As seen here Color temperature - Wikipedia, the free encyclopedia normal daylight kelvin is 5,500-6,000. Can someone explain to me why everyone likes to use 2700k bubls for flowering when in the wild MJ uses normal daylight?
Even the sun produces different color temperatures at different times of the year. The white hot sun of the summer is not the same as the orangish sun of fall and winter. This autumn sun is most closely replicated by a 2700K bulb. Good luck.
I searched google already, but couldent find anything confirming that the kelvin of the sun changed according to seasons
As winter approaches the sun moves lower on the horizon (OK, technically the Earth's equator moves away from the angle of the sun not the other way around), and so the light has to filter through more atmosphere to reach us, that changes the overall light color -- same reason the sunrise and sunset is reddish-orange.
The kelvin of the sun doesn't change. It produces the entire light spectrum from infrared to ultraviolet all the time. What reaches the ground (and your plants) is what matters and varies, as toasty noted, by how much atmosphere the sunlight travels through. Air molecules scatter blue light which is why the sky is blue. Spring and early Summer: sun passes almost directly overhead - longer days and less atmosphere to scatter blue light = veg. phase. Late summer and fall: sun does not rise far off the horizon - shorter days and more atmosphere to scatter blue light = flowering phase. Indoor growers duplicate nature by selecting the lights with the color temperature that the plant experiences outdoors: 6500K for veg, 2700K for flowering and by adjusting the light schedule appropriately.