Frost (Hoar Frost)

Primary reference(s)

WMO, 2017. International Cloud Atlas: Hoar Frost. World Meteorological Organization (WMO). Accessed 1 November 2020.

Additional scientific description

There are two types of hoar frost: hoar frost (proper) and advection hoar frost.

Hoar frost: A deposit of ice that generally assumes the form of scales, needles, features or fans and which forms on objects the surface of which is sufficiently cooled, generally by nocturnal radiation, to bring about the deposition of the water vapour contained in the ambient air. Hoar frost proper is ordinarily deposited on objects at or near the ground, mainly on their horizontal surfaces. Hoar frost is observed especially during the cold part of the year when the air is calm and the sky is clear (WMO, 2017).

Advection hoar frost: A deposit of ice that generally assumes crystalline form and which forms on objects, the surface of which is sufficiently cold to bring about deposition of the water vapour contained in the air coming into contact with this surface, usually through a process of advection. Advection hoar frost is deposited mainly on vertical exposed surfaces. It is observed when relatively warm damp air suddenly invades a region where the temperature of the exposed surfaces is below 0°C and below the frost-point of the advected air (WMO, 2017).

Metrics and numeric limits

Not available.

Examples of drivers, outcomes and risk management

The Australian Government provides guidance on the impact of frost damage (Australian Government, 2014). Clear, calm and dry nights following cold days are the precursor conditions for a hoar frost. These conditions are most often met during winter and spring when high pressures follow a cold front, bringing cold air from the Southern Ocean into settled cloudless weather. When the loss of heat from the earth during the night decreases the temperature at ground level to zero, a frost occurs. Wind and cloud reduce the likelihood of frost by decreasing the loss of heat to the atmosphere. The extent of frost damage is determined by how quickly the temperature takes to get to zero, the length of time temperatures stay below zero, and how far below zero the temperature falls (Australian Government, 2014).

In Australia, frost damage occurs to legume pods and seeds, canola pods, flowers and seeds and cereal grains, flowers, or whole heads if the stem freezes around the flag leaf or in the boot. Flowering wheat, triticale, podding canola and field peas are some of the most sensitive crops to frost. Barley and oats are the most tolerant (Australian Government, 2014).