# Hypothetical: Can we rehabilitate arid/desert landscapes through the artificial precipitation from exhausts of hydrogen powered airplanes?

Note: I know this question sounds fit for aviation stack exchange, but other aspects of this question like cloud formation, atmospheric behavior, precipitation and overall water cycle can be better answered here.

There are aviation manufacturers which are working towards bringing aircrafts fueled with Hydrogen that only/mostly release water vapor (as the major exhaust component). Now, if it proves to be true and safe enough, is it possible to facilitate artificial precipitation in the areas above which the airplanes fly over a period of months, thereby creating enough cloud paths that eventually cause rains.

Can we possibly make areas like Sahara Green using this methodology?

• If you are referring to only the Sahara the Answer is no. May 20 at 23:15
• cloud formation does not equal rain in the same location. You can add a lot of water vapor to hot/dry air and still get no rain. Furthermore, how many airplanes are we talking about? Not sure how many people in the Sahara are in the market for a new aircraft to fly around leisurely all day.
– f.thorpe
May 21 at 5:39
• Does burning Hydrogen make more H2O than released by burning fossil fuels? Can either add enough water vapor to cause desert greening rainfall that otherwise would not occur? Would it be more or less effective than planes water bombing the desert? Don't know, No, and Less would be my answers to those - and No, to the asked question. May 21 at 23:05

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.

• Thank you for the explicit figures! Sounds like Sahara is very far fetched. But, can we cause some precipitation considering the considerable amount of air traffic? May 21 at 18:42
• I think it shows that making a major impact on an area is improbable. That doesn't mean that the effect of aviation is negligible, but it's not about direct precipitation effects, more about albedo. May 21 at 19:00

Jet fuel is hydrocarbons, so their exhaust is partly water, which is responsible for the visible contrails. How Airplane Contrails Are Helping Make the Planet Warmer suggests that the main climate effect is warming, not rain.

• Yes, Warming is not the same as getting wetter. May 21 at 2:31
• The article states that water alone is not responsible for formation of contrails, it is the soot particles which act as the binding surface for water droplets to form the ice crystals. The airplanes in the question burn the fuel cleanly, thus not creating an opportunity for contrail creation. So, even if the exhaust is water, it won't contribute to warming unless it hinders the heat dissipation, which contrails (though for a temporary lifetime) do. May 21 at 19:27
• @lousycoder - If the soot particles are necessary for condensation, it seems that hydrogen powered airplanes can't even contribute the minimal amount of precipitation noted in the answer above. Hydrocarbon powered planes would be more effective, but there's nothing to suggest that they contribute to rainfall.
– Mark
May 22 at 16:16

Making the Sahara Green would be almost Impossible, The Sahara is naturally dry due to Rotation Patterns, In fact, the Sahara was once green, maybe the technique you presented would work for Small Deserts which are caused by Climate change.

There are click-bait articles that disagree with me, This one talks about turning the Sahara into a rainforest which is laughable to me.

Otherwise, terraforming the Sahara is off the list. One comment on the article reads, "I think they should just move around those city’s then and do the rest of the Sahara and change it to a rain forest."

@lousycoder but I think the methodology you presented is smart. The Sahara is huge, you would need a lot of planes and it will have a big price tag, and im sure some countries aren't interested in terraformation.

• Can we expect some amount of artificial precipitation, or at least some contribution/effect on the precipitations through natural clouds? May 21 at 18:44
• Yes, but just one plane won't make a storm cloud, Now I won't go further than this because I don't know much about aviation, but Contrails themselves are part water. @lousycoder May 21 at 23:59