Daylight

Sunlight is the direct beam of the sun after it has been diffused by the atmosphere. Sunlight is welcome in some buildings, such as homes and intermediate spaces for pleasure and because it is an energy source. In others it can cause a problem. In offices it frequently leads to discomfort and disability glare,and to overheating. It is a major factor in occupant dissatisfaction. In sports halls it should be avoided completely, because of problems from glare. In our latitude the sun is difficult to control because the path is lower in the sky. This means that the biggest problems in terms of solar penetration can be in winter.

Skylight

Light from the sky, which excludes any direct sunlight, is termed skylight. It is now the accepted description for daylight. In the UK, the sky is predominantly overcast. For North European countries the European lighting organisation or Commission Internationale de l’Eclairage (CIE) Standard Overcast Sky is used for modelling and calculating daylight in buildings. It allows for the worst case scenario, in terms of minimum levels of daylight, and refers to a completely clouded sky with an average illuminance of 5000 lux, and the zenith three times brighter than the horizon. This sky type is the basis for daylight factor calculations and measurements. However, for at least 80% of daylit hours, the external daytime illuminance will frequently and greatly exceed 5000 lux. A cloudy day with white clouds and sunlight nearly breaking through will have a horizontal illuminance of about 12,000 lux. .On a sunny summer day, with white clouds, the outdoor illuminance can be as much as 100,000 lux. In much of Scotland the external illuminance exceeds 10,000 lux for 60% of daylight hours and this can make a significant contribution to lighting needs.

Seasonal Affects

The changeability of the weather in Scotland gives rise to problems when seeking to integrate natural and artificial light, but benefit can be made of daylight. The different weather conditions in each season greatly affect the length and quality of daylight. In summer the sun rises high in the sky and hours of daylight are long. It is not the most popular time for people to be indoors, in a “black box”, but it is generally believed that daylit sports halls are more likely to encourage year round use. High level sunlight - which is most likely to cause overheating - can be blocked by designed overhangs on south facades, but east and west facades will still receive low level sun. The problem is likely to be more apparent in the west because facilities are used in the evening more than the early morning.

In winter, there are often clear skies, especially in the east. Hours of daylight are short. The sun is low in the sky all day and will enter a building despite any overhangs if adequate shading is not provided.

In spring and autumn the sun rises higher, but dwells for long periods at a low altitude in the mornings and evenings. Rainfall is greatest in the spring and cloud cover can be dense.