Weather News

Frozen Dew, or Frost?

Thomas Hough, Saturday July 23, 2022 - 11:03 AEST

Whilst it may seem like frozen precipitation is a thing of the past with all the rain around currently, there is still plenty of winter left for those frozen crystals to make more appearances.

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 goes from initially 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.

On a cold winter's morning, both can result in a widespread blanket of white across the ground such as in the below photo by Weatherzone's own Andrew Miskelly. As the conditions for their formation are similar to that of radiation fog, it is common for both frost/frozen dew and fog to be present at the same time. For more information on different types of fog and its formation check out one of our other stories: https://www.weatherzone.com.au/news/a-closer-look-at-fog/585186

Both frozen dew and frost are more likely to form on nights where 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 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 a very 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 south-eastern Australia around Tuesday this week.

- Weatherzone

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