Hypothetical: Can we rehabilitate arid/desert landscapes through the artificial precipitation from exhausts of hydrogen powered airplanes?
Rough calculations: A 747 burns about 10000 kg of fuel an hour. Assuming it burned hydrogen at the same rate it would produce about 100000 kg (or litres) of water an hour. Flying at 1000 km/hr that means it produces 100 litres of water per kilometre of flight, or 0.1 litre of water per metre.  Lets assume all of this water drops vertically below the plane in a path 100 metres wide. That means that we have 0.001 litres/m$^2$ of water, equivalent to 0.001 millimeter of rainfall. The plane could then turn around and repeat the pass, so in a 10 hour working day we could have 0.01 mm of rainfall
If we take a threshold of desert very conservatively at 200 mm rain per annum, and fly our plane backwards and forwards along a 1000 kilometre path for 10 hours a day, 5 days a week, it would only create 2 mm of rainfall a year. So to get 'non-desert' along our 100 metre wide strip of desert we would need to fly 100 planes every day!
Looked at another way. The Aswan dam in Egypt irrigates an area of 300,000 square kilometres. The dam releases about 50 cubic kms of water per year. 50 cubic kms  is 5 x 10$^{13}$ litres of water. Our plane produces 10$^5$ litres per hour, so we would need 10$^8$ , or 100 million, hours of flight to produce the same volume.  That would be 100,000 jumbo jets flying 24 hours a day, 365 days a year to irrigate an area equivalent to 3% of the area of the Sahara.