Cloud of the Day – Jet Stream
The jet stream was discovered by Japanese meteorologist Wasaburo Ooishi when making over 1200 balloon observations of high altitude winds between 1923 and 1925. This information was later used when the Japanese launched nearly 9000 hydrogen-filled paper balloons to carry explosives across the Pacific Ocean to North America during the second world war. The remnants of one of these were found near Lumby, British Columbia, Canada as late as 2014.
Jet streams — see video here — form at the tropopause, the boundary between the two lowest layers of the atmosphere, the troposphere and the stratosphere. There are four major jets, two in each of the northern and southern hemispheres. They are referred to as the polar and subtropical jets and they form at the boundaries of the atmosphere’s major circulating air masses. The northern hemisphere’s polar jet flows at the mid- to northern latitudes and is a regular feature of television weather reports for many of us. The southern hemisphere’s polar jet mostly just circles Antarctica. The subtropical jets are weaker than the polar jets and don’t have as much effect on our weather patterns. There are other jets streams that form at particular times of the year or in particular places, but they don’t have much wider effect either.
Jet streams form at the boundaries of air masses where there are steep pressure and temperature gradients. The tendency of the air to move rapidly from high to low pressure down this steep pressure gradient, and its diversion by the Coriolis force results in a strong current of air at the boundary between the air masses. This current flows generally from west to east in prevailing westerlies. Since weather systems also tend to form at the interface between air masses, it is common for those systems to follow the jet stream. The polar jet streams track north and south with the seasons in concert with the Sun. The streams are quite concentrated phenomena, being only a few hundred kilometers wide and less than five thick.
The wind speed in a jet is often a hundred kilometers per hour and can exceed four hundred. It is easy to see how this could affect the flight of aircraft by reducing or prolonging flight time, depending on whether the flight was with or against the flow. Before this was understood, aircraft were known to take longer than anticipated to reach their destination, sometimes running out of fuel before arriving.
The jet stream is not straight, but rather meanders in its flow from west to east. These meanders look like waves and are called Rossby waves. These waves also travel from west to east, carrying the different weather on their north and south sides across the land below. Recently, probably due to climate change, Rossby waves have been stalling their eastward movement for unusually long periods, subjecting areas to prolonged rainfall or heatwaves. These extreme weather conditions are becoming more common.
In a future post we will cover related phenomena such as the Southern Oscillation, el niño/la niña, the polar vortex and the Dust Bowl.