Situation Map
Updated Daily 6am - 6pm
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Weather maps are usually printed in most daily newspapers (eg: The Press). The maps will contain a key to aid in the interpretation of the symbols. Note that "troughs" or "fronts" can result in sudden changes of weather. Notwithstanding the general pattern that one is able to obtain from the maps, New Zealand’s topography results in markedly varied weather conditions within localised areas.
Find out about local conditions and idiosyncratic weather patterns if unsure.
Learn to interpret maps by reading them regularly at home; - try predicting the weather yourself.
Wind: The strength and general direction of the wind can be determined from the weather map. The prevailing wind in New Zealand is generally from the NorthWest.
Wind flow is anti-clockwise around a high-pressure system (anti-cyclone).
Wind flow is clock-wise around a low-pressure system (depression)
The spacing of the Isobars will give an indication of possible wind strength. Generally, the closer the spacing between the isobars, the stronger the wind.
In New Zealand, a Southerly Wind or front will usually result in colder air moving onto the country. Snow may fall at higher altitudes, regardless of the season.
New Zealand’s mountain ranges have a large influence on localised effects of the weather. Gaps in the main divide will usually result in a higher prevalence of stronger winds (eg Wellington and Cook Strait). Areas east of the main divide are generally drier in the usual prevailing conditions.
Temperature: As a general rule, air temperature drops by between 6.5 degrees and 10 degrees Celsius per 1000m increase in altitude. A hike above the bush-line (usually between 1000m and 1300m) can therefore result in conditions where the temperature is about 8 to 10 degrees less than that at sea level. Stronger winds at the higher altitude can increase the cooling effect of the wind.
http://www.nzlsar.org.nz/sar-wx.html
General Weather Information
New Zealand straddles the 40 degree south latitude, which the sailors refer to as the "Roaring Fourties'. This name comes about because of the strong westerly winds which were very helpful for pushing the sailing ships. Therefore New Zealand's weather is dominated by these westerly winds which flow across the Tasman sea collecting moisture.
The airflow is forced to rise and cool as it passes over the country's alpine backbone. The winds then release their load of water on the western slopes of the alps in the form of rain or snow. The air, now drier, then descends to the east, warming as it goes and creating a warm, dry wind over the eastern plains.
This weather pattern follows approximately a 6-10 day cycle. The westerly airflow is caused by depressions (bringing cold fronts) that pass to the south of the country.
Between these cold fronts, ridges of high pressure (anti-cyclones) bring light winds and generally fine weather. Sometimes a depression will travel directly over New Zealand, bringing large amounts of rain, but if a slow moving anticyclone passes over the country, long periods of fine weather can be experienced.
The only thing certain about New Zealand weather is its uncertainty.
Long storms are quite possible, like one experienced by two climbers in November 1982 when they where trapped on the summit ridge of Mt Cook for 14 days, but as a rule a storm will only last for two to three days, and is often followed by a couple of days of fine weather.
The South Island's free air freezing level is usually about 1650 metres in winter and 3200m in summer, but within the Alpine regions it is usually lower.
During the winter period it can get cold in the Alpine regions. Mt Cook village average winter temperature is 2 degrees Celsius while in summer it will average 14 degrees Celsius.
As a general rule the temperatures declines by approximately 6.5 degrees Celsius every 1000 metres altitude gained. In Christchurch temperatures climbing into the 30's are not uncommon during the summer months.
In the North Island the weather patterns are not so pronounced, and the temperatures are generally a few degrees warmer than in the South Island.
Common Weather Patterns
Nor'westerly
Northwesterly airflow over the South Island will bring rain to the West Coast, high winds in the mountains with rain sneaking over to the main divide for a few kilometres. The further east of the divide, the drier and less clouded the skies become.While the winds will increase in intensity until the Nor'west gets to the east coast, where it is very warm and dry. From the eastern plains you will see dark grey clouds obliterating the Southern Alps, while ridges of tightly formed arches will be found above the inland eastern plains, clear skys will be found on the eastern coast. Advice: head to the east coast crags, take your sunblock and swimming gear.
