Our Weather and the Formation of Winter Precipitations

The weather is determined by the rotation of the earth around the sun. Due to the approximately round shape of the earth, the sun`s rays hit the surface of certain regions of the globe in different angles. Near the equator, the almost vertical rays of the sun create a warm climate. Near the poles, due to the slanting rays of the sun, temperatures are much lower.

Due to the elliptic earth`s orbit around the sun – especially in the so-called temperate zone between the equator and the poles – there are the four seasons. When, on 22 December, the earth reaches the furthermost point from the sun, this is called midwinter – the shortest day of the year. After that, in our latitudinal lines, winter begins. The sunlight is less intensive then and it gets colder.

These changes of temperature and the calendric beginning of winter, are increasingly influenced by sudden changes in weather and by climate change. Between the air layers above the surface of the earth and the atmosphere, there is a permanent exchange between cold, descending air masses and warm, ascending air layers. Cold air streams ascend, for example, in the atmosphere, to compensate a low pressure area. All weather phenomena like rain, wind, storm and cloudless sky, are the result of movements of the air that meteorologists try to forecast with the aid of satellite images.

When moist air cools – depending on the temperature – rain or snow occurs. In the wintry “normal case”, the moist air is already cooled off in higher air layers, where it crystallises into snowflakes. However, a snowflake is not a frozen drop of water. When raindrops freeze on their way down, hailstones are formed. Snowflakes originate in “cool clouds”, from so-called nuclei of crystallization, which are dust or soot particles. Little by little, more and more cold water vapour freezes solid to this nucleus so that, finally, a visible snow crystal forms.

Depending on through which layers of temperature a snowflake falls, its structure changes. In warmer clouds, leaflets are formed – in colder ones prisms develop. A crystal which forms at -25 degrees, initially resembles a prism. As soon as it passes a warmer layer (between -12 and -16 degree), it gets the characteristic shape of a star at its ends. This shape can be explained by the freezing characteristics of water.

However, in a certain constellation of air layers, each winter service is overwhelmed: when a warm air layer slides above a cold air layer which is about 300 to 400 metres thick. The raindrops that cool while passing the cold air near the earth, freeze on the icy ground and form a dangerous, constant coat of ice, the so-called black ice. Then the risk of accidents for pedestrians and other road users is extremely high.