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Frost Hollows | 
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There are some places in the UK that are particularly prone to frost. These are known as 'frost hollows' or on a smaller scale 'frost pockets'.
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Forecasters often inform us that in rural spots temperatures could fall low enough to give a touch of ground frost, even when air temperatures on weather charts are above freezing. Air over the ground on hill and mountain sides is cooler than the air at the same altitude above the surrounding valley.
On calm clear nights, the air near to the ground cools as heat radiates back into space until just after dawn. This results in a temperature inversion where temperatures increase with height above the ground. Air over the ground on hill and mountain sides is cooler than the air at the same altitude above the surrounding valley. This causes the cold dense air to flow down the hill side to the valley floor (cold air drainage) resulting in a katabatic wind.
Katabatic winds are generally light, and often blow unnoticed. However, some areas in the world see katabatic winds of incredible velocities. Outflowing winds in Antarctica can reach speeds of 120mph. As a result of katabatic winds, temperatures in valleys and low lying areas can be significantly lower...
As a result of katabatic winds, temperatures in valleys and low lying areas can be significantly lower than on the surrounding higher ground. They can be low enough for ground frost to form while the surrounding hill sides remain frost free, or for frost to be more severe than in nearby areas at higher altitude.
These frost prone areas are known as 'frost hollows'. This effect can be enhanced when the cold air drainage is obstructed by obstacles such as hedges or railway embankments. Temperature differences can be as much as 8°C between a valley bottom and land at 200m (700ft). Ground frost can form even when air temperatures (which are measured in Stevenson Screens 1.2m above the ground) are as high as 4°C. Large temperature ranges can also be recorded in frost hollows. Rickmansworth, a very well know frost hollow, recorded the largest daily temperature range in England when, on 29th August 1936, the temperature climbed from 1.1°C at dawn to 24.9°C within 9 hours! Other well-known frost hollows in the UK are the Welsh Marches, the Glens of Scotland, the Pennine Valleys, the Vale of Evesham, Shrewsbury and Redhill. Frosts are often seen here earlier in the autumn and later in the spring than on the surrounding higher land. Water holds heat more effectively than land, as does darker bare soil.
It can be very difficult to grow delicate plants in gardens in frost hollows, although there are some things that can be done to prevent damage by light frosts. Water holds heat more effectively than land, as does darker bare soil. So, moist, firm and bare soils will absorb more heat in the day and lose heat more slowly overnight than dry loose and weed covered soils.
If cold air drainage is obstructed by a barrier, such as a hedge, removing this can help prevent frost. On the other side of the coin, building a barrier to prevent drainage of cold air into a hollow can also help, as can planting less sensitive or taller plants in more prone areas. So if you are a keen gardener, check the topography of the land around your perfect garden before you buy a property - it could be harder to cultivate than you realise. | |||||
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