The jet stream plays a vital role in the global climate system and is a key factor in the UK’s changeable weather.
High above our heads, around 5 to 7 miles above the Earth’s surface, a powerful ribbon of air races from west to east. This is the jet stream; a fast-flowing current of air that plays a crucial role in shaping the weather we experience on the ground.
How Does the Jet Stream Work?
The jet stream flows within the upper levels of the atmosphere, just below the tropopause, and can reach speeds of over 200 miles per hour. It acts like a high-altitude conveyor belt, influencing the movement and development of weather systems such as areas of high and low pressure.
Sometimes the jet stream flows in a relatively straight path, much like a fast-moving river. At other times, it meanders and loops, slowing down the movement of weather systems and making their paths less predictable. These meanders can lead to prolonged periods of wet or dry weather, depending on where the jet stream is positioned.
The jet stream can also intensify weather systems. When it flows over an area of low pressure, it can act like a vacuum cleaner, drawing air out of the top of the system. This causes the pressure to drop further, strengthening the system and often resulting in stormier conditions.
Conversely, when the jet stream slows and buckles, it can allow areas of high pressure to dominate. These systems typically bring calmer, drier weather with light winds and clearer skies.
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What Causes the Jet Stream?
The jet stream exists due to temperature differences between the equator and the poles. In the northern hemisphere, cold polar air lies to the north of the jet stream, while warmer tropical air lies to the south. This temperature contrast creates a pressure gradient, which drives the high-altitude winds.
The Earth’s rotation also plays a role. The Coriolis effect causes moving air to deflect to the right in the northern hemisphere and to the left in the southern hemisphere. This deflection causes the jet stream to flow from west to east.
The position and strength of the jet stream vary with the seasons. In winter, the temperature contrast between the equator and the poles is greater, strengthening the jet stream and often positioning it over the UK. This is why we tend to see wetter and windier weather during the colder months. In summer, the contrast is weaker, and the jet stream typically shifts northward, allowing for more settled and drier conditions.
How Does the Jet Stream Affect Flights?
The jet stream also has a significant impact on aviation. It flows at altitudes similar to those used by transatlantic aircraft. Flying with the jet stream can reduce flight times and save fuel, while flying against it can lead to delays and increased fuel consumption.
However, the jet stream can also cause turbulence, especially where wind speeds change rapidly or where the stream meanders. This turbulence is similar to the choppy flow of a river and is an important factor in flight planning.
Interestingly, aircraft engines operate more efficiently in the colder air found on the polar side of the jet stream, which can also influence flight routes.
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Why Does It Matter?
Although the jet stream is invisible to the naked eye, its influence is felt every day. From steering storms across the Atlantic to shaping our seasonal weather patterns, the jet stream is a key player in the Earth’s climate system. Understanding its behaviour helps meteorologists forecast the weather more accurately and helps us prepare for whatever conditions may come our way.
You may have seen some mentioning in the media of a 'Caribbean jet stream' and how it might have been responsible for the warmer temperatures we've experienced recently. However, a 'Caribbean jet stream' is not a meteorological term. Explaining this, Met Office Chief Meteorologist Matthew Lehnert said: "A ‘Caribbean jet stream’ is not a meteorological term and is certainly not what is behind the warmer temperatures we expect to see this week. The jet stream is a ribbon of air high in the atmosphere that helps to steer weather systems around the globe.
"A subtle shift northwards in the jet stream later this week will result in pressure rising in the south and temperatures climbing into the mid 20s Celsius for some of us, but this is nothing to do with the Caribbean. The airmass that brings us the warmer weather on Friday into Saturday this week is being drawn from the Azores."
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