ICY COVERING OF CRYSTALS TURNS THE COUNTRYSIDE WHITE ON A CLEAR NIGHT
Onclear, still winter nights, a delicate covering of crystals turns the countryside white. The phenomenon is hoar frost. Leaves, flowers and stems are covered with delicate patterns. Spiders’ webs are outlined with a fragile coating. Fine feathers and needles turn grass silver and give trees a ghostly frame.
The name hoar comes from the old English word, hor or har, for white or grey. These frosts form on cold, cloudless nights, with little or no wind. The conditions must be right for crystals to form directly from water vapour present in the air. This process is called sublimation and happens when air changes to a solid without passing through a liquid stage. A true hoar frost is formed solely by sublimation, and is neither frozen dew nor droplets of water. In practice, the frost is often a combination of frozen dew and sublimation.
On these nights, the ground, and the air just above it, steadily cool as they rapidly lose heat to the atmosphere by radiation. Eventually the dew point is reached – this is the temperature at which water vapour in air condenses into liquid water. Both the air temperature and the dew point need to be below the freezing point of water for hoar frost to form.
When the radiation persists, and the temperature continues to drop, the air becomes supersaturated with moisture. The excess moisture is deposited in the form of needle-like crystals that bind themselves onto surfaces.
A crystal world
If there is a lot of moisture available during the long winter night, hoar frost is formed in a thick layer. This can resemble a covering of snow. The pure white is caused by the reflection and refraction of light as it interacts en masse with the frost’s crystalline structure.
An area of high-pressure during the winter will provide the quiet conditions suitable for the formation of a hoar frost. An old weather rhyme says ‘clear moon, frost soon’. When it occurs on successive nights, and does not melt during the day, an ever thicker coating of crystals are formed. This can be 1in (2.5cm) or more long. Conversely a cloudy sky or windy conditions inhibits radiation, preventing a frost.
It is possible for a lawn to be coated with hoar frost while trees and shrubs are frost-free. This happens when the ground temperature is several degrees colder than the air higher up, which remains above freezing point.
Places that are particularly susceptible to a frost are known as frost hollows. As air chills its density increases. On a clear night, slopes in undulating country lose heat by radiation cooling the air above them. The cold air runs downhill like water into hollows and valleys. It is known as a katabatic flow, from the Greek word katabatikos, meaning downhill. If it is trapped by embankments across the valley or by natural spurs jutting outwards, a lake of cold air can form. The valley floor is coated with a thick hoar frost whereas higher up may be frost free. During March 2012 there was only one air frost at a weather station in east Surrey situated above a downland valley. A weather station close by on the floor of the valley recorded 16 air frosts.
Hoar frost in the garden
Gardeners can inadvertently create frost pockets by building a solid fence across a slope. The type and texture of soil is also a factor in contributing to the likelihood and severity of a frost. Sandy soils lose heat more rapidly and reach the frost point more readily than a clay soil. Moist soils will not cool as briskly or lose as much heat.
Hoar frost can be harmful to more delicate plants, shrubs and trees. Water freezes within the plant’s cellular structure, breaking down the walls. A vulnerable time is just after dawn when the air is often at its coldest. Rays from the rising sun are cast onto foliage, causing a rapid defrosting. This ruptures the sides of the plant’s cells when the sap expands. By putting fleece covers over plants, frost is prevented from forming on the plants themselves and averts that sudden thawing. Plants inside a glass cold frame get some protection from a coating of hoar frost. The covering raises the temperature inside by several degrees, giving some protection from radiation.
The longer the duration of the frost, the more a plant suffers. It is the buds and flowers that are most tender. One plant that is susceptible is Magnolia soulangiana. A mild winter induces the tree to flower early, then a spring frost scorches it.
In an average year, many places will have twice as many grass or ground frosts than air frosts. Air temperatures measured at 4ft (1.2m) can often be up to 40C higher than those on the lawn, flower bed or vegetable patch. A thick hoar frost can blacken potatoes while the blossom high on an apple tree may be untouched, remaining just above freezing.
The effects of nature’s frozen tracery are unpredictable, but its beauty remains one of the wonders of the winter.
Words: Ian Currie Photography: Alamy