The only thing that is certain about the South Island weather is that it will be changing rapidly.
The majority of weather systems arrive over the South Island having originated in Australia or the SubAntarctic seas somewhere south of Tasmania.
These systems give us three general types of weather:
Anti-Cyclonic (High Pressure System)
Let’s take a look at each one:
High Pressure System
In the summer, these produce light winds and average to strong thermal strength in the mountains (Southern Alps). The convection in the mountains quickly draws in a sea breeze from the NE and across the Canterbury plains. Christchurch and the Plains will generally only be good for coastal soaring in this NE sea breeze.
In the winter, these systems are more common and can last many days. However, the persistence of the high pressure (descending air) causes strong inversions which limit thermal activity. The NE’r sea breeze is generally a lot weaker. In these conditions, the Port Hills thermic sites can be flown for longer before the sea breeze arrives.
The famous NW’er is a Foehn wind which we experience most during the spring and autumn equinox. You can check the Metservice mountain forecast for the likelihood of this wind. The NW’er brings warm dry air as the westerly air mass is dried and compressed by passing over the Southern Alps and descending into the Canterbury plains. The NW’er creates a system of wave lift and turbulent rotor bars downwind of the Alps. The crest of the waves normally form cloud that give rise to the smooth lenticular clouds at high altitudes. The rotor clouds are marked by ragged cumulus clouds in between the wave crests.
In Christchurch, something odd happens when the NW is around 30-80 km/h at 2000 m in the Alps. One of the NW’er waves hits the ground around Christchurch airport and bounces back up. This encourages the NE sea breeze circulation to start and you can find that although there is predominant NW wind, it is actually NE at low altitude. Here, you need to be careful. The NE sea breeze is cold and dense, so it stays at low altitude. This makes it possible to fly low lying coastal sites (Taylor’s Mistake) but 100 m higher on the Port Hills (Gondola) it is turbulent NW. So, only fly low coastal sites in these conditions. Do not be tempted to fly higher Port Hills sites even if the NW feels like 10-20 km/h. It is turbulent and can quickly change. When the NW strength increases past 80 km/h at 2000 m in the Alps, the NW wave normally pushes over the city completely. Do not fly in these conditions.
Immediately after the NW wind, SW fronts and winds hit. Sometimes as a gust front. The SW front can bring rain but being a cold front the poor weather clears quickly, leaving us with a cold and dry airmass. The SW wind normally drops quickly during the day and leaves us with our best thermic soaring conditions.