Weather is an important factor in aviation. It is often most critical during landing and take-off. Many people have experienced turbulent shaking of aircrafts, or closed airports due to fog problems. What is considered as critical weather conditions in aircraft operations depend on aircraft model, terrain, runway length and choice of navigation signals. Good knowledge of weather conditions is essential, both in the planning and the actual aircraft operation. Descriptions of wind conditions such as the risk of side wind, tail wind, strong wind and turbulence are important. In addition, knowledge of visibility and cloud height at the airport is essential. In winter, slippery runways, large snowfall and icing may cause problems. This requires knowledge of temperature and precipitation at the site.