The troposphere can be divided into two sections, the upper layer, known as the Free Atmosphere, and the lower, or Planetary Boundary Layer. The Boundary Layer usually runs up from the surface to about 1000m above it (sometimes more, sometimes less) but in any case it's a relatively thin layer in which the air movements and temperatures are influenced not only by major weather patterns but also by localised effects relating to the interaction of the air with the planet's surface.

These localised effects in the Boundary Layer include frictional drag as winds cross land areas, eddies and veering due to hills and headlands, convection initiated directly by heat radiation from sun-warmed ground, heat-radiation from warm sea water and so on. These factors are all forcing mechanisms that set air currents in motion, such as the thermals enjoyed by birds and glider-pilots alike.

Above the Boundary Layer, winds are directed by the Coriolis Effect working in conjunction with pressure-gradients. The Coriolis Effect is the force exterted by the Earth's rotation, and in weather terms its importance is the effect it has on the atmosphere. In the Northern Hemisphere, it causes airmasses to be deflected to the right - the opposite happens in the Southern Hemisphere. Thus, in the Northern Hemisphere, warm and cool airmasses around a developing low-pressure centre start to circulate in an anticlockwise (cyclonic) direction. So the Coriolis Effect is what makes depressions rotate; on a larger scale it helps to maintain the prevailing west-to-east airflow around our hemisphere.

In upper-air meteorology, pressure-patterns are as important as they are down here at the surface. Atmospheric pressure is simply an expression of the force applied by a column of air upon a fixed point of known area:

p=F/A where p = pressure, F = force and A = area.

Pressure used to be measured in millibars but the internationally accepted (SI) unit is the pascal (Pa). Meteorologists use the hectopascal (hPa) because the numbers are the same whether expressed in hectopascals or millibars. Makes life easier!

The greater the altitude, the lower the atmospheric pressure, hence the rarified air encountered in high mountain ranges. In upper-air meteorology, goings-on aloft are observed with satellites and directly sampled by weather-balloons carrying measuring instruments. The results of the balloon ascents, called soundings, are plotted on charts at several pressure-levels:

Pressure at any given height can change quite drastically as weather-systems move through. This goes on at the surface too. In the case of the UK, as an Atlantic low-pressure system is replaced by a large high-pressure area, the pressure over a few days at sea-level can change from maybe 970 hPa to 1030 hPa. The same applies aloft, but unlike surface charts, where the data are plotted in terms of pressure, the upper air data are plotted in terms of geopotential. Geopotential is the height above sea-level where the pressure is, say, 850, 500 or 300 hPa, and is measured in Geopotential Metres (gpm or gpdm). In an area of high pressure (an anticlyclone) the 850hPa level will be at a higher altitude than in an area of low pressure, so that although a different method of measurement is being used in the upper air, the resulting charts will look just like the sea-level pressure charts in terms of distribution of high and low pressure systems. So they make it possible to examine forecast data for the upper troposphere as well as close to the surface.

Finally, just as surface air has various physical properties (warm or cool, moist or dry etc), the properties of the upper air are important too. While convection - the vertical transport of heat and moisture - is obviously important to the storm-chaser working at the surface, the horizontal transport of upper air with certain proprties into an area is also of great importance. For example, storm formation in an unstable lower troposphere is markedly encouraged if cold dry air is present aloft. The process by which this cold, dry air moves horizontally into an area is known as cold air advection. Advection is simply the horizontal transport of air - and with that air comes a set of physical properties including temperature, moisture, stability and so on. The term will crop up in the text below quite a few times!



WEATHER SYSTEMS ALOFT - THE POLAR FRONT AND THE JET

The interaction of warm tropical air and cold polar air is what drives our mixed bag of weather all year round. It also, very importantly, plays a major role in maintaining out planet's heat balance. For a variety of reasons, the change in temperature with latitude is not even, but is instead rather sudden across the boundary between the tropical and polar air. This boundary, between the two contasting airmasses, is known as the Polar Front. It is the collision-zone where Atlantic depressions develop and their track is largely directed by its position. The steep pressure-gradients that occur aloft in association with this major, active air-boundary can result in narrow bands of very strong high-altitude winds, sometimes exceeding 200 miles per hour, especially just below the tropopause. These are known as jets or, specifically in association with the Polar Front, the Polar Jet.

The jet is readily picked out on upper-air wind charts (below). This one is a GFS forecast chart for windspeeds and direction of flow at the 300 hPa pressure level, in other words at an altitude a little higher than the summit of Everest. Highest winds are red, weakest blue. The most obvious thing that immediately catches the attention is that the jet doesn't always run in a straight, west-east line, even though that's the prevailing wind direction in the Northern Hemisphere.

Instead, it curves north and south in a series of lobes, any one of which can half-cover the Atlantic. These large features, which are high-pressure ridges and low-pressure troughs, are known as Longwaves, of which there are several present at any given time along the Polar Front. High pressure is exerted by warm air masses, which expand in all directions as far as they can, so that beneath one the atmosphere is thicker. Warm air pushing north delineates the ridges. Cold air, by contrast, is nowhere near as thick, so that it exerts lower pressure, hence forming troughs where it pushes south.

