By Daniel Collins
Despite what they say on Broadway, the rain in Spain does not stay mainly in the plain. Most of New Zealand’s rain doesn’t fall in the plains either, but in the mountains, particularly the Southern Alps. This is why the West Coast is so wet. The plains are in fact quite dry in comparison. In an earlier post, I mentioned how much precipitation falls on each region, but now I want to zoom in further.
Look at the map of rainfall below (actually, it’s total precipitation). While the average annual precipitation depth over the West Coast is 5.5 m, some parts of the alps get a lot more. You can also make out a small circle of high precipitation around Mt Taranaki on the west coast of the North Island. The driest places, in terms of precipitation, are inland Otago and Canterbury’s Mackenzie Basin. The Canterbury Plains are pretty dry too, as well as the pocket of land around Napier and Hastings. So just as each region receives more or less rainfall than the national average, so too does each catchment and even each valley.
Now look at the topographic map. You’ll see a strong correspondence with the rainfall map: mountainous locations tend to receive more rain than their lower and flatter neighbours. This is known as orographic precipitation, from the Greek word for mountain, ‘oros’. When moisture-laden air approaches hills and mountains, it is forced up and over the obstructing terrain. As it rises, the air cools and expands, so much so that the air temperature may drop to the dew point. (In thermodynamics, this process is called adiabatic cooling.) When this happens, the water vapour in the air starts to condense into clouds; when the cloud droplets are large enough, it starts to rain or snow. And so, broadly speaking, where the topography rises sharply, the more it will tend to rain, provided that the air is sufficiently moist. This casts a rain shadow over the downwind areas, which is why central Otago, the Mackenzie Basin and the Canterbury plains are so dry.
Of course, this picture is just an annual average and the topography is only one of the many factors at play here. In future posts we’ll talk a bit more about how the picture changes over time: among seasons, years and even decades. We’ll also describe the monitoring network that paints this picture for us, how this picture has changed in the past, and how we are striving to improve it even further.