How James Glaisher discovered the jet stream

Among many other, more stereotypical features, Scotland is famous as a place where palm trees can grow. And grow they do, with several proud, albeit slightly weather-beaten examples dotted around the mountainous islands and coasts of the western Highlands, blissfully unaware that they share latitudes with the southernmost Canadian tundra. This feat is often attributed to the gentle, warming influence of the Gulf Stream, bringing tropical seawater right up to the mouths of the lochs. But perhaps there is more to this story.

The Gulf Stream is indeed a great ocean current, transporting huge quantities of warm, tropical water northward. It’s like a river within the ocean, and it carries a thousand times the water that our greatest rivers on land do. But the Gulf Stream lies over 3000 kilometres away from Scotland. It begins life in the Gulf of Mexico and then tracks up the eastern coasts of Florida, Georgia, and the Carolinas before peeling away from the shore at Cape Hatteras. Soon after, it breaks up into a myriad of smaller, meandering currents and is lost in the swirling masses of the Atlantic Ocean.

Relatively warm seawater from the south does indeed help to keep European winters mild, but while some of this water will have passed along the Gulf Stream recently, much has arrived courtesy of quite different ocean currents. And it certainly can’t be described as tropical any more.

The myth of the Gulf Stream was already widely believed in Victorian times, when two lesser-known explorers were pushing back the frontiers of knowledge. This was the age of the aeronauts, when daring celebrity-scientists probed the atmosphere to ever greater heights in perilous balloon ascents.

James Glaisher and Henry Coxwell are best known for a dramatic ascent in 1862, in which they launched from Wolverhampton and reached heights above the top of Everest within an hour. While clearly impressive, this was perhaps not Glaisher’s finest hour. He is immortalised in a famous sketch of the day, frostbitten and “insensible at the height of seven miles” while Coxwell rescues them, incredibly by climbing up into the balloon’s rigging to release a trapped rope with his teeth.

Glaisher and Coxwell went on to perform many less eventful but highly successful ascents, recording invaluable data of the upper atmosphere. Glaisher, already a fellow of the Royal Society, would later lead the Royal Meteorological Society as its president. On one trip in 1864 he noted a characteristic warm, south-westerly wind blowing above the country, and his thoughts proved to be well over a hundred years ahead of their time. Writing in the British Quarterly Review in 1871, he noted:

The meeting with this SW current is of the highest importance, for it goes far to explain why England possesses a winter temperature so much higher than is due to our northern latitudes. Our high winter temperature has hitherto been mostly referred to the influence of the Gulf Stream. Without doubting the influence of this natural agent, it is necessary to add the effect of a parallel atmospheric current to the oceanic current coming from the same regions – a true aerial Gulf Stream.

Glaisher was right: the prevailing south-westerly winds that dominate across much of Europe are at least as important in maintaining our mild winter climate as the ocean currents are. These winds bring relatively warm air from parts of the Atlantic further south, just enough to protect the brave palm trees of the north. And as Glaisher suspected, the prevailing winds we feel down at the surface are just the underbelly of a giant atmospheric current above us. His “aerial Gulf Stream” is now also widely known. It is none other than the jet stream.

These two great currents are often confused but the distinction is clear: while the Gulf Stream is a giant river of seawater flowing along the American coast, the jet stream is a veritable river of air which snakes its way around the whole northern hemisphere, profoundly shaping the regional climate patterns of the planet below. A great divider, the jet stream forms as a sharp barrier between warm, tropical air to the south and cold, polar air to the north. Surges of cold in much of Europe, as exemplified by the dreaded Beast from the East, arise when the jet stream is diverted south, starving us of Glaisher’s warm south-westerlies and leaving us stranded on the cold, northern side of the jet.

Today we are well aware of the jet stream, and its swings and meanders are the principal actors in countless weather forecasts. A new generation of scientists is probing the unknown, albeit largely from the safety of a computer terminal. But as greenhouse gases accumulate and the world warms, a new threat is emerging. Climate change is altering the balance of temperatures between the warm and cold sides of the jet, and the future of the jet stream itself hangs in this balance.