Fog and Cloud Cover

A thick cloud of tiny water droplets suspended in the atmosphere at or near the earth’s surface which obscures or restricts visibility (to a greater extent than mist; strictly, reducing visibility to below 1 km)

Radiation fog happens when ground loses its heat by radiation. It becomes cold and cools the air in contact with it. If this lowers the air temperature below the dew point, water vapour condenses out as droplets, resulting in fog if there is a light wind, or dew/frost if there are calm conditions

Radiation fog is most common in autumn and winter when there is a long night giving the land time to cool. It occurs at night and early morning after a prolonged period of cooling. It doesn’t occur over the sea as the sea has insufficient diurnal variation. It forms first in the valleys due to katabolic effect and is common in anticyclones and ridges where the air remains in contact with the ground for a prolonged period.

Radiation fog is the most common fog a student pilot conducting his flight training will encounter on regular basis and it is the student’s responsibility along with the flight instructor to understand the effects of radiation fog and how to avoid visual illusions caused by it.

Advection fog forms when war moist air flows over a cold surface. It can occur over land or sea and the conditions necessary for it to form are:

• A wind of up to 15 knots
• A high relative humidity so little cooling is required to bring the air to saturation
• The cold surface over which the air moves must have a temperature lower than the dewpoint of the warm moist moving air

Advection fog is common over land areas in winter and early spring when the land is colder than the sea and over sea areas in late spring and early summer when the land becomes warmer than the sea.

This type of fog is much more persistent than radiation fog and can last several weeks. Dispersal comes when there is a change of air mass or an increase in wind speed beyond that described in the conditions above.

Hill fog is really a stratiform cloud that forms when there is orographic lifting in stable conditions. The cloud stays next to the surface obscuring the tops of the hill or mountains

Fog has been a neglected topic for most of the flight academies in the past which has caused a considerable number of flight accidents. Sky Team aviation academy focuses on teaching the principles of meteorology and fog, in particular, to ensure safe flying for its students and flight instructors.

Cloud cover refers to the fraction of the sky obscured by clouds when observed from a particular location. Okta is the usual unit of measurement of the cloud cover. The cloud cover is correlated to the sunshine duration as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places.

In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code, there is an extra cloud cover indicator ‘9’ indicating that the sky is totally obscured (i.e. hidden from view), usually due to dense fog or heavy snow.