Strong Longwave patterns are more prevalent in winter than in summer. This is because during the winter there is a much bigger temperature gradient between the warm Tropics and the frozen North.

In addition, there are similar, but much smaller ridges and troughs, known as Shortwaves. These are less obvious on many charts but are important because they can still have significant effects on the weather at surface. As this chart shows, Longwaves are by contrast pretty hard to miss! But if you look carefully, there's a shortwave trough crossing the middle of Italy.



HOW THE JET AFFECTS OUR WEATHER - POSITION, SHAPE, VORTICITY AND SHEAR



Several factors are important with regard to the Polar Jet and its effect on our weather. Firstly there is its position relative to the UK. If it sits well to our north, we can expect mild and breezy weather, and occasional settled spells. The Atlantic Storms are passing by to our north, so they only clip us. However, if it runs straight across the UK we can expect depressions to run straight over us, with wet, stormy weather likely. If it sits to our south, depressions take a much more southerly course, bringing storms to Continental Europe, and, in winter, the risk of heavy snow for the southern UK as the prevailing winds associated with low pressure systems tracking to our south will be from the east.




Secondly, there is the direction in which the jet flows. A standard E-W flow passing over the UK will be accompanied by a steady succession of frontal systems, rolling in from the west and passing away east into the North Sea. The resulting weather pattern will be one of frontal rainbands followed by convective showers, followed by a brief dry interlude as a transient ridge of high pressure moves through. Standard British weather in other words. This type of setup is known as "zonal".



If the lobes (the ridges and troughs) are very large then there is a marked N-S looping within the generally westerly airflow. This can give fairly slow alternations between warm and cool conditions. But if a lobe grinds to a halt over one area, the whole thing slows up completely and the weather settles for a while into one form or another. This weather setup is known as "blocked". During some winters, a block forms in the Atlantic when high pressure extends all the way from the Azores up to Greenland. Provided the block is far enough west, it can induce a cold northerly flow over the UK if there is low pressure near Scandanavia, as shown in the simple diagram below. This weather setup can produce frequent snow-showers, and sometimes more widespread snow caused by the formation of Polar Lows, small depressions that develop within the cold air and move south over the UK.



Blocks in the wrong place, however, can lead to days of dismal grey weather with nothing much of interest going on!!


Then there is vorticity advection. The jet flowing around a lobe of cold polar air (an upper trough) orientated NE-SW, first runs SW, then S, then SE, then E, then NE - i.e. its motion is anticlockwise, or cyclonic. Watch a floating twig in a slow-moving river. As it turns a LH bend it will slowly spin in an anticlockwise direction. It's spinning because the water upon which it floats is spinning. You can't necessarily see the water doing this but the floating twig gives the game away! Vorticity is a measure of the amount of rotation (i.e. the intensity of the "spin") at a given point in a fluid or gas. And, in the air rounding an upper trough, anticlockwise vorticity is induced. This is known as Cyclonic Vorticity (or frequently as Positive Vorticity).

Positive vorticity encourages air at lower levels to rise (which is what happens in cyclones after all). Rising air encourages deepening of low-pressure systems, assists convection and thus generally leads to heavier precipitation. Thus, as an upper trough moves into an area, what is going on is the transport (advection) of air with positive vorticity. The process, positive vorticity advection, is usually abbreviated to PVA. When forecasting, look on the upper air charts (300 & 500 hPa) for the approach of an upper trough. PVA will be at its most intense just ahead of the trough, so that's where the most interesting weather will be!

The reverse, anticyclonic or negative vorticity advection (NVA) will occur between the crest of an upper ridge and the back of the trough, due to the same process but with a clockwise (anticyclonic) spinning motion induced into the air as it runs up around the ridge. In such areas air is descending instead of ascending. Descent is very adept at killing off convection. Thus as the upper trough passes, severe weather becomes increasingly unlikely to occur. The timing of upper troughs and ridges is thus of considerable importance in severe weather forecasting. All other parameters may be in place for a big summer thunderstorm, but then along comes an upper ridge at the best time of day for storm formation and the whole thing fizzles out!

Vorticity is affected by other factors too. Shear is important. It's caused by winds of different speeds running side-by-side. Air in an environment with stronger winds to the south and weaker ones to the north will be given enhanced cyclonic spin. Since windspeeds for different pressure levels are available via sites such as Wetterzentrale, this can be factored into a forecast.

Shear in which windspeed increases occur with height (speed-shear) may also be estimated from the charts. Some speed-shear is pretty much normal as you will notice when climbing a mountain. A breeze at the bottom can be a near-gale at summit-level. There's less surface friction up there which is one factor. But in the upper troposphere the proximity of the jet can bump the windspeed up massively. Speed-shear is important in convective situations as it literally whisks away the "exhaust" of a storm, thus helping to prolong it. It's a bit like an open fire drawing well. Strong speed-shear occurs when the jet is racing overhead. In this environment, cumulonimbus anvils may stretch for many miles downstream due to the icy cirrus of the anvil being dragged downwind. When there's hardly any speed-shear the storm-tops have a much more symmetrical shape to them.

