On 10 October 2022 I had a climber booked for eight o’clock to strip deadwood out of a 22-metre Lombardy poplar behind a villa on Straven Road, Riccarton. The MetService forecast the evening before read north-west 45 km/h rising to 70 in the afternoon, with a strong wind watch for the plains. At seven I looked at the arch of cloud sitting over the Alps, felt the air already warm and dry at 14 degrees, and cancelled. By two in the afternoon Christchurch Airport was logging gusts in the nineties and a limb the thickness of my leg came out of that poplar on its own. Nobody was in it.
What a nor’wester actually is
A Canterbury nor’wester is a föhn wind. Moist westerly air rides up the Southern Alps, drops its rain on the West Coast, and comes down the eastern side compressed, warmed and dried. Over the plains it arrives as a hot, gusty north-westerly with humidity that can fall below 20 percent and temperatures that jump 8 to 10 degrees in an hour. The tell is the nor’west arch: a clean band of clear sky between the mountain cloud and a high sheet further east. When the arch is visible at breakfast, the wind is coming by lunch.
The important word is gusty. A nor’wester is not a steady 60 km/h you can lean into. Mean speeds of 50 can carry gusts of 90 to 110, and the gust factor in the lee of the Port Hills or between two-storey houses is worse. The August 1975 event flattened shelter belts across the plains and remains the reference storm for anyone who writes about Canterbury trees. Smaller versions happen most springs.
The numbers I look at
The forecast mean speed is nearly useless for tree work. I look at the forecast gust, the timing of the peak and the timing of the southerly change behind it. My own rule, stricter than most contractors run, is no rope work above 10 metres if forecast gusts exceed 50 km/h, and no chainsaw aloft at all above 40. The arboriculture code of practice says to stop in high wind without naming a number, which leaves it to whoever is signing the job sheet. I would rather be the one who called it off.
Two other things go on the sheet. Humidity: when the dew point drops sharply through the morning, the wind is already coming down the slope even if the flags are still. And the soil: a nor’wester is dry, but if the preceding fortnight was wet, the root plates of shallow-rooted species are sitting in soft ground and the wind gets leverage it would not have had in a dry January. NIWA’s climate records for the Canterbury stations are where I check what the previous month actually delivered rather than what I remember of it.
Which trees I would not be in
Poplar and willow first, because their wood is brittle and the limbs are long levers. Macrocarpa and radiata pine in old shelter belts, because the rows have been thinned by decades of removals and the survivors were never meant to stand alone. Anything in full new leaf, which in Christchurch means September to November, when the sail area is at its maximum and the season’s wood has not hardened. And anything with an existing lean toward the south-east, because that is the downwind side and the direction it will go.
Lean is not automatically a fault. Trees that have grown in wind for fifty years put on reaction wood and are often the safest thing on the block; the problem is a lean that is new. I wrote about that distinction in the piece on wind-sculpted trees on a Wellington hillside, and the same reasoning applies on the flat here, with less slope and more sail.
What to do with the day instead
A cancelled climbing day is not a wasted one. Ground crews can chip, stack and clear. I walk the site before the wind arrives and photograph every tree I have concerns about, then walk it again between the nor’wester dying and the southerly arriving, because that half-hour of calm is when new cracks and lifted root plates are easiest to see. The southerly, cold, wet and from the opposite direction, is what actually finishes off trees the nor’wester has loosened.
The money side is simple. A two-person climbing crew costs NZ$180 to NZ$280 an hour on site, so a cancelled day is NZ$1,500 to NZ$2,500 that someone has to absorb. Every quote I review should carry a weather clause saying who that is. Most do not.
Frequently asked questions
At what wind speed should tree climbing stop? There is no legal number in New Zealand; the code of practice says stop in high wind. Most careful contractors stop rope work when gusts reach 50 km/h and chainsaw work aloft at about 40 km/h, and a nor’wester forecast with gusts above 70 km/h should cancel the day outright.
Do nor’westers blow trees over? They break limbs more often than they uproot whole trees, because the ground is usually dry. The wet southerly that follows, arriving from the opposite direction on a tree that has already been loosened, does most of the uprooting.
Which Christchurch trees are most at risk in a nor’wester? Poplars, willows, old macrocarpa and radiata shelter belts, and any tree in full spring leaf with a lean toward the south-east. Trees that have grown in wind for decades are usually more stable than they look.