Southerly
The country's snow storms come from the southerly quarter. A southerly storm progresses up the east coast, bringing a big drop in temperatures and a cool rain (summer), or a snow storm (winter). If the storm comes from the southeast during winter the low country to the east will get a heavy dumping of snow and many of the country's high roads and passes are closed.During winter it is rare for a southerly to last longer than three days while during the summer a southerly does not last very long, ranging from hours to a couple of days. Southerlies also bring good news for climbers. Advice: If in a hut, find a good book, if not in the mountains, make preparations for a trip and start walking in to a high hut during the tail end of the bad weather.
Southerly Clearing
After the southerly front has passed through, climbers will start to see increasing patches of blue skies on the southern horizon. This is very good news. The clouds will be moved away by the cool southerly airflow and a period of fine weather will follow. The initial period will be cool but unbelievably fine while as the fine spell progresses air temperatures will rise as the air flows moves into the westerly quarter. Advice: Climb. Pick an ice climb, save the rock climbs for the warmer westerly.Westerly
A light warm westerly airflow does bring some cloud cover on the western side of the divide. This is often referred to as "West Coast Crud' by climbers. Early morning the West Coast is often clear of cloud. As the day develops, white fluffv cloud starts forming over the rain forests at about 1200m. As the morning progresses the "Crud" intensifies, and rises up the valley systems onto the snowfields.At about midday anybody east of the divide will see the "crud" flowing over the divide at its low points. Anything flowing over the divide will quickly disperse within a few kilometres of the divide. The "Crud" will start to disperse during the evening leaving a clear night. Advice: Climb. On the western side of the divide you may be engulfed by cloud, but don't panic, just make sure you can navigate yourself back to the hut in no visibility.
Westerly Deterioration
As the westerly portion of the weather cycle progresses, the "west coast crud' becomes more intense and will not retreat during the evening. High cloud can be seen moving eastward above the "westcoast crud" layer. As the sky greys this high cloud intensifies and the air temperatures will rise. The increase in night temperatures is particularly noticeable. At this time, the wind velocity increases and arched shaped lenticular clouds are seen in the sky, or sitting over major summits. These lenticular clouds, known as 'Hogs Backs', indicate high winds at altitude and are the fore warners of a nor'westerly. Advice: abandon high camps, and head for reliable shelter while you can still move. Last minute ascents can often be sneaked in on the more easterly mountains like the Malte Brun Range at Mt Cook, or Cloudy Peak, or the Arrowsmiths.Two other weather phases that do not occur very often are worthy of mention.
Easterly
This often brings light warm rain with very little wind to the east coast, and sometimes as far inland as the Alpine regions. Fine weather is experienced on the Westcoast. Most east coast farmers are overjoyed with this rain. Advice: Climb on the West Coast. Avoid the east coast rock.Northeasterly
Perhaps one of the rarer weather occurrences. This brings an extremely heavy downpour of rain (and flooding) to the east coast region, in particular to East Cape (North Island). Advice: go to Fiordland, and the Darrans. It will most likely have stopped raining there.Blue Skies and Sunburn
Most northern hemisphere climbers will find the skies over New Zealand extremely blue. This is primarily because of the relatively low level of pollutants in the air. This means sunburn can be a major problem. During the summer period weather forecasts often include a "Burn Factor'. This represents the number of minutes before unprotected skin suffers sunburn. Periods may be as short as 12 minutes during the summer months, and shorter times should be expected when at altitude or on a glacier.
Sources of Weather Information
An accuracy rate of over 80% is being claimed for forecasts for the following day, while the forecasters are claiming over 70% accuracy two days in advance. One point to remember when looking at weather maps in New Zealand; Anticyclones (High pressure areas) rotate in an anticlockwise direction, and lows rotate clockwise, the opposite to the northern hemisphere.