While people in the yang world cannot live without the sun, the beings in the yin world cannot bear to be exposed in the daylight, even at the time there is no sun, no rain, as which would cause them to melt. It is why when ghosts parade in the human world, whether for celebration or protest, they have to pop up an umbrella to hide in shadows. Ghosts' umbrella movement – ink painting by Luo Pin, an 18th-century Chinese artist. https://www.viewofchina.com/ghosts/