Observations
Along with many in the Fayette and Shelby County areas today, we lost power. I took the opportunity to evaluate the amount of light we are getting at Greyhaven when it’s overcast and there’s no other source of light available.
East facing office window: 10 lux.
South facing living room window: lower, 120 lux; upper 250 lux.
Standout southern window: lower, 220 lux; upper 440 lux.
West facing windows: lower, 500 lux; upper, 700+ lux!
North facing bathroom window: 275 lux.
Champ of the day, west facing shower window: 700+ lux!
Thoughts
When it is overcast, the light source is the sky, which diffuses light from the hidden sun. At this time, access and angle to the sky is key. Our western windows have no nearby trees, so that is why they are catching so much light. The standout southern window producing twice the light as the rest on the south seem to be doing so much better because it is near where the trees stop. When pointing my meter toward the trees viable from it, the light levels go down. Also note the advantages of angle: the upper part of windows are consistently catching twice the amount of light as the lower. This is because they have a better view of the sky, which is the dominant light source right now.
The Memphis area has some degree of sunshine between 60%-70% of the time. This means daylighting strategies should aim at using and managing sunlight, whereas regions to the northwest and northeast need to assume an overcast sky the majority of the time, which requires a different design strategy. (To learn about the difference between sunlight and daylight, see our page on Solar Orientation.)