Updated: Dec 11, 2019
According to Acupuncturist & Qigong instructor, Jeffrey Chand, "According to traditional Qigong theory, practicing outdoors in the elements builds resilience. If we regularly practice in nature, our body attunes to weather, humidity, cold, heat, and light and becomes stronger. This can be very beneficial!
However, for some, it may cause too much agitation if the "wei qi" is not strong. Wei qi is the defensive energy of the body. Strong wei qi = strong immune system.
So, if we have a strong wei qi then practicing outside can be wonderful, but if it is not strong, then an indoor practice may be better.
A good way to measure this is looking at your body's response to wind. Wind challenges the wei qi. If wind doesn't bother you, then there is a good chance your wei qi is strong."