The fourth angel poured out its vessel on the sun, and it was given to him to afflict men with heat and fire. And men were scorched with great heat, and they blasphemed the name of God.(Apocalypse 16:8-9)
After three vessels had already been poured out on the earth, the fourth was poured out on the sun, which in turn rained down fire and heat on the earth. The illuminator conveyed a sense of the intensity of the human suffering through the figures' desperate gestures. The heat even forces John to take refuge by covering his head with a cloak. So that he can still see what is happening, he props up the front of his cloak with his staff.