Possible Clues:

• To go up in the air and fly over Pisa
• Rio paves way to get out into the air
• Turn to mist and spray over Pisa
• Disappear into thin air with Virginia on river approaching hill
• Post Office goes after Virginia on housing estate road for gas annihilation
• What sweat will do via pores in transformation
• To change physical state of substance, divide 5, as e, by Pi /2
• Blast into gaseous form - via pores (anag)
• That's the way to make great gas of it and the river flood at last