Route Climatology — Page 454, Lesson 423

I want to walk you through the Route Climatology chapter now, starting with the Southern Hemisphere routes. Let's begin with Polar Front Depressions. In the Southern Hemisphere's disturbed temperate zone, travelling polar front lows are located well south of the typical route. But even though the main low is far south, the troughs and secondary lows associated with it can bring frontal weather as far north as the Australian South Coast and New Zealand. Just like in the Northern Hemisphere, these eastward-travelling fronts alternate with temporary ridges and anticyclones. Cold frontal activity can be quite severe on the Sydney to Auckland sector. Now let's look at Surface Winds along this route. Starting at Singapore, the SW Monsoon Wind will soon back to SE — that's a backing wind, meaning it shifts anticlockwise. This happens because of the Coriolis effect as the route crosses the Equator. These SE winds are the trade winds, and they remain as far as Darwin and beyond. However, at Darwin itself, strong local sea breezes may blow from the north. On the Darwin to Sydney sector, the SE trade winds gradually veer — that's a veering wind, shifting clockwise — to southwesterly. This happens to conform with the anticlockwise rotation around the Central Australian winter high-pressure system. Towards Sydney and beyond to Auckland, the wind direction is locally governed by the location of those travelling polar front depressions to the south, but it's generally westerly. South of the Australian land mass, these westerlies encircle the globe largely unimpeded by land, so they strengthen. Here they are known as the 'Roaring Forties' — named for the principal latitude band, roughly 40°S, in which they blow. Sea breezes can affect Sydney even in winter. Now Visibility. Between the frequent equatorial showers over Singapore and the Indonesian islands, visibility is good until near 05°S. Beyond that latitude, haze appears, caused by the dry dust-laden SE trade wind blowing from Australia. Beyond Darwin, the dusty outflow from the interior maintains haze. Near large cities, visibility may be reduced to 1 to 2 kilometres by industrial smoke haze. Over the two islands of New Zealand, the clear air gives good visibility in between cold frontal precipitation. Radiation fog can occur inland, especially over the colder South Island. Advection fog — also called sea fog — can occur off the South Island east coast, over the cold Antarctic Drift sea current. Finally, Cloud and Precipitation. Daily convective cumulus with showers over Singapore and the Indonesian islands gives way to quieter weather towards Darwin. The anticyclonic Australian interior is dry. But rising over the mountainous east coast, the onshore SE trade winds can give orographic cloud and rain. The Sydney area and the Tasman Sea route are affected by the disturbed temperate region lows to the south. They are crossed by fronts which bring moderate to heavy precipitation, interspersed with highs that give several days of cool fine weather. Let's look at the upper wind charts for these seasons. Here we have the July — winter — upper winds for the Southern Hemisphere , and the January — summer — upper winds . And here's the January surface weather chart with wind velocity, and upper wind velocity shown in yellow .