Over the past several mornings as large parts of NSW, Northern VIC and Southern QLD have frozen in one of the harshest cold snaps (for minimums) we’ve seen in the last 5 years… if not longer. Many people have come to us asking where is the snow? or, why isn’t it snowing? we’re seeing temperatures of -10ºc – wheres the closest snow?
While its accurate to associate cold with snow.. the requirements for the temperature to plummet are actually a massive hinderance for snow to occur. Its usually the same process time and time again for these massive cold snaps to occur. A large, slow moving, high pressure system will dominate the region in question (whether thats South East AUS, NSW, Southern QLD, Inland AUS.. it doesn’t really matter). This high will bring clear, cloud free skies along with very light winds and an abundance of dry air – whether its at the surface or just above the surface. This combination allows for all the heat of the day to escape and the temperature to plummet. The light winds allow for dew to set as frost with no disturbance and we wake up to blue skies, white lawns and frozen taps.
A slice of the atmosphere from this morning (Monday, July 16th) at Glen Innes. You can see a massive amount of dry air dominating the atmosphere with 0% cloud coverage – this equates to absolutely no snow, but frigid temperatures. – Image via BSCH
For a snow event to occur, while the actual daytime maximums are most likely colder… the nights are nearly always warmer than whats experienced in these cold snaps. For snow to occur, you need precipitation. That means you need moisture in the atmosphere which also creates cloud coverage. The cloud coverage traps any heat from coming in during the day (thus why Guyra can have 2ºc maximums sometimes), but also stops any heat from escaping. So the temperature will sit between about -3 and 3ºc for the majority of the time. The increased moisture allows rainfall to occur which in the right conditions will turn to snow.
Slice of the atmosphere for Wednesday morning, July 18th, which is favourable for Snow over the Tasmanian Highlands… and possibly even some big snow flakes. – Image via BSCH
As you can see, the process for either a severe frost or snowfall is virtually completely opposite. Even for the Snowy Mountains its quite rare to see snow falling when the temperature drops below about -7ºc, for the Northern and Central Tablelands of NSW and into Northern VIC and Southern QLD, it would almost never happen as the region doesn’t normally get that cold and therefore it NEEDS the clear skies for heat to escape.