Constant pressure charts
At cruise the altimeter is on 1013 hPa, so the pressure at your level is fixed and it is the height of the pressure levels that varies. Upper wind charts are built the other way round from a surface chart: height is drawn on a surface of constant pressure.
Reading a contour chart
- The contour lines are isohypses, joining points where that pressure level sits at the same height above mean sea level, labelled in decametres, tens of metres. An isohypse marked 522 on a 500 hPa chart means the 500 hPa level is 522 decametres, about 5220 m, above MSL.
- High contour values mean high pressure at that level, low values mean low pressure.
- Closely spaced contours mean a strong gradient and a strong wind. The wind blows parallel to the contours, with Buys Ballot giving the direction: back to the wind in the Northern Hemisphere, the lower contour on your left.
- Lines of equal wind speed on the chart are isotachs.
Charts are produced at standard levels: 850 hPa near FL050, 700 near FL100, 500 near FL180, 400 near FL240, 300 near FL300, 250 near FL340, 200 near FL390, and higher. Upper winds are stronger than surface winds for the same pressure gradient because the air is less dense: at 20 000 ft, for the same gradient, the wind is roughly double the surface value.
The thermal wind and how jet streams arise
The pressure differences that drive the upper winds are made by surface temperature differences, so the upper winds are called thermal winds. Where the temperature contrast is sharp, the thermal wind piles up into a jet stream.
Buys Ballot, extended
The thermal wind itself is the vector difference between the geostrophic wind at an upper level and the one lower down. It blows parallel to the isotherms, or the thickness lines, with the cold air on the left in the Northern Hemisphere, and it is what has to be added to the lower wind to build the upper wind.
Jet streams
- The strongest winds are just below the tropopause, where the pressure surface has its steepest slope and the temperature stops changing with height.
- A current of this fast air is a jet stream once the wind exceeds 60 kt, the WMO threshold.
- Caused by a large horizontal temperature gradient. The bigger the contrast between the two air masses, the stronger the jet.
- Exam dimensions: about 2000 miles long, 200 miles wide, 2 miles deep. That gives a width to length ratio of 1:10, a depth to width ratio of 1:100, and a depth to length ratio of 1:1000. Speeds of 350 kt happen but are rare.
The jet stream families
Five named jets come up in the exam, and the two that matter most over India are the subtropical jet in the dry season and the tropical easterly jet in the monsoon.
| Jet | Core level, height | Latitude, direction | Notes |
|---|---|---|---|
| Subtropical, STJ | 200 hPa, about 12 km | 25 to 45°N, westerly | The one that blows more or less constantly through the Northern Hemisphere. Sits near the subtropical highs, moves seasonally. Over India October to May, mean 27°N, as far south as 22°N in February |
| Polar front, PFJ | 250 to 300 hPa, 9 to 12 km | 40 to 65°N, westerly | Runs with the polar front depressions. Moves north and weakens in summer, south and strengthens in winter |
| Polar night | 50 hPa, top of the stratosphere | High middle latitudes, westerly | Mid winter only, average 150 kt |
| Tropical easterly, TJ | 100 to 150 hPa, 15 to 16 km | About 13°N, easterly | Its distinctive feature is its easterly direction. Monsoon only, June to August, over peninsular India, running west from the South China Sea across southern India to Africa. Average 60 to 80 kt |
| Arctic | 400 hPa, about 20 000 ft | Around 60°N, westerly | Transient, forms during Arctic air outbreaks over North America and northern Eurasia |
Clear air turbulence, movement and recognition
The jet stream carries its own turbulence in the sheared air around its edges, and it is the reason clear air turbulence is largely a jet stream phenomenon.
Where the CAT is
If CAT is met at high altitude, slow to the rough air penetration speed and descend, because the buffet margins are small up near the jet.
Movement and recognition
- Jets move with the sun. The subtropical jet follows the heat equator north in the northern summer. The polar front jet moves north and slows in summer, south and strengthens in winter as the front sharpens.
- From the ground, look for wind blown wisps of cirrus streaming at right angles to the lower cloud.
- In the air, a temperature change, a rising wind speed, a change of drift and CAT all point to a jet.
- On charts, the upper wind charts and the Significant Weather chart show the jet directly. Forecasts are built from radiosonde contour charts, confirmed by in flight reports, which are vital over the oceans and deserts.