A jet stream is a narrow band of strong wind that flows in the upper atmosphere, specifically in the troposphere and stratosphere. Jet streams occur in both hemispheres and blow from west to east. These winds reach speeds of up to 250 miles per hour (400 kilometers per hour). They play a crucial role in driving and influencing weather patterns around the globe.
Main Jet Streams: Location and Formation
There are two main jet streams in each hemisphere: the polar jet stream and the subtropical jet stream.
- Polar Jet Stream: This occurs at mid-latitudes, typically between 50°N and 60°N in the Northern Hemisphere and between 50°S and 60°S in the Southern Hemisphere. It forms due to the temperature difference between the cold polar air and the warmer temperate air.
- Subtropical Jet Stream: Found at around 30°N and 30°S, this stream forms due to the temperature difference between the warm tropical air and the cooler temperate air. It’s generally weaker and found at higher altitudes than the polar jet stream.
In terms of altitude, jet streams usually occur between 23,000 and 39,000 feet (7,000 to 12,000 meters) above the Earth’s surface. Each jet streams stretches for thousands of miles, but is only about 3 miles thick and a few hundred miles wide. Jet streams are strongest in the winter because temperature contrasts are greater.
Other Jet Streams
Apart from the main jet streams, there are secondary jet streams like the equatorial jet stream, which occurs near the equator. The equatorial jet stream is most prominent in the stratosphere.
Furthermore, Earth isn’t the only planet with jet streams. For instance, Jupiter boasts multiple jet streams, driven by the planet’s rapid rotation and internal heat. Saturn also has jet streams, with its most prominent one being the hexagon-shaped jet stream around its north pole.
How a Jet Stream Works
The two key players in jet stream formation are temperature gradients and the Earth’s rotation.
- Temperature: Jet streams primarily form due to differences in temperature between adjacent air masses. The Sun unevenly heats the Earth’s surface. For instance, the equator receives more direct sunlight than the poles. This creates temperature differences between the equatorial regions and the polar regions.
- Rotation: The Earth’s rotation introduces another complexity. As air moves from high-pressure areas to low-pressure areas, it doesn’t move in a straight line. Instead, because of the Earth’s rotation, it’s deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection due to the Earth’s rotation is called the Coriolis effect.
Combining the temperature-driven pressure differences with the Coriolis effect results in the formation of jet streams. In the upper atmosphere, the temperature difference between the cold air from the poles and the warm air from the tropics becomes significant. This temperature difference creates a pressure difference. As the air flows from high to low pressure, the Coriolis effect makes it spiral and flow in a westerly direction. This westerly flow of air, concentrated in a narrow band, is a jet stream.
Jet streams don’t flow in a straight line. They meander, creating ridges (northward bulges) and troughs (southward dips). Also, jet streams are not static. They vary in strength, location, and altitude depending on the season, temperature differences, and other factors.
Historical Perspective
The discovery of the jet stream is attributed to pilots during World War II. They noticed that their flights were unusually delayed or accelerated when flying at high altitudes, especially during bombing raids over Japan. Study of the phenomenon led to the identification of these high-speed air currents.
German meteorologist coined the term Strahlströmung (meaning “jet current”) in 1939. The term “jet stream” comes from the fact that these streams were first identified using jet-powered aircraft.
Jet Stream Effects and Uses
- Weather Influence: Jet streams play a pivotal role in guiding and influencing weather systems. For example, the movement or “meandering” of the polar jet stream results in prolonged periods of cold or warm weather in certain regions. The subtropical jet stream helps the Hawaiian Islands resist the effects of hurricanes.
- Aviation: Commercial aircrafts use the jet stream to save fuel and time. Flights going with the flow of the jet stream are significantly faster.
- Climate Systems: The position and strength of jet streams influence phenomena like the North Atlantic Oscillation, which has effects on European and North American winters.
- Potential Power Generation: High altitude wind power generation is a potential use of the jet stream.
- Military Applications: In World War II, the Japanese Fu-Go balloon was a weapon that used the jet stream to cross the Pacific Ocean. Such balloons could deliver a variety of payloads, including biological weapons.
Jet Stream and Climate Changes
- El Niño: During El Niño events, the jet stream over the Pacific becomes stronger and shifts southward, leading to increased storminess in the southern U.S. and drier conditions in the Pacific Northwest.
- La Niña: Conversely, during La Niña, the jet stream remains further north. This produces drier conditions in the southern U.S. and wetter conditions in the Pacific Northwest.
- Global Warming: As the planet warms, the temperature difference between the polar regions and the tropics decreases. This weakens the jet streams and produces more pronounced shifts north and south. These “wavy” patterns in the jet stream may prolong weather conditions, such as heatwaves, cold snaps, or rainy periods. Climate models consistently predict that global warming shifts jet streams toward the poles.
References
- Archer, Cristina L.; Caldeira, Ken (18 April 2008). “Historical trends in the jet streams”. Geophysical Research Letters. 35 (8). doi:10.1029/2008GL033614
- Francis, Jennifer A.; Vavrus, Stephen J. (2012). “Evidence linking Arctic amplification to extreme weather in mid-latitudes”. Geophysical Research Letters. 39 (6): L06801. doi:10.1029/2012GL051000
- Lewis, John M. (2003). “Oishi’s Observation: Viewed in the Context of Jet Stream Discovery”. Bulletin of the American Meteorological Society. 84 (3): 357–369. doi:10.1175/BAMS-84-3-357
- Mann, Michael E.; Rahmstorf, Stefan (2017). “Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events”. Scientific Reports. 7: 45242. doi:10.1038/srep45242
- Woolings, Tim (2020). Jet Stream – A Journey Through our Changing Climate. Oxford University Press. ISBN 978-0-19-882851-8.