Without experience, it is almost impossible to forecast weather in the backcountry. Clues that the weather will change are dependant on the geography, so a recent, longer range weather forecast provides the best basis for decision making: nowadays such forecasts are reasonably accurate. Always stay up to date with the forecast and warnings, and always be prepared for bad weather.
Heavy rain creates the potential for rivers and streams to rise rapidly. Even prolonged periods of steady, moderate rainfall can cause a slow rise in rivers and streams to potentially dangerous levels. Thunderstorms in the headwaters of small watercourses often cause flash flooding; if you hear thunder and you are on the bed of a confined stream it would be prudent to climb out.
Heavy snow brings the risk of avalanche. Like rain, large accumulations of snow can be the result of steady falls over a period of many hours, or intense falls over a short time. Moderate-to-heavy rain can quickly turn into snow if the air temperature falls below 4°C. Alternatively put, snow will fall to about 300m below the freezing level.
High winds Often a period of very strong winds will have a steady build-up, but a much quicker abatement, though the easing may only be temporary.
- If thunderstorms are involved, a period of very strong winds may only last a few minutes to an hour or so, and have a sudden onset,
- In the absence of rain, or thunderstorms, wind tends to flow around the mountains in a similar fashion to water around rocks: the strongest winds might be about saddles and exposed ridges rather than on the higher peaks,
- Down-slope winds can be fierce,
- Beware of gusts, which can lift you off your feet: stay low, and seek shelter in vegetation, or behind a knoll.
Lightning Strike Lightning does kill people in New Zealand. During thunderstorms, avoid ridges and peaks.
Extreme cold The air temperature drops by about 2°C per 300 m of altitude. Wind chill though, is related to wind speed and humidity as well as air temperature. For example, in strong winds (50km/hr) at 0°C, the loss of body heat would be roughly equivalent to -10°C and no wind. Hence, in strong winds, especially in clear, cold conditions, hypothermia and frostbite can develop quickly.
Poor visibility Cloud about the ridge-tops (perceived as fog) can occur just about any time, but is more common in windy situations. Snow falling at even a moderate rate also reduces visibility considerably and can make distinguishing the ground from the sky difficult.
Alpine weather indicators
Signs of approaching bad weather:
- North-west wind,
- High cirrus cloud (mares’ tales) building up, followed by lower stratus cloud (typical “cloudy day” clouds),
- Hogs backs clouds forming on, or above the higher peaks,
- Falling barometric pressure.
Signs of improving weather:
- Wind turning south-westerly,
- Increasing breaks in the cloud from the south,
- Falling temperature,
- Rising barometric pressure.