Sky Illuminance Level

"Design Sky values are derived from a statistical analysis of dynamic outdoor sky illuminance levels. They represent the horizontal illuminance value that is exceeded 85% of the time between the hours of 9am and 5pm throughout the working year. Thus they also represent a worst-case scenario that you can design to and be sure your building will meet the desired light levels at least 85% of the time. Design sky values vary from around 12-15,000 lux at the equator down to around 3-4000 lux at a latitude of ±60°, as shown in Figure 1 below." to find out more...
This is important to generate real time rendering too. Some basic settings for different position for sky illuminance.
Passage and Image via http://wiki.naturalfrequency.com/wiki/Design_Sky