Dr Roger Brugge, University of Reading: Talk to Newbury Astronomical Society, 1st June 2018
The existence of jet streams was first suspected in the 1930s from studies of cloud systems, the motion of storms, and aircraft flight times, especially of propeller-driven aircraft. It is a fast, narrow air current in the troposphere, at an altitude of 10 to 15 km depending on latitude and the season. The wind speeds in the jet streams can be up to 150 knots, and the streams can be thousands of kilometres long and several hundred kilometres wide. There are two jet streams in each hemisphere: the subtropical jet stream, which is consistent and stable, and the polar jet stream, which is highly variable.
In a simplified model of atmospheric circulation, hot air rises at the equator and cold air sinks at the poles, giving a single rotation cell in each hemisphere. The pressure gradient leads to winds, which are deflected by the Coriolis force so they blow mainly along isobars (lines of equal pressure). In the real world, these factors combined with the Earth’s rotation, axial tilt, seasonal changes and irregular landmasses with mountain ranges lead to the formation of three rotation cells in each hemisphere: a polar cell, the Hadley cell near the equator and the Ferrel cell in mid-latitudes. Rising air at the equator and near 60 degrees latitude between the Ferrel and polar cells causes low pressure, cloud formation and rain. Sinking air at the poles and between the Hadley and Ferrel cells at around 30 degrees latitude causes high pressure and clear conditions. The jet streams are mainly found in the upper atmosphere near the cell boundaries, although they meander in latitude and vary in height.
Jet streams both affect and are affected by the weather. They move towards the poles in summer, as they are affected by temperature and pressure changes. They steer low pressure systems around: the jet stream moves a lot of air very quickly, and air rising to replace it leads to low pressure. When the jet stream is to the north of Scotland, in summer, depressions pass north of the UK, giving the worst weather in northern England and Scotland. When the jet stream lies over the UK, in winter, Scotland receives cold easterly winds and the south gets westerlies from the Atlantic. Strong jet stream generally mean quieter weather, whereas when the jet stream is weaker and meandering, lows can form more rapidly and winds tend to be stronger. Kinks in the jet stream can cause very rapid changes in the weather. Climate change is leading to arctic warming and reducing the N-S temperature gradient, which is likely to result in slower and less stable jet stream, and thus more severe weather in the long term. The El Nino effect modifies equatorial ocean temperatures, and thus also affects the location and strength of the jet stream.
The jet streams affect aviation, mainly due to the wind strength, and flight planners for commercial airlines try to avoid them to minimise flight times. They are also sources of severe turbulence.
The weather is the biggest hindrance to astronomy, although clear is not necessarily better. The jet stream are sources of wind shear, turbulence and rapid temperature variations, all of which cause bad seeing due to the resulting variations in refractive index of the air. The southern edge of the jet stream has smoother air flow, but also ice formations such as cirrus clouds. The northern edge of the jet stream can be very clear but also very turbulent. It is best to be as far south of the jet stream as possible, although locations that are too far south, in the high pressure regions, can suffer from poor transparency due to dust carried up near the equator sinking back to earth: this can generate mist or fog. Too far north (in the UK) the North Atlantic Drift brings warm water which heats the surface air. When this mixes with cold air from the east, it again causes turbulence and bad seeing.
Notes and summary by Chris Hooker.