Wherever you are in the world there is likely to be snow above you. Even in summer, or over the tropics, ice crystals are found in the highest cirrus clouds as temperatures are far below freezing. It is a fact that most of the precipitation on Earth starts life as ice crystals in higher parts of the atmosphere and whether they fall as rain or snow depends on the temperature of the air that these crystals fall through before reaching the ground.

One of nature's finest sites is a landscape covered in thick, fresh snow. Snow is common in winter over Northern Europe and North America and is permanent on some mountain tops (as well as at the poles). Mount Kilimanjaro is located just 3 degrees south of the equator but is still high enough to maintain a permanent snow cap.

Snowflakes form when ice crystals stick to one another. The largest snowflakes develop between 0 degrees C and -5 degrees C, rather than much colder conditions. At these warmer temperatures the snow can contain more moisture and hence have bigger flakes. As snow falls the melting process extracts latent heat from the surrounding air, causing it to cool and refreezing the snow flakes. The melting and refreezing allows the snow crystals to combine into bigger flakes.

Snow that falls in temperatures well below freezing will be fine and powdery, ideal conditions for skiing, whereas snow forming in temperatures much closer to freezing tend to be larger, wetter and stickier. In mountains the build up of snow causes dangerous avalanches which can sweep all before them in a destructive wave. It is estimated that there are over one million avalanches every year, though, thankfully, most of these rumble away in inaccessible areas where there is little contact with man.

More common in populated areas, but just as dangerous, are blizzards. Blizzards are a combination of heavy snow, freezing temperatures and strong winds. A blizzard can easily bring a city to a complete standstill, and with the associated whiteout conditions, when heavy snow and low cloud make it impossible to distinguish between the sky and the ground, it is extremely difficult to travel even small distances.

In China last winter (2007-2008) the worst winter storms for 50 years engulfed huge areas for more nearly 5 months from October 2007 onwards. With power lines ripped down, and the traffic infrastructure at times being brought to a complete standstill there was over $8 million worth of damage. In northwest China 1.65 million people were left with either frostbite or snow blindness, or both. 500,000 livestock were killed and 3 million others were starving. Also 350,000 people were in need of food and 130,000 had run out of fuel. On the Zhuhai-Beijing freeway there was a 75km traffic jam with over 10,000 cars stranded while soldiers worked to clear ice from the road.

This explains why blizzard warnings are so important in countries where these winter storms are common. An accurate forecast can ensure people are indoors and not travelling when the blizzard strikes, reducing fatalities and alerting the emergency services before the storms onset.