Christmas in Australia occurs in midsummer, yet many of our Christmas trappings and traditions relate to cold midwinter weather and the classic snowy white Christmas. This is not surprising since most Christmas traditions, and most of us, originate from Britain and Europe.
The winters and summers of each hemisphere are opposite. This is because the Earth is tilted on an axis of 23.5 degrees, and when its lower half tilts towards the Sun, the angle of the Sun’s rays striking the Earth’s surface concentrate heat in the Southern Hemisphere, and spread the heat in the Northern Hemisphere. However other factors can effect local weather, and ironically on Christmas Day in 2006, snow fell on Mt Buller, while according to the UK Met Office and the UK bookmakers, there was no “official” white Christmas in Britain that year.
Low temperatures and atmospheric moisture do not always produce snow; there are a number of forms of cold precipitation.
Hail is produced only by storm clouds, which have intense updrafts and low temperatures in the upper cloud layer. Super cooled water droplets in the storm cloud, freeze on contacting condensation nuclei, such as dust. The frozen droplet is then lifted by the updraft into the cold upper reaches of the cloud and a layer of ice forms around it and creates a hailstone, it then falls and is lifted again. Each time it ascends the hailstone accumulates another layer of ice, like an onion. When it becomes too heavy, it falls to the ground. If the updraft is strong, the hailstone can make many ascents and may reach weights of 0.5 kg.
Sleet consists of transparent ice pellets and may be a mixture of frozen rain and partially melted snowflakes. Sleet occurs when a layer of warm air lies above a below-freezing layer of air closer to the ground. Sleet from rain occurs when raindrops freeze into pellets as they pass through the cold layer. Sleet from snow occurs when snowflakes partially melt as they pass through the warm layer but then freeze into pellets as they pass through the cold layer.
Snow forms in cold clouds when water vapour condenses around nuclei and freezes directly into ice crystals. A snowflake is an aggregate of ice crystals and because of the variations in the conditions of each snowflake’s creation, the size and shapes of snowflakes also show infinite variation, however each snowflake has hexagonal (6-line) symmetry and this never varies.
The typical White Christmas scenes may be picturesque, but to survive, plants and animals have to adapt to harsh conditions. To avoid drying out over winter, deciduous plants shed their leaves, shut down and live off stored nutrition in their roots. Evergreen plants such as holly and ivy retain their leaves through winter. Such plants brightened the bleak mid winter landscape, and it is easy to understand why people adopted these cheering evergreens, first into the pagan celebration of the winter solstice, and later as a part of Christmas tradition.