November 2004  Wind speed and direction in the mountains

How strong does the wind have to be to knock over a tree … or even a person? MetService Weather Ambassador Bob McDavitt provides a guide to measuring wind speed both in the mountains and on land.

Mountain weather forecasts give wind direction to the nearest octant (in degrees true), plus or minus one octant (an octant is an eighth of a circle or 45 degrees). Speed is given in kilometres per hour, sometimes abbreviated to kph or even to “k” – but please don’t confuse this with “knots”! If you hear a weather forecast with knots in it, it’s either a marine or aviation forecast, not a mountain one. Speed is given to the nearest 10kph and carries a precision of plus or minus 10 kph. The brief mountain forecast uses the “Beaufort equivalent descriptive” as follows:

Strong = 45 to 60 kph, gusts to 90 kph or more

Gale = 60 to 90 kph, gusts to 120 kph or more

Storm = over 90 kph, gusts to 150 kph or more

In the lowlands wind gusts can be 50% stronger than the sustained average at low levels. This percentage gets less with altitude and above about 1000 metres the gust factor fades.

Land Wind Speed guide   

Kph (gusts)Impact
Less than 10Leaves rustle, smoke drifts
10 to 20Twigs move, flags flap
20 to 30Branches move, raises dust and loose papers
30 to 40Small trees shake
40 to 50Large branches shake, power lines whistle
50 to 60Trees shake, hard to walk into the wind
60 to 70Twigs break, lifts dirt and sand
70 to 80Branches break
80  to 90Twists signs and TV aerials
To 100Mature tree in wet soil falls, breaking branches are big enough to take down power lines weak roofing iron rips off, planes are grounded
To 110Boats break off moorings, roofing tiles lift 
To 120Windows blown out, power lines snap
To 140People tumble

Only the general wind can be read from a weather map and put in the mountain forecast. Terrain effects swing the breeze a lot and may as much as halve or double its speed. Wind over land acts like water over rocks; it runs around corners, gallops through gaps, contorts along coastlines and dips and dives over hills and dales. During quiet sunny days, air is drawn inshore as the land heats up (“the land sucks”). On clear nights, the land cools and this cools the air near the ground so that it drains downhill and accumulates in pockets. An inversion forms between the chilled air near the ground and the un-chilled air aloft.  The chilled air stops moving, and that’s what makes dawn the calmest time of the day.  

Sometimes an island or a range of hills may block the incoming wind like a dam, but once this dam is full the overflow sinks over the down-wind slopes and speeds up as it drops to sea-level, turning what might have been a sheltered valley into a wind channel. Be especially wary of “rivers of wind” at the end of mountain chains and through gaps like Cook Strait and Manawatu gorge. Be especially wary when there is a high pressure system on the weather map – it brings stable air that is very difficult to lift. Rather than tumbling up-and-over a mountain range, stable air squeezes around the shoulders or through the passes.  What might look like a “fine” weather map, can easily result in a gale at the pass. 

The force (or push or pressure) of the wind increases with the square of the speed (its power, as used in wind turbines, rises with the cube). Thus doubling the wind speed increases its punch four-fold. The wind force in Newton/square metre is half the density (1.25 kg/m3) times (speed m/s)2 times shape factor(about 1). For practical purposes divide this by 9.8 to get kgforce/square metres. Then a 45kph wind has a dynamic pressure of about 10 kgforce (or 98 Newton) per square metre and if it hits you suddenly it’s like some one throwing a 10kg air bag at you. That’s OK, but double the speed to 90 kph and the force jumps to about 100kgfper square metre, like being hit by two 20kg packs at once. Anything over 90 kph is worth avoiding. When the wind is gusting around 140 kph the sideways-pressure thrusts are around 100kgf – strong enough to knock over a 100kg human.. The strongest wind gust measured in New Zealand was 250 kph at Mount John in Canterbury on 18 April 1970. 

MetService issues severe weather warnings for strong wind when the forecast includes widespread areas (1000 square km or more) with a minimum mean speed of 90 kph or frequent gusts exceeding 110 kph. These warnings are available at www.metservice.com and from there you can arrange to receive them as they issued via email. 

Bob McDavitt retired from the MetService in 2012. FMC thanks Bob for his valued contribution as the Bulletin weather columnist over many years. This column was originally published in the November 2004 FMC Bulletin

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