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Over the last few weeks there have been reports of hail, sleet and even snow occurring in South-East Qld.  Hail, sleet and snow, are forms of cold precipitation, all three forms consist of the same material – ice – and yet they are different and occur in different conditions. Why?

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.

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Why does attentiveness to nature matter? In a very fundamental sense, we are what we pay attention to. Paying heed to beauty, grace, and everyday miracles promotes a sense of possibility and coherence that runs deeper and truer than the often illusory commercial, social "realities" advanced by mainstream contemporary culture. ... Our attention is precious, and what we choose to focus it on has enormous consequences. What we choose to look at, and to listen to--these choices change the world. As Thich Nhat Hanh has pointed out, we become the bad television programs that we watch. A society that expends its energies tracking the latest doings of the celebrity couple is fundamentally distinct from one that watches for the first arriving spring migrant birds, or takes a weekend to check out insects in a mountain stream, or looks inside flowers to admire the marvelous ingenuities involved in pollination. The former tends to drag culture down to its lowest commonalities; the latter can lift us up in a sense of unity with all life. The Way of Natural History, edited by Thomas Lowe Fleischner and published by Trinity University Press (Texas)