That is oddly written.

'Mercury levels' here really means temperatures. High air temps reduce lift generated by a wing; they also reduce thrust (air is less dense). It means that an aircraft that has the range to fly xxx-yyy with a full passenger load at 80 degrees F from a given runway length may lack the range to do so at 110 degrees F. Offload passengers (tough when load factors are typically >80%), cargo, or offload fuel and make a fuel stop on the way.

Range charts for aircraft are based on a 'standard day' and that is actually a pretty cool air temp - more like Seattle in May than Phoenix in July.