At least some of the water (in the liquid or solid form) falling from a cloud vaporizes as it passes through air below the cloud base. if the water completely vaporizes before reaching the earth's surface, the shaft of falling liquid or ice particles is known as virga.

Dry or unsaturated air

Explanation:

Virga is defined as a type of precipitation that falls from the lower side of a cloud and before it strikes the ground surface, it gets evaporated.  

This type of precipitation usually occurs when the rain falls through a region of dry or unsaturated air, which absorbs the water vapour (or moisture content), thereby resulting in a formation of a falling liquid shaft (ice particles).  

This type of precipitation generates microbursts that are dangerous for the aircraft and planes. These microbursts help in the elimination of heat from the prevailing air, as a result of which the cold air sinks causing severe turbulence.

an obstruction