The shapes isobars make, and the wind that follows them
A surface weather chart is nothing more than pressure drawn as contour lines. Learn to read the five shapes those contours make and you can forecast the day, because each shape carries its own wind, its own vertical motion and its own weather. This page is the grammar. The three pages after it are the vocabulary.
Isobars
An isobar is a line joining places of equal pressure. Before the line can be drawn, every station's reading has to be reduced to a common level, mean sea level, so that a hill station and a coastal one can be compared. That reduced value is QFF, the station pressure brought down to mean sea level using the actual temperature of the air column. Isobars are lines of QFF, not QNH, and on an analysis chart they are spaced at a fixed interval, usually 2 or 4 hectopascals.
The pressure gradient
The pressure gradient is the rate at which pressure changes with horizontal distance, measured at right angles to the isobars, always from high pressure towards low. It is the force that drives the wind, so the spacing of the isobars is a direct picture of wind strength.
Buys Ballot's Law
In the nineteenth century the Dutch meteorologist Buys Ballot set down the link between wind direction and pressure in a single sentence.
The wind blows almost along the isobars, not across them. Friction near the surface turns it a little towards the low, which is why air spirals gently inward to a depression and outward from an anticyclone rather than simply circling. One more term you will meet throughout the chapter: advection is the meteorologist's word for the horizontal movement of air.
The five systems
Every pattern on a surface chart is one of five named shapes: a depression or low, an anticyclone or high, a trough, a ridge, and a col. The rest of this chapter takes them in that order.
Depressions and troughs
A depression is a region of comparatively low pressure shown by roughly circular, concentric isobars with the lowest pressure at the centre. It is also called a low, or a cyclone, though in Europe the word cyclone is kept for tropical revolving storms and cyclonic circulation simply means flow around a low.
How air moves through a low
Buys Ballot's Law puts the Northern Hemisphere wind anticlockwise around a low, and friction turns it inward, so at the surface air is converging on the centre. That air has nowhere to go but up. It rises through the depression to medium and high levels and then diverges outward at altitude.
The link between the top of the column and the surface is worth stating on its own, because it is asked directly. If air is diverging aloft, it is being removed from the top of the column faster than it is being supplied, so the surface pressure falls and the depression deepens. Convergence aloft does the opposite and the low fills.
Two kinds of depression
- Frontal, or Polar Front, depressions. Large scale systems of the temperate latitudes, formed where warm, moist sub tropical air meets cold polar air. They move roughly west to east, deepen and then decay, and their average life is 10 to 14 days. These are the depressions that give north west Europe its changeable weather.
- Non frontal depressions. Smaller and able to form almost anywhere. Thermal depressions form over land heated strongly in summer. Tropical cyclones form over warm sub tropical oceans. Cold air lows form over the sea in polar or arctic air in winter.
Depression weather
| Element | In a depression |
|---|---|
| Cloud | Extensive, often from low level to the tropopause |
| Precipitation | Continuous, intermittent or showery, light to heavy by type |
| Visibility | Poor in precipitation, otherwise good in the ascending air |
| Temperature | Depends on the type of depression and the season |
| Wind | Usually strong, stronger the deeper the low and the closer the isobars |
Joshi adds a comparison that is easy to remember and often tested: visibility in a low is generally better than in a high, because the rising air disperses haze, the exception being while it is actually raining.
Troughs
A trough is an extension of a low, a tongue of lower pressure reaching out from the centre. Its isobars are sharply curved, drawn V shaped, and the pressure along the trough line is lower than on either side. Crossing a trough the wind backs abruptly, changing direction as it passes the line. The weather in a trough is much like the weather in the depression it belongs to. Troughs very often lie along the cold front of a Polar Front depression, and the monsoon trough along the Indo Gangetic plain is a large scale example.
Winds around a low blow in anticlockwise direction in N hemisphere, converging towards the centre. There is convergence and upward motion at a low. Hence a low is associated with bad weather, visibility is better than a High.IC Joshi, Aviation Meteorology, chapter 2
In a depression air is converging at the surface, rising from the surface to medium to high altitude then diverging at medium to high altitude. The weather associated with a trough will be similar to that of a depression.Oxford ATPL Vol 9, 4
Anticyclones, ridges and cols
An anticyclone, or high, is a region of relatively high pressure with roughly circular isobars and the highest pressure at the centre. The isobars are widely spaced, so the gradient is slack and the winds are light. In the Northern Hemisphere they blow clockwise, veering as they go, and diverge outward at the surface.
How air moves through a high
The circulation is the mirror image of a depression. Air converges aloft, descends through the anticyclone, and diverges at the surface. That descent has a name.
Subsiding air is compressed as it sinks and warms as it is compressed. A layer of this warmed air sitting on top of cooler surface air is a subsidence inversion. It acts as a lid: haze, smoke and moisture collect underneath it and cannot disperse upward, so an anticyclone can give fair, dry weather and poor visibility at the same time.
