- Written by Nadia O'Carroll
Firstly we have to consider the water or hydrological cycle. The amount of water on Earth is constant, water cannot be created, and because water cannot escape from the atmosphere, the amount of water cannot be reduced. Although the amount of water on Earth never changes, water constantly moves, circulates and changes its state between liquid (water), solid (ice) and gas (water vapour).
The hydrological cycle circulates water, it is driven by the Sun and its main processes are:
Evaporation – heat from the Sun transforms water on the Earth’s surface into water vapour,
which then rises into the atmosphere.
Transpiration – plants take up water from the soil, then release water into the atmosphere through their leaves.
Condensation - water vapour combines into tiny droplets of liquid water, these tiny droplets float in the sky and are seen as clouds. Condensation also occurs, when water vapour contacts a cold surface, this produces dew, frost and the fogging we see on mirrors and windows.
Precipitation – water droplets merge until they become too heavy to float and are pulled by gravity from the atmosphere down to the Earth’s surface. The main forms of cold participation are:
Hail is produced only by storm clouds, which have intense updrafts and low temperatures in the upper cloud layer. Supercooled 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 .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.