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No difference whatsoever on the glide ratio.To get maximum glide ratio you must fly at the optimum angle of attack and as long as you mainain that angle of attack then the glide ratio will be maximum.  With higher temperatures and/or lower atmospheric pressure the true airpspeed to maintain the best glide angle of attack will be slightly higher, say aboout 5 knots at 3000 feet but the indicated airspeed will be constant no matter the atmospheric conditions. Since the plane might be going a lilttle bit faster then it will be comming down a little bit faster but the glide ratio will stay the same, only Sully might have reached the river 5 or 10 seconds sooner, but would have splashed down in the exact same spot. gl

This is a good answer Gary.  Interestingly, the initial impact might have been less survivable in July, because there would have been more energy to dissipate.  Wikipedia quotes the touchdown IAS as 125kts and the air temperature as -7C.  Neglecting the wind velocity, which appears to have been about 8kts across, this means the aircraft struck the water with a TAS of about 120kts.  If it had been July and the temperature had been 25C, the touchdown TAS would have been 127kts or 6% greater.  However, energy is proportional to the square of the speed, so this would have resulted in a 12% increase in kinetic energy to dissipate.  However 2, it’s not the speed that does the damage, it’s the deceleration, and how that might be affected by a landing on water is less predictable. DaveP