Whilst we are all familiar with snow and know the difference between it and rain – how many know the difference between two other winter phenomena, frozen dew and frost.

Before we get into the main differences between the two, we first need to remind ourselves of the dew-point - the temperature at which saturation occurs. As surfaces cool below this temperature, water vapour in the air begins to condense on them. This is known as dew.

Frozen dew is simply dew that freezes, due to the air temperature reaching or dropping below freezing. Dew, and thereby frozen dew, is most likely to form on blades of grass and other objects near the surface, as this is usually where the coldest air is found.

Whereas frozen dew initially goes from water vapour in the air, to liquid (dew droplet) and then to ice (frozen dew), frost goes straight from water vapour to ice.

Frost forms when the dew-point temperature is at or below freezing, meaning any water vapour skips the liquid phase and goes straight to ice, a process called deposition. Frost, white frost and hoarfrost all refer to the white crystals of ice that form in this way.

Both frozen dew and frost are more likely to form on nights when skies are clear and wind is either very light or calm. Clear skies allow rapid cooling of the Earth's surface and calm winds mean little to no mixing of the air, meaning the coldest air will be at ground level. It follows then that both frozen dew and frost are associated with high pressure systems, where winds are lighter and skies generally clearer.

For either of these phenomena to form however the air must be cold enough. An ideal setup would be for a strong cold front to move across a region, with high pressure building behind it. The cold front would introduce this cold airmass, with the high pressure behind leading to clearer skies and light winds.

Although not the coldest of airmasses, a similar setup is expected over southeastern Australia around Wednesday this week as a weakening front crosses the southeast, introducing colder air across the region. High pressure is then expected to build across the southeast into the weekend, with fog and frost possible over parts of NSW’s Central and Southern Tablelands, and South West Slopes.

A cold airmass across the southeast on Wednesday 14th June in the wake of a front