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Why is there no such technique used where cool water gets sprinkled on a runway?

Cooling down the air above the airport would decrease air density and therefore lead to a shorter starttime for airplanes.

This image explains exactly what I mean.

(related article: https://www.boldmethod.com/learn-to-fly/performance/prevent-a-density-altitude-crash-on-takeoff/) air density

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    $\begingroup$ Humid air is less dense than dry air. The drop in temperature has to compensate for that. The biggest drop is possible if evaporation happens right before air is ingested by the compressor. That function is performed by water injection for improving thrust in hot and high conditions. $\endgroup$ – Peter Kämpf Dec 9 '19 at 21:31
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    $\begingroup$ What happens when the aircraft can't climb out of the more humid/cooler air because the hot air is just above it? $\endgroup$ – Ron Beyer Dec 9 '19 at 22:22
  • $\begingroup$ Why would you want to decrease density? That's backwards. $\endgroup$ – TomMcW Dec 12 '19 at 16:49
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There are a few problems with this idea:

  1. The water evaporating would increase the humidity, likely canceling any cooling benefit.

  2. The effect would be localized in a very small region just above the pavement.

  3. The effect would be temporary as any breeze would quickly blow the cooled air away.

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Another problem is now your balanced field length takeoff plus landing performance has to be based on "wet runway" parameters which adds significant penalty, more than negating any benefit you got from evaporative cooling.

Now if you were in the desert you might get some kind of benefit with a fogging system that blew atomized water fog over the runway where it evaporated immediately without making the surface wet, like is done with outdoor terraces at bars. The temperature drop from that evaporation might give some tiny performance benefit. If you could find someone to pump all that water along a 5000 ft runway, at the water utility rates you'd be paying in a desert environment.

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  • $\begingroup$ It would be a lot cheaper to remove cargo (people/baggage) to be within performance limits for hot/high operations than to create a weather pattern around an airport for the purpose of taking off (and possibly crashing as they may not be able to climb out of the hot air) $\endgroup$ – Ron Beyer Dec 9 '19 at 22:23
  • $\begingroup$ Actually it made me think of it as kind of the inverse of this wild and crazy concept that was used quite a lot in WW2 youtube.com/watch?v=gAIjxaJ2_Ag You sure has hell wouldn't want to drift off to the side! $\endgroup$ – John K Dec 9 '19 at 22:27

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