The conditions of the sky of daylighting evaluation are classified as either overcast, clear or partly cloudy. Each classification has characteristics that influence daylighting design.

Overcast sky--
                                                       
The overcast sky is defined as one in which the position of the sun cannot be determined because of the dense cloud cover; the light is diffuse and relatively even over the sky dome. The overcast sky is approximately three times brighter at the zenith than at the horizon, and the illumination is evenly distributed around the zenith.

Therefore, the top of the sky dome is the main source of illumination. The overcast sky is frequently used as the minimum design condition, through the actual amount of illumination in fact varies with the altitude of the sun and the density of the cloud cover.

                   
                                                 Overcast sky
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Clear sky--
Illumination from the sun's direct rays is extremely intense compared with those diffused through the sky dome; because direct sunlight is so powerful, it is a potential source of glare. Excluding the sun, the clear sky is less bright than the overcast sky. The distribution of light from a clear sky, (except the area immediately around the sun), is just opposite to that of the overcast sky, i.e. three times brighter at the horizon than at the zenith. Therefore, without direct or reflected sun rays, building openings that face the top of the sky dome, receive less light on clear days than on overcast days.

The illumination from a clear sky varies with the position of the sun, as well as the amount of water vapour in the atmosphere. Therefore, the amount of illumination available to any surface changes throughout the day and the year.

Sunlight reflected from the ground usually represents 10 to 15% of the total illumination reaching a vertical window. It can even account for more than 50% when the window is shaded from direct radiation. Reflected light from the ground can be a valuable source of daylighting during clear sky conditions because it reflects off the ceiling (usually in light colours) and penetrates deep into the room.

 
                                                                  Clear sky
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Partly cloudy sky--
                                                   
Partly cloudy sky is the situation most commonly occurred. It falls somewhere between the extremes of overcast sky (that has a few clear spots) and mostly clear sky (that has a few clouds). Partly cloudy sky frequently exhibits very different amounts of illumination across the sky dome as the cloud cover changes over time.

The assessment of daylighting is different because the light source (the sun) is subject to continuous variation. Both the illuminance in a room and the character of lighting undergo extensive changes as the external conditions vary. Some European countries, like Germany and the UK, have considerably more daylight hours with an overcast sky than with a clear one. Hong Kong, however, has more clear sky daylight hours than an overcast sky will be discussed here because the approach is comparatively simple and it represents the minimum design condition.

               
                                Partly Cloudy Sky

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