Snow is naturally made in clouds just as in snow cannons at ski areas: a mist of water droplets is sprayed outwards into chilly air. As the spray leaves the nozzle, ice crystals grow in the thinning air and snow forms. The recipe is to mix humid air and cold air in a zone of falling pressure.
Clouds are a sign of rising motion. When wind blows in from the sea perpendicular to a mountain range some air is deflected upwards. The resulting clouds tend to have their maximum updraft at around 600 metres altitude and slightly upstream of the mountain crest. This process forms showers that produce snow where the clouds grow higher than the freezing level. The side of the mountain that faces the wind gets this snow, but some ski fields are more sheltered than others.
A large portion of the CHILL mountains receive snow from the south and southwest. Treble Cone and Temple Basin are close to the main divide and receive snow from the west, parts of the Craigieburn Range close to the divide also receive from the west. Hanmer Springs from the south and south east.
If the air has come from the Southern Ocean, there may be a deep enough unstable layer to produce snow, usually in brief localised bursts. In September 2010 a series of such squally systems brought snow intense enough to temporarily damage the roof of Stadium Southland. Check the Metservice blog for this sort of snow map
Large low pressure systems, as seen by isobar circles on a weather map, can be the best snow-makers. The magic numbers for optimal snow is to feed moist air into the middle cloud layer at around 4000m altitude with a wind flow from the northwest, and then cool these clouds to between -14 and -17 degrees Celsius, by thinning the air (increasing the number of surface isobars) and/or by an undercutting of chilled air fresh from the Southern Ocean.
The severe snow in Canterbury in June 2006 is a good example, see the online newsletter for more details.
Use the snow maps (links above) as references when checking the 3-day rain model on metservice.com. When the purple line is displayed, any precipitation within it is likely to fall as snow. Note that when cold air pools about inland valleys, the next snow event may beat this purple line.
For the latest mountain forecasts, check metservice.com or m.metservice.com on your mobile.
