March 2004  The Föhn Effect and ‘God’s Own Wet Zone’

Where in New Zealand lies ‘God’s Own Wet Zone?’ Former MetService Ambassador, Bob McDavitt, explains in layman terms how to understand orographic precipitation, freezing level, and the best recipe for snow.

In the last Bulletin I gave all the weather schedules that trampers need to know to check weather to go. Now, what about the mountain forecast areas? In the North Island these work well with the national and forest parks. Commercial (MetNet or MetFax) forecasts are prepared for the Urewera once a week, for Egmont/Mount Taranaki daily and for the Tongariro area four times daily. The Brief Mountain Forecast (free on www.metservice.co.nz) may also mention the Raukumara, Kaweka, Kaimanawa, Ruahine, or Tararua Ranges.

The map shows the myriad of South Island areas: Nelson Lakes, Canterbury High Country and Southern Lakes four times a day, and Arthurs Pass, Mt Cook, Mt Aspiring and Fiordland once a day. Please distribute this map widely!

So, what’s orographic precipitation?

“Orographic” means “to do with mountains” and “precipitation” means “falling out”. Evaporation-condensation and precipitation are the three basic processes that make up the “water cycle”.

Sunlight shining on water helps to excite some of the molecules to kick themselves out of the liquid and into the air, floating off thanks to their buoyancy. This is evaporation. When air blows unobstructed over the sea towards New Zealand it picks up more and more clear water vapour.

When this moist air hits a mountain range, most of it squeezes around the shoulders and through the gaps but some goes over the tops. This air cools as it rises into the lowering pressure aloft. When its falling temperature reaches the “dew point”, the clear water vapour molecules start to settle on bits of dust or salt which are always in the air, condensing into droplets small and buoyant enough to float. These droplets make clouds. If cloud droplets start to coalesce and/or turn to ice, they no longer float but fall out of the cloud and are called “precipitates”. The process is “precipitation”.

Precipitates from a cloud may be ice particles, snow flakes, hail, sleet or rain. This depends on the moisture, stability and speed of the incoming air, how directly it hits the ranges and the steepness of the slopes. Usually the greatest fallout or precipitation is over the steepest slopes on the upwind side of the crest. In New Zealand this happens to be around 1200m altitude along the western side of the Southern Alps, which has been called “God’s Own Wet Zone”.

It takes time for droplets to grow and for precipitates to develop and fall, so as the wind gets stronger more raindrops end up downstream from the crest. This “spill-over” is the main way our southern hydro lakes get their rain. Also, as the air sinks down the lee slopes it warms and dries creating a rain shadow. This is called the föhn or foehn effect, from a local name for a wind in the European Alps. In a mountain range with many crests, these combined effects may cause the wettest area to be between crests.

Combinations of these orographic effects are highly variable for individual storms. The storm that brought the February floods was forecast to produce about 200mm of rain on the Tararuas. This happened, but the winds were so strong and the moisture source so big that it also produced 200mm of rain over the Manawatu.

What’s the freezing level?

Usually mountain rain starts off as ice particles or snow flakes which start to melt only when they fall through the altitude at which the air temperature is zero Celsius. Forecasts call this the “freezing level”, but really it should be the “melting level”. It has a lot to do with do with the melting of falling ice or snow and not much at all to do with freezing anything. Being warm blooded, we normally feel that conditions are “freezing” well before we encounter this lofty level because of the chilling effects of wind and rain.

Melting takes time, during which melting snow flakes or ice particles can fall an extra 200 to 300m before they liquefy into raindrops. That’s why the forecast snow level is always lower than the forecast freezing level. If enough moisture feeds into the system to keep precipitation going the air starts to cool, because heat is taken up by some evaporation of the falling rain. This brings freezing level closer to the ground and allows snow to reach still lower levels. In heavy precipitation snow can penetrate to 500 metres below the freezing level, especially about confined valleys.

Recipe for snow

The best way to make rain is to take moist air and cool it. The best way to make snow is to take cold air and moisten it. That’s how snow machines do it – we wait for the air temperature to drop below about 2°C, then squirt a fine mist in the air. It lands as man-made snow. The difference between these recipes explains why patterns that look the same on a weather map may have different outcomes depending on how the ingredients come together.

New Zealand is in the mid-latitudes, half-way between the warmth and moisture of the tropics and the cold dry air of Antarctica. When a low pressure area forms in these latitudes it draws in surrounding air, which spirals in to the centre of the low. In the southern hemisphere the earth’s rotation makes this a clockwise spiral. The clockwise-rotating air masses pull warm moist air southwards on their eastern sides and cold dry air northwards on their western sides.

The leading edge of an invasion of warm most air is called a “warm front”, while the leading edge of an invasion of cold air is called a “cold front”. The zone between a warm front and a cold front is a “warm sector”. Normally the depressions are south of New Zealand, so all we see on the weather map is the cold fronts travelling northwards on their western side. If the freezing level is close enough to ground level a cold front will bring some snow, but usually it’s too dry for much fallout.

The best snow-producing weather pattern for New Zealand starts with a high pressure (anticlockwise) system centred near or south of Tasmania so its southerly winds reach us, feeding in cold air which lowers the freezing level. Another necessary factor is an incoming low deepening in the Tasman Sea with its warm sector crossing New Zealand. The cloud in this warm sector moistens the cool air allowing snow to reach the ski fields. A good example of all this can be seen on one of our best recent snowstorms in mid June 2002.

This year’s snow show

The weather pattern in the Pacific Ocean is still neither El Niño nor La Niña, so our weather is swinging from one regime to the other. We started the annual cooling trend for land, sea, and air a little earlier than normal and frontal activity should strengthen as usual during April and May. Chances are we’ll get appreciable snow before the end of June.

If you have any queries please send them to me at mcdavitt@metservice.com. See you in August!

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 March 2004 FMC Bulletin

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