"You don't need a weatherman to know which way the wind blows." Bob Dylan

Understanding the weather  

The following generalisations give an idea of how weather affects the South Island's Southern Alps. In New Zealand most weather systems come from the west. Weather coming from the northern quarter (ie northerly or northwest) tends to be warmer and from the southern quarter (ie southerly or southwest) is colder. No matter where they come from they have picked up moisture as they crossed the mid lattitude oceans to our west. When these systems hit the South Island they are forced to rise over the mountains. The act of rising causes air to cool, and this cooling causes some of the water within moist air masses to condense, creating rain. As the air continues to rise it might cool to zero degrees Celsius and most moisture condensing above this point will fall as snow.   Having dumped most of it's water, the weather system then descends the eastern side of the mountains, warming as it descends, creating warm dry winds on the lee side of mountain ranges. This behaviour explains why the West Coast of New Zealand is so much wetter than the East. Back to top of page

Rain?  

A consistent surprise to those new to tramping in New Zealand, is just how much it can rain. A quick look on the web will find you varying sites proporting the worlds wettest place to be in India, Colombia or Hawaii. The average annual rainfalls quoted for these places seem to be somewhere between 10 000mm and 13 500mm (393-531 inches). The highest rainfall within a 365 day period in New Zealand is over 18 000mm, recorded in the Crop river in Westland (Source: Te Ara - The Encylopedia of New Zealand). The highest annual average rainfall here is possibly at Frews Hut, collecting around 14 000mm a year (Source: National Rural Fire Authority - www.nrfa.govt.nz). It is difficult to compare worldwide rainfall stats as they are subject to different monitoring regimes and are averaged over different periods, BUT, the point I make is that New Zealand is one of the worlds wettest places... are you and your equipment prepared for this?

Check out this NIWA (National Institute for Water and Atmosphere) climate page for an approximate climate overview of the areas you are likely to go tramping in. Back to top of page

Reading a mountain weather forecast  

The mighty mountain weather forecast is a valuable tool when planning a venture into the back country. They are easily accessible on the net Met Service Mountain Forecast and at Department of Conservation Visitor Centres. To read one of these gems there are a few terms that must be understood

• Freezing level: The altitude where the air temperature is zero Celsius. At any point higher than this altitude, the temperature will be below zero. The lower the freezing level, the colder it will be at any given altitude.
  
• Wind speed: Measured in kilometres per hour and usually quoted for 1000 metres and 2000 metres of altitude. A 40km/h wind is unpleasant if it is cold or if you are walking along a narrow ridge. At around 60km/h it becomes difficult to walk, unexpected gusts could knock you over and your loose pack straps and jacket toggles will start slapping the heck out of you. By 70km/h it is usually hard to make progress on foot and there is a increasing risk of falling tree branches. At 80-100km/h twigs and small stones start to become projectiles, forest may sustain significant damge and most people will struggle to stand up.
  
• Cyclones/Lows/low air pressure: Lows rotate clockwise and are formed by rising unstable air. They generally signal poor weather.
  
• Anticyclones/Highs/high air pressure: Highs rotate anticlockwise and are formed by sinking stable air.  Highs generally indicate good weather.
• Cold Fronts: A cold front is a distinct package of colder air moving within a weather system. Cold fronts usually bring a rapid deterioration in the weather. They often give you little warning and tend to bring mores severe weather to the mountains.  Cold fronts don't usually carry a lot of moisture but if they come into contact with warmer, wetter air they can still cause heavy rain or snow.
• Warm fronts: A warm front is a distinct package of warmer air moving within a weather system. Warm fronts generally unleash less energy and develop slower than cold fronts. They can however carry more moisture than a cold front and can lead to heavy rain or snow with little wind.
  

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DIY weather forecasting