case the correct question was “How do we measure the beach gradients and runnels, without the Germans knowing?” and proposed instead inferring the details from aerial photographs — which actually worked. By reading archive documents, he worked out crucial details of the geography of the French beaches which were used in deciding where to launch assaults.

Another important problem for D-Day related to the weather was how to prevent flat-bottomed boats laden with heavy vehicles from sinking; simple stormy conditions could easily overwhelm them off the coast of northern France. The solution was to create an “artificial harbour” by making temporary breakwaters to dissipate the energy of the waves. Bernal didn’t invent the idea, but he gave a memorable demonstration involving newspaper boats in a bathtub that convinced senior naval officials.

Stagg and the meteorologists couldn’t see two weeks ahead, but on June 19 a terrible storm hit the Channel, and continued for several days through the summer solstice. Artificial harbours were destroyed, with dramatic knock-on effects: the loss of supplies for troops at the front in northern France. But if this had happened on the day of the original invasion it would have been totally disastrous. Science never has all the answers, but it is our best guide when quantifying the uncertainty of the future. We may not know the exact weather in two weeks’ time due to the chaotic local atmosphere, but we can definitively predict the general and global trends of climate change, which demand radical action and dwarf the weather on any particular day; even D-Day.

Bernal would have agreed. In his earlier career he had hoped that through “the planned mobilisation of people, politics, science and social hope” humanity might achieve a better world. In the observation of historian Eric Hobsbawm, it took war to demonstrate to those in government that Bernal was correct when he predicted the need for “big science” to tackle major social problems: “War and only war would give science and technology…the resources and the support structure that propelled both into the second half of the century.”

In the present day, similar collective and decisive action is required on climate change, but the warnings of scientists are being ignored. We hope that future historians will look back and reflect that it was the global response to climate change that forced a union of “people, politics, science and social hope” to create a more just society.