Five kinds of anticyclone
- Cold anticyclone. A permanent feature over the poles and a winter feature over cold continents, such as the Siberian high. The surface air is chilled, becomes dense, and draws more air down from above, raising the surface pressure. It is a shallow system.
- Warm anticyclone. The sub tropical highs, such as the Azores high in the North Atlantic. In the three cell circulation, the Hadley cell, the Ferrel cell and the Polar cell, air flowing towards the poles at the tropopause piles up in the sub tropics and is forced to descend, giving permanent, deep, warm highs over the sub tropical oceans. Their pressure surfaces bulge upward, which is the signature of a warm high.
- Temporary cold anticyclone. A ridge of high pressure in the cold air between two frontal depressions. Because the depressions are moving quickly, it lasts up to about 24 hours.
- Blocking anticyclone. A near stationary high that blocks the usual eastward march of frontal depressions and forces them onto northerly tracks in the Northern Hemisphere. Usually an extension of the warm sub tropical highs, or of the Siberian high in a European winter, it can persist for weeks.
- Ridge, or wedge. Isobars extending outward from a high, always rounded, never V shaped. A region of fair weather and a brief fine interlude between two depressions. Winds veer crossing a ridge.
Anticyclonic weather
| Element | Summer high, and cold highs in winter | Warm high in winter |
|---|---|---|
| Cloud | Little or none, except at the edge | Extensive low stratus, limited vertical extent |
| Precipitation | None | Possibly drizzle |
| Visibility | Moderate, with haze | Moderate to poor, mist and fog likely |
| Temperature | Depends on type | Relatively warm |
| Wind | Light | Light |
So the answer to "what is it like in the centre of a summer high" is calm winds and haze, not thunderstorms and not dense cloud.
Cols
A col is the region of slack, almost level pressure trapped between two highs and two lows. It is an area of stagnation with light, variable winds. Its weather turns on the season: poor visibility and fog in autumn and winter, and thunderstorms in summer.
H is associated with fair weather but visibility is poor due to subsidence. In N hemisphere winds blow in a clockwise direction, veer, and diverge outwards. Due to the downward air motion in an anticyclone the descending air is compressed and after sometimes the cold anticyclone warms up.IC Joshi, Aviation Meteorology, chapter 2
Within an anticyclone, at high altitude we have air converging, then descent of air within the anticyclone, subsidence, and divergence at the surface. Ridges of high pressure are indicated by isobars extending outwards from an anticyclone and always rounded, never V-shaped as seen in a trough.Oxford ATPL Vol 9, 4
Movement, the upper air charts, and the systems over India
Knowing the shape is half the job. The other half is knowing how the shape moves, because a forecast is really a statement about where each system will be tomorrow.
How the systems move
- Frontal depressions move rapidly, roughly west to east.
- Non frontal depressions vary with type and location, from nearly static to moderate speeds.
- Anticyclones are slow moving and persist in much the same place for long periods.
- Cols tend to be static, lasting a few days before another system replaces them.
The vocabulary of change: a depression deepens as its pressure falls and fills or decays as it rises. An anticyclone builds as its pressure rises and weakens or collapses as it falls. Systems tend to keep their shape from one day to the next, which is what makes tracking them useful.
Upper air charts
A surface chart draws pressure on a surface of constant height, mean sea level. An upper air chart turns that around: it draws height on a surface of constant pressure. The contour lines, called isohypses, are labelled in geopotential metres, and a dip in the contours towards the equator is an upper trough while a bulge towards the pole is an upper ridge, the same shapes seen aloft.
| Pressure level | Approx height | Flight level |
|---|---|---|
| 850 hPa | 5 000 ft | FL50 |
| 700 hPa | 10 000 ft | FL100 |
| 500 hPa | 18 000 ft, about 5.5 km | FL180 |
| 400 hPa | 24 000 ft | FL240 |
| 300 hPa | 30 000 ft, about 9 160 m | FL300 |
| 200 hPa | 38 000 ft | FL380 |
| 100 hPa | 53 000 ft | FL530 |
The systems that bring India its weather
Joshi's synoptic appendix lists the systems that matter over the subcontinent:
- Western Disturbance. A low or a cyclonic circulation in the middle and lower troposphere that forms over the Mediterranean, Caspian or Black Sea and travels across north India, bringing winter rain to the plains and snow to the hills. With two or more closed isobars at the surface it is called a Western Depression.
- Induced low. A low that develops to the south of a Western Disturbance under its influence.
- Trough in the westerlies. A wave in the mid latitude westerlies moving west to east, generally affecting the northern parts of India.
- Easterly wave, or trough in the easterlies. A shallow trough in the tropical easterlies moving east to west, clearer in the upper winds than at the surface, bringing cloud and showers to peninsular India.
- Monsoon trough. The semi permanent low pressure trough along the Indo Gangetic plain in the monsoon season. Where the trough sits decides where the rain falls.