Directional shear in the upper atmosphere basically means that up there the winds are blowing in a different direction to what they are at the surface. This can be of importance in severe storm development, including the formation of tornadic supercells.

DIFFLUENCE AND CONFLUENCE

Both troughs and ridges can be either diffluent or confluent. These terms deal with the way the air flows into, and out of, these features. Diffluence (or divergence) involves the air moving rapidly into a ridge or trough but slowing down upon leaving it. In the case of troughs, diffluence permits the conditions ripe for intense cyclonogenesis. Confluent troughs are those in which the air leaving the trough area is faster-moving than that entering it, and these tend to have stable, anticyclonic weather in their wake. These features can be picked up using upper-air windspeed charts.

JET STREAKS, THEIR ENTRANCES AND EXITS

Within the overall circumglobal wind-field of the jetstream, there occur local bands with much stronger winds than elsewhere. These are called jet streaks (and sometimes one may be referred to as a jet max). They form in response to localised but major temperature-gradients, and consist of narrow zones in which the pressure-gradient is especially steep as a consequence, shown on pressure charts by the isobars being very close together. They move along the lobes, following the troughs and ridges, and affect these in their passing, strengthening them as they move in and weakening them as they move out. They also influence the weather below even if moving in a fairly straight line when there are few longwave ridges/troughs about.




Fast jet streaks with winds as high as 200 knots pull in air upstream (to their west) at what is called an Entrance Region and throw it out downstream (to their east) at what is called an Exit Region. These are further subdivided, as in the diagram above, into Left (to the north) and Right (to the south). A mass of air heading into a jet streak's Entrance region is accelerated by the force of the pressure-gradient operating to its left. During this process there exists an imbalance of the two forces controlling the situation - the pressure-gradient force to the north and the Coriolis force to the south. The pressure-gradient force is the more powerful of the two in this area. It forces the air to be pulled to the left (northwards), a bit like you might expect a car to constantly veer left if the front passenger-side tyre's a bit flat and all other tyres are at the correct pressure.

In the Exit region, the air is leaving the jetstreak and entering an area with a more relaxed pressure-gradient. It slows down, but in the opposite scenario to the process going on in the Entrance region it is in this case made to veer to the right (southwards) because the Coriolis force is the stronger one here.

With such narrow zones of high winds, considerable shear occurs resulting in induced vorticity of both positive and negative types. These are distributed differently on the north and south sides due to the following reason: within a jet streak an air parcel will flow parallel to the isobars but in its entrance and exit regions it instead flows across them - due to the deflection process explained above. This can cause either convergence or divergence. This is shown in the diagram below, in which the red arrows show the prevailing airflow direction, the black lines are the tightening isobars within the jet streak and the blue arrows show the manner in which the air entering or leaving a jet streak is deflected due to the pressure gradient force (Entrance region) or the Coriolis Force (Exit region).



Convergence involves winds from different directions coming together in a given area. Divergence involves winds from a given area spreading out from that point because they are blowing in different directions. In the western, Entrance region of a jet streak, this results in convergence within its northern (left) part and divergence within its southern (right) part.

Positive vorticity (as described above) is associated with divergence, so not surprisingly the Right Entrance region is a likely area for PVA to be present, encouraging convection and cyclonic development below. With the Exit region the opposite is the case, in that the Left Exit region is where any PVA will be found and the Right Exit region is one of NVA, implying descent and uneventful conditions. Jet streak Left Exits are essentially at the leading edge of a jet streak, so they arrive on the scene first, causing surface weather-fronts to become active as they over-run them, or allowing convective storms to develop vigorously. This occurs because as the air in the Left Exit diverges (moves apart), something has to replace it, so new air rises from below in order to achieve this: in turn, lower-level air converges (comes together) to replace that and a deepening low-pressure system, or intense convection, results.



Let's have another look at that upper-air (300 hPa) wind-chart. We now know that for cyclonic or convective weather to be supported, we need to look for the area ahead of an upper Longwave Trough axis, for smaller Shortwave Troughs and for Left Exit and Right Entrance regions of a jet-streak. These are all nicely shown on this chart. It would suggest unsettled conditions to be most encouraged in the following areas:

a) southern of Greenland (Left Exit of jet streak/ahead of Longwave Trough axis)
b) to the SW of Portugal (PVA ahead of Longwave Trough plus cold air aloft in upper vortex)
c) the S of the UK (PVA ahead of approaching, diffluent Longwave Trough axis)
d) the S of Italy and the sea to the SE (PVA ahead of Shortwave Trough)

The weather that day showed this to be not far off the case. An active low-pressure system was present over the S of Greenland. High pressure in the mid-Atlantic gave way to slack low pressure essentially following the upper trough. Flabby, slack highish pressure sat over most of continental Europe. An active cold front brought some squally rain to England and Wales, while thunderstorms broke out widely in the Atlantic to the W and S of Portugal and another thundery area developed over S Italy and Sicily and moved east. All of these events were supported by what went on aloft, so in conclusion, it's well worth learning to understand these initially confounding plots of what's going on way up in the Troposphere!