Fronts and depressions

Weather is driven by how the Sun's energy is distributed and moves. Warm air rises, is less dense and carries more moisture; cold air is heavier and drier. Air tends to move from high pressure (H) towards low pressure (L). An air mass is a large body of air that takes on the characteristics of the area where it formed: if it has crossed the sea it is maritime and moist, if it has crossed land it is continental and dry.

In the figure you can see the air masses arriving over the British Isles. Tropical continental air comes from North Africa in summer: it is hot and dry. Polar continental air reaches us from the pole across land, from the east or north-east: it is dry and, in winter, very cold. Polar maritime air arrives from the north-west, warmed from below over the warmer sea and loaded with moisture: it brings showers, some of them hail, with clear spells in between. Continental air masses change little on their journey, while maritime ones are modified a great deal. To find out which air mass is arriving, follow the isobars back to where the wind originates. These examples are written for the UK: for the Mediterranean they must be adapted.

At our latitudes the Atlantic depressions form on the polar front, where cold air from the pole meets warm tropical air. Aloft, the jet stream meanders between the two. Where a trough forms, air aloft leaves faster than it arrives and the surface pressure falls. Warm air meeting cold air has to rise, and with the Earth's rotation a wave with two fronts forms: a depression. It is a little like two gears: between two highs turning clockwise, the low turns anticlockwise.

The life cycle of a depression has five stages, the steps shown alongside. First comes the polar front, then the young wave, with a cold sector, a warm sector, a warm front and a cold front. Then comes the deepening: the central pressure falls and the cold front begins to catch up with the warm front. With the occlusion the occluded front forms and the depression is mature. Finally the depression fills: the central pressure rises and the occlusion closes like a zip. The cold front moves faster than the warm front and overtakes it: that is how the occlusion arises.

A front is the line of contact between two different air masses, like oil and water that do not mix. The same rule holds at every front: it is the warm air, being lighter, that rises over the cold air, which is denser. At a warm front the climb is a gentle ramp, as the cross-section in the figure shows: warm air slides slowly up over the retreating cold air.

That is why high cloud, the cirrus, can appear hundreds of miles ahead of the front at the surface (up to about 800 km, roughly 430 miles). Then the sky closes in, in order: cirrostratus, altostratus, nimbostratus. The closer the front, the lower the cloud base, the poorer the visibility and the steadier the rain. The first warning of a warm front, then, is the high cloud, long before the rain.

The cold front is a much steeper wedge than the warm front, as the figure shows: cold air undercuts the warm air and forces it upwards abruptly. Deep cloud forms, cumulonimbus, with a narrow but intense band of rain, sometimes with hail and thunderstorms. As it passes, the wind veers sharply (clockwise), often with squalls, and the pressure rises suddenly. Behind the front the air is colder, with sunshine and showers and very good visibility between them. The cold front moves faster than the warm front, which is why sooner or later it catches it up.

The cold front moves faster than the warm front and at some point catches it, as the cross-section in the figure shows. The warm air, squeezed between the two, is lifted and detached from the surface: at the surface the cold air arriving from behind meets the cool air that was ahead. This is the occluded front, and on the chart it is a purple line with alternating triangles and semicircles. It appears when the depression is mature: afterwards the central pressure rises and the occlusion closes like a zip.

The figure shows the depression seen from above. At the centre is the low (L); around it, the isobars, the lines joining points of equal pressure. From the centre run the warm front, ahead, and the cold front, behind, with the warm sector between them. Remember that air tends to move from high pressure to low pressure.

The wind, however, does not blow straight towards the low: the Earth's rotation (the Coriolis force) deflects it. Aloft it blows along the isobars (the geostrophic wind). Near the surface friction slows it and makes it spiral in towards the centre of the low at a moderate angle. The closer the isobars, the stronger the wind; where they are wide apart, the wind is light. In the northern hemisphere the wind circulates anticlockwise around a low and clockwise around a high.

This rule tells you where the centre of the depression is, even without a chart. In the figure the yellow dot is you, with the wind at your back: in the northern hemisphere low pressure is on your left and high pressure on your right. In the southern hemisphere it is the opposite. It is also used to check on the chart the wind direction you have worked out.

The figure follows a depression passing north of the boat, in the northern hemisphere, with time running from left to right. Before the warm front cirrus appears, then cirrostratus, altostratus and nimbostratus; the pressure falls steadily, the wind backs (anticlockwise) and freshens, the rain becomes continuous, the cloud base lowers and visibility falls. As the warm front passes the wind veers (clockwise), the pressure stops falling, the rain turns to drizzle and the humidity rises.

In the warm sector wind and pressure are steady, the air is humid, with patchy drizzle and probable fog. As the cold front passes the wind veers sharply (to the right), often with squalls, the pressure rises abruptly, and heavy rain arrives, possibly with hail and thunderstorms. Behind the cold front the wind direction is steady, but the wind is stronger and gustier; the pressure rises and then levels off, the temperature is lower, and between showers there is sunshine and very good visibility. In short: backing before the warm front, veering at the fronts, then the wind settles. The directions in the figure are only an example: what matters is how the wind turns.

The synoptic chart is the weather chart for the surface, for example those issued by the Met Office. The figure shows a simplified chart: the isobars (usually every 4 hPa), the centres of high (H) and low (L) pressure with their value in hPa, and the fronts with their symbols. There is often also a geostrophic wind scale, so you can read the wind strength from the spacing of the isobars. The title gives the validity: “T+12 valid 12 UTC” is a 12-hour forecast for 12 UTC.

To read it, follow four steps. First, find the lows and highs and the direction in which the wind circulates around them. Second, follow the fronts and work out where you are in relation to the warm sector. Third, look at the spacing of the isobars near you to estimate the wind: close isobars, strong wind. Fourth, use Buys Ballot's law to check the wind direction.

How fast is a system moving? In Met Office sea-area bulletins, as reported by the Royal Meteorological Society, the speed at which systems move is expressed in standard words, in knots: slowly, 0 to 15 knots; steadily, 15 to 25 knots; rather quickly, 25 to 35 knots; rapidly, 35 to 45 knots; very rapidly, over 45 knots.

Four recurring mistakes. The first: it is the warm air, being lighter, that rises over the cold air, which is denser and heavier, and never the other way round; the figure shows this at every front. The second: veering is a clockwise shift of the wind (for example from NE to SE), backing an anticlockwise one (from NW to SW). The third: the anticlockwise circulation around a low applies in the northern hemisphere, and in the southern hemisphere it is the opposite. The fourth: the first sign of a warm front is high cloud, long before the rain.