Dew And Frost
When a jug full of iced drink is taken out of the refrigerator, water droplets condense on the outside of the container (provided the jug is made of a material which is a good conductor of heat, such as metal). This happens because the jug is at a lower temperature than the dew-point of the air.
'Dew-point' is defined as the temperature at which the air, when cooled, will just become saturated. For example, on a summer's day when the air temperature reaches 18°C, the dew-point might typically be 8°C. By sunset the air temperature may have fallen to 12°C, but the dew-point will still be around 8°C. During the night the temperature continues to fall and if it reaches, say, 7°C the temperature of the ground is below the dew-point of the air and droplets of moisture begin to form - this is dew. Since the air is now being 'robbed' of some of its water vapour, the dew-point of the air will actually start to fall very slightly.
Next morning, as the incoming solar radiation gathers strength, the dew will evaporate and the grass will become reasonably dry (and suitable for sitting on during the day). However, in winter, when calm conditions prevail, the daytime evaporation may be so slow that dew may persist all day.
Hoar-frost is composed of tiny ice-crystals, 'feathery' in appearance when well developed. It is formed by the same process as dew, but occurs when ground temperatures are below freezing point. Consequently, when the grass is covered in a white hoar-frost at dawn it cannot be assumed that there has necessarily been an air frost.
Sometimes dew forms during the evening and subsequently freezes to become hoar-frost with globular ice on the grass.
Snow
Snowflakes can be formed by the collision of ice crystals within clouds. This is known as the process of aggregation and usually accounts for the larger snowflakes that are seen to fall. Smaller snowflakes are formed by the Bergeron-Findeisen process. Supercooled water droplets (i.e. those with a temperature below freezing) are 'picked up' by the falling ice crystals. The ice crystals grow at the expense of the water droplets.
For snow to reach the ground the air temperature must be no more than 2°C. One would expect the falling snow to melt as soon as the temperature rises above freezing, but this is not so. As the melting process begins, the air around the snowflake is cooled. At temperatures above 2°C the snowflake will melt to become 'sleet' or rain. In this country, the heaviest falls of snow tend to occur when the air temperature is between zero and 2°C.
- Individual ice crystals and snowflakes can be the shape of prisms, plates or stars - but all have 6 sides.
- 30cm of fresh fallen snow has about the same water equivalent as about 25mm of rainfall.
Hail
There are three different phenomena which affect the British Isles that could loosely be described as hail.
- Snow pellets are beautifully white but are easily crushable between the fingers. They are occasionally called 'soft hail'.
- Ice pellets are quite moderate in size and are composed of clear ice, sometimes conical in shape.
- Hailstones are whitish in appearance and vary greatly in size. If a hailstone is cut open, a layered structure like and onion is sometimes apparent.
Large hailstones fall from deep cumulonimbus clouds. The cloud base may be 2,000 feet above the ground with tops at 30,000 feet. Much of the cloud will be composed of supercooled water droplets. As the hailstone falls it will collect tiny water droplets which freeze and form a layer of ice. Perhaps the hailstone will then be caught in a vigorous updraught. As it is carried back higher into the cloud, it collects more minute water or ice particles to form another layer of ice. Thus layers build up on the hailstone (made of alternate layers of clear and opaque ice) and the cycle may be repeated until the stone is so big that it falls to earth.
Hail showers are quite common over the British Isles in westerly and northerly airstreams in spring, but really large hailstones tend to occur in the south and are very much a feature of summer months.
The largest hailstone recorded in the British Isles weighed 142 grams (5 oz) and occurred at Horsham, West Sussex on 5th September 1958. Certainly anything approaching golf-ball size is remarkable, but hailstones can grow large enough to dent cars, shatter greenhouses and even injure people.
The USA, Canada, central Europe, the southern parts of India and China all experience large hail. So too do land areas in the southern hemisphere. The world record (as quoted in the 1994 edition of The Guiness book of Records) occurred in a hailstorm in the Gopalanj district of Bangladesh on 14th April 1986. The hailstones weighed up to 1kg (2lb 4oz) and were reported to have killed 92 people.
Other features in the Weather Basics series:
- Winter's Weather - Rain, Drizzle and Fog
