Rising air is the source of all dramatic weather. In New Zealand, the strongest upward air motion usually occurs when strong winds blow directly against mountains. The quickly rising air expands as it encounters lower pressure at higher altitude. This causes the air to cool rapidly and its water gas to condense, forming clouds and heavy rain.
The height of the Southern Alps and the prevalence of westerly air streams over New Zealand are the reason that more days with heavy rain occur over the west of the South Island than anywhere else in New Zealand.
The heaviest falls are usually halfway up the Alps. At lower altitudes, condensation and raindrop formation have only just begun; higher up, the air has begun to run out of water vapour.
On some occasions, however, the strongest upward motion and the heaviest rain occur in the coastal zone. This happens when the low-level air is particularly stable, and so resists upward motion. Instead of rising, this air is deflected southwards by the land, and blows parallel to the Alps in a strong current of air known as a barrier jet. The barrier jet acts like a line of virtual mountains for the incoming air, which is forced to rise over the barrier jet before rising over the actual mountains. Consequently, strong upward motion begins over the barrier jet itself, and heavy rain falls along the coast.
The same phenomenon occurs on a smaller scale over the southern North Island. If the air is stable and a strong north-west air stream is blowing across the sea between Wanganui and Nelson, then a low-level north-east barrier jet develops against the Tararua Range. Out to sea, a branch of the north-west air stream rises above the barrier jet, and a cloud forms in this rising air which extends back from the ranges out over the coast.
The area west of the southern Tararua Range has a bad reputation for icing aircraft, and it is likely that the cloud which forms over the north-east barrier jet is partly to blame. Icing occurs on aircraft when they fly through clouds with abundant droplets of supercooled liquid water at temperatures between about -5° C and -15° C. The droplets freeze as they hit the aircraft's skin, and spread over it.
Particularly badly affected are the leading edges of wings. Ice changes their shape, and so affects their aerodynamic properties. Some aircraft have heating systems or moveable parts which can break the ice off. If an aircraft does not have an ice-protection device it must be quickly piloted away from the atmospheric zone in which icing is occurring, either by descending to warmer air or rising to levels where it is so cold that there are no liquid droplets in the clouds, only ice particles (which just bounce off the aircraft).
Many propeller-powered aircraft have a protective device that releases alcohol near the hub of the propeller when ice forms on the blades. The alcohol slightly melts the ice, which is flicked off as the propeller spins.
The barrier winds described so far form in stable north-west air streams that typically spread over New Zealand as an anticyclone moves away. Another phenomenon occurs when an unstable showery air stream covers the country and is in the process of stabilising as an anticyclone approaches from the west. The atmosphere stabilises from above as air sinks from high levels. Where the warmer, sinking air meets the cooler air below it, the temperature typically changes by about 4° C across a thin layer called an inversion.
In shower clouds, the rising air cannot usually push up through the inversion, which acts as a cap to the clouds. The closer the anticyclone approaches, the lower the inversion sinks, until eventually the clouds are too shallow for raindrops to form inside them.
However, if a range of mountains blocks the southerly air stream, as at Kaikoura, a curious thing happens. Once the inversion sinks below the level of the mountain tops the southerly air stream has difficulty rising over the mountains. As the air piles up against the range, the sea-level pressure next to the coast goes up two or three hectoPascals compared to further out to sea. This causes the wind at the base of the mountains to blow partially out to sea as a south-westerly.
About 25 km out to sea, in a line running parallel to the coast, the southerly wind blowing over the open ocean collides with the southwesterly, as if running into a hill. The air rises, giving the clouds a boost of energy so that they can punch up through the inversion, and, about a kilometre into the warm air aloft, grow tall enough for rain to form.
Although these showers are fairly light, the clouds are travelling along in the southerly air stream and often all pass over the same place, such as Wellington. So, six hours or so after showers first clear and when it is just beginning to seem that fine weather is there to stay, more rain passes over.
The Kaikoura barrier wind is at its strongest when a south-east air stream is blowing perpendicular to the ranges. Then, because the Kaikouras lie parallel to but a little to the west of the Tararua Range, the barrier jet has another interesting consequence. Spilling off the end of the mountains, the air pours through Cook Strait as a southerly wind in excess of 100 km/h, then funnels up behind the Tararua Range and over northern Manawatu.
Acting like a hill of air, this southerly flow prevents the south-east air stream crossing the tops of the Tararuas from descending. Then, whatever rain or snow is falling from the southeasterlies continues, or even intensifies, over the northern Manawatu, instead of the usual rain-shadow effect, whereby the rain stops on the downwind side of the ranges.
Barrier winds are not the only examples of hills of air. All cumulus clouds are, in a way, hills of air towering up through the sky. From a distance, cumulus clouds can even be mistaken for mountains. In fact, in the early days of European exploration of the Pacific, there were several occasions when, as ships were forced to turn back for home—supplies exhausted, crews ravaged by disease and starvation—sailors swore that the clouds rising above the receding horizon were actually the high mountains of the undiscovered Southern Continent.