Where the air pressure at the surface is low, air rises and becomes cooler. The water vapour in it begins to condense into tiny drops of water or, if it is cold enough, into tiny ice crystals. If there is enough water or ice, rain or snow begins to fall. This is why areas of low pressure (sometimes called depressions) are associated with bad weather.
The trough of low pressure that moves over New Zealand when an anticyclone moves away to the east will often contain several bands of cloud and rain, known as fronts. Ahead of the first front, north or north-west winds will often bring air from the tropics or sub-tropics. The tropical air will have a large amount of water vapour and consequently produces heavy rain when it rises over the hills and mountains.
If the front moves slowly, then the rain may last long enough to cause flooding. In these situations, on the West Coast of the South Island, rainfall in excess of 200 millimetres in 24 hours or less is often recorded on the slopes of the mountains.
Behind the first front there will usually be westerly winds with scattered rain. The next front in the trough will typically have much less rain. Behind this second front the wind flow is likely to be south-west, originating over the cold ocean near Antarctica. As this cold air moves over the relatively warmer seas near New Zealand, thunderstorms are likely to develop, with hail and possibly snow to low levels. Occasionally there will be a sequence of fronts embedded in the south-west flow, each with its own burst of cold air and thunderstorms.
When the trough is deep (the air pressure is much lower than either side of it), the winds around the trough will be strong or gale force.
Lows form when the air currents in the middle and upper atmosphere of a trough diverge, so that more air leaves one place than arrives to replace it, and the air pressure falls. This often happens east of Australia. There are several reasons for this:
- There is a warm ocean current running down the coast, which feeds heat and moisture into the air.
- Air in the westerly flow crosses a mountain chain just before it reaches the Australian coast, and this favours divergence aloft.
- During the winter, some of the air rising in the northern hemisphere monsoon spills across into the southern hemisphere. This increases the high-level winds, known as the jet stream, where wind speeds can exceed 300 kilometres an hour. In turn, this increases the high-level divergence in the trough and so accelerates the pressure fall at sea level.
Many of the lows that form in the Tasman Sea bring unsettled weather to New Zealand. When the lows are deep, they have strong easterly winds on their southern flank. Where these blow from sea to land, the air is lifted as it rises over the hills. This adds to the upward motion already taking place within the low, thereby causing heavy rain. Such lows are able to bring rain to eastern areas that are sheltered from the prevailing westerlies.
In winter, if very cold air feeds into the low centre then heavy snow can fall in eastern areas. This happened over Canterbury during the winter of 1992.
New Zealand’s worst 20th-century snowstorm
Late July 1939 saw widespread snow when a deep trough lay east of New Zealand, allowing cold south-westerlies to bring Antarctic air over the country. It snowed from Cape Maria van Diemen in the far north to Southland, where flooding occurred when the thaw set in. Dunedin was worst affected. There was snow a metre deep in some of the hill suburbs, which ran short of food. In Auckland on 27 July, 5 centimetres of snow fell on the summit of Mt Eden, and the Bombay Hills shone white for most of the morning.
In Gisborne, snow fell for nearly three hours, and in Masterton, the snow lay 15 centimetres deep. Snow fell to sea level at Castlepoint, and drifts in the hills closed the road inland.
Snow was not the only problem during the devastating cold snap. Overnight frosts caused water pipes to burst in Palmerston North and Hastings. At Paremata, just north of Wellington, eight hectares of the harbour froze over. Tidal waters also froze at Ōpōtiki, in the Bay of Plenty.