This is because the altitude causes climate difference and affects the growth of vegetation. May is the midsummer in most parts of China, with flowers and plants in full bloom. However, Tianshan Mountain, located in the northwest frontier, is still covered with snow, and there are no willows and flowers, which shows that Tianshan Mountain (Qilian Mountain) at an altitude of more than 4,000 meters has neither summer nor characteristics of spring and autumn.
This is a typical altitude difference, which leads to the difference of phenology.