Named reference. It converts a sumti into another sumti. The converted sumti points to the referent the name of which is the referent of the unconverted sumti.
x1 is a specific kind of bad weather characterized by large dark rain clouds that block out most of the daylight, large, cold rain drops and an overall dark, uncomfortable and unsettling atmosphere, optionally accompanied by icy wind, such that a change in the weather conditions would be a relief to anyone present, both physically and emotionally.