The rain shadow (the dry side of the hills)

Stand on the eastern crest of the Mount Lofty Ranges and look back the way you came, and you have just watched a weather system get robbed. All winter, moist westerly air blows in off Gulf St Vincent and the ranges take it apart: forcing it upward, cooling it, wringing the water out of it as rain, long before it ever reaches the ridge you are standing on. Squeeze a cloud like that over a few hundred vertical metres and you get the drenched, ferny side of the range behind you — catching well over a thousand millimetres a year in the high country, against the few hundred the plains below make do with (Gill, McKenna & Wouters 2014). But whatever the cloud has left by the time it finally clears the summit is spent. It rolls down the far slope already wrung dry, and the country on that side gets whatever is left over, which is not much.

That is a rain shadow, and it is not a metaphor; it is the same plain physics that plays out behind every mountain range on Earth standing across a prevailing wind. The Mediterranean engine that hands the western, seaward slopes their wet winters and killing-dry summers (Cowling et al. 1996) hands the eastern lee an even harder bargain: a summer just as dry, stacked on a winter that was thin to start with. Drive over the crest and down the eastern side and you can watch the vegetation do the arithmetic within a few kilometres — tall wet stringybark forest thinning, then giving out altogether, into a low, grey-green scrub of multi-stemmed, lignotuber-rooted eucalypts standing over saltbush or spinifex. That is the mallee, the driest and toughest community on the whole gulf-to-range transect, and its extent and its inhabitants were counted, quadrat by quadrat, across a landscape most people only ever cross at highway speed (Foulkes & Gillen 2000).

It helps to think of the rain shadow as a mirror. Everything the wet side of the range does with its surplus — fern gully, swamp, tall forest, a refuge for species the baking plain cannot support — the dry side does in reverse, building an entirely different, equally specialised community out of scarcity. Read the rain shadow correctly and you are reading the same mountain twice: once as a gift, and once, a few kilometres further on, as a debt still being paid.