Wind chill factors tell us how cold a wind 'feels' against our skin rather than simply the ambient air temperature. Even in summer its effects can be felt during a rest stop as clothing dampened with perspiration is exposed to the wind and body temperatures fall.
Add some altitude, exhaustion and lack of food and it can happen even more quickly.
With even fine days occasionally turning nasty, the answer is to be prepared with the right clothing to stop temperature loss as soon as possible; layered clothing being the best method for regulating body temperature.
Wool or technical synthetic fabrics provide excellent insulation and moisture wicking qualities. Cotton has the opposite effect, trapping moisture against the skin and allowing the thermal boundary layer to be stripped by the wind.
A light base layer followed with fleece, and a wind and waterproof jacket over both, provides the most effective method of keeping out the wind and trapping warm air against your body.
Gloves, rain and fleece pants are prerequisite at altitude or in extreme cold and can be layered in the same way. A beanie can be pulled from the pocket of your pack or jacket and worn to keep your head warm.
Make sure you've read the Think Before You Trek brochure by the NSW Police.