Roughly ten kilometres above your head, higher than most aircraft cruise, a ribbon of ferociously fast wind snakes around the entire planet. It can blow at speeds that rival a racing car, it meanders in great loops that shift from day to day, and though you will never feel it directly, it exerts more control over your daily weather than almost anything else in the atmosphere. This is the jet stream, and understanding it is one of the great keys to making sense of why the weather does what it does, especially in the middle latitudes where most people live.
The jet stream exists because of temperature contrast. Our planet is heated unevenly: the tropics receive intense, direct sunshine and stay warm, while the poles receive weak, glancing sunlight and stay cold. This sets up a great temperature difference between the warm tropics and the cold poles, and the atmosphere is constantly trying to even it out by moving heat from the equator toward the poles. Where the warm tropical air and cold polar air meet, in the middle latitudes, the temperature changes sharply over a relatively short distance, and this sharp contrast, combined with the rotation of the Earth, generates a band of powerful wind high in the atmosphere flowing from west to east. That band is the jet stream. Its very existence is a signature of the boundary between warm and cold air masses.
Because it marks the boundary between warm and cold air, the jet stream is not a straight, tidy line. It buckles and waves, forming great northward and southward loops that shift and travel around the globe. These waves are enormously important, because they determine which air mass sits over any given place. When the jet stream loops northward over a region, it allows warm tropical air to push up beneath it, bringing mild, often settled weather. When it dips southward, it drags cold polar air down into the region, bringing cooler, often more unsettled conditions. The position of these loops, and the way they migrate and change shape over days and weeks, is a large part of what determines whether you get a warm spell or a cold snap, a settled week or a stormy one.
The jet stream also acts as the great steering current for weather systems, which is perhaps its most direct influence on daily life in the mid-latitudes. The areas of low pressure that bring wind and rain, and the fronts that separate air masses, are guided along by the jet stream like leaves carried on a river. Storms tend to form along the jet and travel with it, so its position determines the storm track — the path along which one weather system after another is delivered. If the jet stream is positioned directly over your region, you can expect a procession of weather systems, wind and rain as storms are steered right across you. If it lies well to the north or south, you may sit in the calmer air on one side, either the mild side or the cold side, out of the main firing line. This is why forecasters watch the jet stream so closely: knowing where it will lie and how it will wave tells them where the storms will go.
The strength and behaviour of the jet stream vary with the seasons, which helps explain the changing character of the weather through the year. The temperature contrast between tropics and poles is greatest in winter, when the poles are at their coldest, so the jet stream is strongest and most active in the cold season, driving the vigorous storms and rapid changes typical of winter weather. In summer, as the contrast weakens and the whole pattern shifts poleward, the jet stream often lies further north and blows less fiercely, which is part of why summer weather tends to be more settled in many mid-latitude regions. The jet also shapes air travel: because it blows from west to east, flights riding along it in that direction get a substantial speed boost, while those flying against it battle a headwind, which is why an eastbound transatlantic flight is often noticeably shorter than the westbound return.
Some of the most significant weather events arise when the jet stream falls into unusual patterns. Sometimes the waves in the jet grow very large and then stall, locking into place for an extended period in what is called a blocking pattern. When this happens, the same weather persists for days or weeks because the systems that would normally move through are held stationary. A block can trap a region under persistent high pressure, bringing a prolonged heat wave and drought in summer or a stubborn cold spell in winter, while a neighbouring region stuck under a stalled loop endures relentless rain and flooding. Many of the most damaging weather extremes, from lingering heat waves to prolonged wet spells, trace back to a jet stream that has locked into a stuck configuration rather than progressing normally.
For all its remoteness, the jet stream is worth picturing whenever you try to understand a stretch of weather. A mild, settled spell often means the jet has looped north of you, letting warm air build; a cold, stormy one often means it has dipped south or lies right overhead, steering systems across you; a period of weather that simply will not change may mean it has become blocked. You cannot see or feel this river of air overhead, but its meanders are writing much of the story of your weather, day by day. Learning to think about where the jet stream lies turns the seemingly random procession of warm and cold, wet and dry into something with a visible, comprehensible cause high in the sky.