What is the Mountainwatch Snow Line?

June 16th, 2009


This is the blue line you will see in all of our 3-Day Video Forecasts. It provides a forecasted indication of whether precipitation in alpine areas will be snow or rain.

When the this line moves north of a ski-field area in Australia or New Zealand (or south of a ski-field in the Northern Hemisphere) it is very likely that if snow does fall, it will be able to reach ground level in alpine areas without melting.

Technically, the Mountainwatch Snow Line is an “isopleths”, which displays a depth of 5400 metres between the 1000hPa and the 500hPa pressure levels in the atmosphere. The thickness of this 1000-500hPa layer is largely dependent upon the average temperature in the layer and to a lesser extent on the moisture content. During a cold outbreak, the air is very dense and so this layer is thinner. When the thickness is less than 5400 metres it indicates that the air is cold enough that frozen precipitation (e.g. snow) will be able to reach ground level in alpine areas without melting.

Definitions of Techincal Terms:

A line on a map connecting points with the same value for variables such as temperature or air pressure.

Pressure Level:
A level in the atmosphere defined by a set pressure. The height of a pressure level will vary depending upon the temperature and humidity of the atmosphere. On average, pressure levels are highest at the equator where the air is relatively warm and lowest at the poles where it is relatively cold.

Pressure is defined as force per unit area. Atmospheric pressure is analogous to the weight of the column of air directly above the point where pressure is being measured.