Putting on a front

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Fronts adorn most weather maps, and can be recognised as lines decorated with solid black triangles or half-moons. Put simply, they mark areas with abundant cloud and usually some rain. As they pass over an area, there will generally be changes in both wind and tempera­ture.

Fronts have traditionally been described as the boundaries between air masses of different temper­ature. When cold air is advancing and replacing warmer air, the front is designated as a cold front (a line with triangles); when warmer air is advancing, the front is designat­ed as a warm front (a line with half moons).

The traditional explanation for the formation of cloud and rain was the fact that cold air is denser than warm air, and, when the two con­verge, warm air is forced to rise. In the case of a cold front, cold air burrows under warm air, forcing it to rise; in the case of a warm front, warm air rides up over cold air. Either way, the rising air is cooled by expansion, causing water vapour to condense and form clouds and rain.

This explanation is now acknowledged to be only part of the story. In fact, a lot of cloud is formed ahead of the front by other mechanisms. Studies of satellite and radar images have given rise to the idea that cold fronts are preced­ed by “conveyor belts” of air, thousands of kilometres  long, that start at the earth’s surface in or near the tropics and slope upwards as they move towards the poles, reaching altitudes of six kilometres before turning away from the front and descending again.

Warm, moist air rising up the conveyor belt soon cools enough to form extensive cloud sheets, which eventually produce rain. Sometimes the warm conveyor belt will actually cross over the leading edge of the cold air.

This causes a narrow  band of very heavy rain.

In the traditional view, the change of wind and temperature associated with a front was expected to happen almost instantaneously. We now know that fronts are actually zones about 50km wide in which there are usually a number of wind and temperature changes.

Nevertheless, in the east of New Zealand some cold fronts are experienced as a very abrupt change of temperature as the wind turns to the south. In the South Island, this happens because the north-west wind ahead of a southerly change is very warm, since the air has crossed the Southern Alps and has been warmed as it descend­ed to the coast. By contrast, the air in the southerly has not crossed the mountains but has come straight off the sea.

The strength of a front can be temperature across it. Like many atmospheric processes, frontal develop­ment is accelerated by positive feedback mecha­nisms.

Fronts are created and intensified by wind patterns that blow warm air and cold air towards each other. As the tempera­ture difference increases in an intensifying front, the pressure begins to fall, causing the wind to increase and changing the distribution of the atmos­phere’s vorticity. This results in a new pattern of upward and downward air currents that in turn give rise to horizontal currents that intensify the tempera­ture difference across the front even faster.

Another positive feedback that intensifies the temperature difference across the front is the heating of the air in the warm conveyor belt when water vapour condenses to form cloud droplets—a reaction which releases heat.

One of the most intense fronts experi­enced in New Zealand in recent years crossed the North Island early in March 1990. The rain ahead of the front caused floods in Taranaki and Wanganui on Saturday, March 10, and in Mana­watu, Welling­ton, and Wairarapa three days later. In Taranaki, the Waitara and Oakura Rivers broke their banks, and many people had to be evacuated. Slips closed many roads and a freight train was derailed. The Wanganui River also overflowed its banks, flooding parts of the city and rising to within one metre of the record flood of 1904.

The front that caused this damage had exception­ally moist tropical air ahead of it. When the strong northerly winds ahead of the front lifted this air over the Taranaki hills, torrential rain fell out of it into the catchments of rivers in both Taranaki and Wanganui.

The front had developed from Tropical Cyclone Hilda as it moved into the North Tasman Sea the previous week. The air behind the front was only a few degrees cooler than the air ahead of it, so it was not entirely appropriate to call it a cold front. The major difference lay in the moisture content of the air—the air ahead of the front contained about four times as much water vapour as the air behind it.

In situations like this one, the front is sometimes designated as an occlusion, and marked on weather maps with both triangles and half-moons. Traditionally, an occlusion was said to have occurred when a cold front caught up with the warm front ahead of it, and so lifted all the warm air above the earth’s surface. In fact, this is a rare occurrence, and the occlusion symbol is now widely used for fronts where there is no pro­nounced temperature contrast at the earth’s surface.

Radar enables us to study the workings of fronts by looking at the distribution of water droplets inside the clouds. They show that, although there are similarities, no two fronts are the same Sometimes, a pattern of lines of strong echoes parallel to the front is seen on the radar screen. These lines are thought to be formed by gravity waves (in a similar way to ripples forming when a stone is thrown into a pond). As one line of thunderstorms pushes vigorously up into the sky, the air immediate­ly ahead of it tends to sink, suppressing any shower cloud. This sinking air then helps the air ahead of it to rise, causing another line of showers with strong radar echoes.

Fronts get a share of most of the excitement going on in the weather Each time we find new ways to probe inside them, we find they have a new story to tell us about how the atmosphere works.

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