During the Second World War, British pilots were fighting more than the German Messerschmitts. They were also fighting against the weather—more specifically, with fog.

Fog was responsible for a large number of losses of RAF aircraft returning from bombing missions over Germany. Since most of these raids took place at night, fog would often obscure large areas of the ground making it difficult for the pilots to see the airfields and the runways. In these cases, the pilot would point his airplane towards the sea and then, while still over land, the crew would bail out by parachute leaving the aircraft to harmlessly crash into the ocean. With bombing raids involving several hundred aircraft, a significant number of bombers were lost to fog this way.

An Avro Lancaster of the RAF’s No 35 Squadron takes off with FIDO petrol burners on either side of the runway at Graveley, Huntingdonshire in May 1945.

Fog has always been and continue to be a serious hazard in aviation. Fog leads to poor visibility, and visibility is crucial for making a safe landing. Henry Garrett Houghton, an electrical engineer at MIT, the United States, considered fog “the greatest hindrance to the development of aviation.” He believed that combatting fog would be the first breakthrough for humans trying to use science to modify weather, and a stepping stone to controlling it.

In 1934, Houghton demonstrated that fog could be removed appreciably by spraying the air with calcium chloride. For his demonstration, Houghton built an apparatus consisting of a 100-foot long pipe fitted with downward spraying nozzles that he suspended 30 feet in the air. When the fog rolled through, he sprayed the misty cloud with 2.5 gallons of calcium chloride solution per second. In just three minutes, Houghton’s machine turned an area with visibility of less than 500 feet into one where “buildings more than a quarter-mile away were visible.”

Houghton’s methods, though successful, was less than practical for commercial use in airports because of the vast quantities of calcium chloride required. Besides, calcium chloride is corrosive against the aluminum-alloy bodies of airplanes. Houghton’s research in fog dissipation, however, didn’t go to waste. It morphed into a new research field called cloud physics, which explores atmospheric condensation and precipitation.

Image credit: Popular Science Aug 1945

By the 1940s, it became apparent that the only proven method of dispersing fog on a massive scale is by heating. So Prime Minister Churchill called the Petroleum Warfare department, an organization originally tasked with developing various creative uses of fire to thwart enemy invasion, and instructed them to develop a tool to defeat fog. The weapon that bore out of it was called FIDO—”Fog Investigation Dispersal Operations". When the apparatus proved a brilliant success, it was renamed “Fog Intensive Dispersal Of,” retaining the original code name.

FIDO consisted of two parallel pipelines running along either side of the runway through which fuel was pumped and sprayed out of burner jets positioned at regular intervals along the pipeline. Before an aircraft was due to land, a ground personnel with a flaming torch would run or drive along the pipes lighting the gasoline or kerosene vapors. Flames would shoot up all along the pipes burning with a fierce white-yellow glare, and the heat they generated would evaporate the suspended fog droplets in a matter of minutes. The first successful trial of the FIDO system was in 1942 in Hampshire, when a dense fog of 50 yards' visibility was cleared by petroleum burners in an area about 200 yards square to a height of 80 feet. Before long large-scale FIDO systems were routinely clearing the air to a height of several hundred feet. The glow of the burners could be seen from a hundred kilometers away.

A member of staff at Blackbushe airport, Surrey ignites burners in preparation for aircraft to take off in fog in November 1952

The first operational use of FIDO took place in November 1943, after a little over a year of experimenting. Four Halifaxes landed successfully after a bombing expedition to the Ruhr, on a night when the visibility was only 100 yards prior to the lighting of the FIDO system. The FIDO revolutionized the war. It enabled more than 2,500 Allied aircraft to operate from more than 15 fog-covered airfields throughout Britain, made possible the bombing of Berlin thirty-six nights in succession, and enabled Allied bombers to take to the air during the Battle of the Bulge in December when the entire Europe was enveloped in dense fog.

After the war, there was a plan to install FIDO at Heathrow airport, but the idea was dropped because of the high operating costs—FIDO used huge quantities of fuel, as much as 450,000 liters per hour, while longer airfields used twice as much. Today, a modern aircraft can land in practically zero visibility, thanks to onboard computers and modern inventions such as radar and GPS.

FIDO pumps at RAF Graveley May 1945