It`s not as bad as you think....

[#1]

 

A misty morning arrival for a @British_Airways 777 at @HeathrowAirport. https://www.jetphotos.com/photo/8862762  © Wael Al-Qutub

[#2]

It'll never pass an M.O.T. with emissions like that.

[#3]

It's still cleaner than a volkswagen..

(and skoda lol)

[#4]

@shutter: That's a good photo!  The rising sun is just at the right stage to make it look really dramatic.

Here's another:

The reduced pressure on the top of the wing (lift) causes a drop in temperature.  At cruising speeds it's spread over the top surface, but slow speeds require high angles of attack when the reduced pressure area is concentrated near the front. Because the area is smaller the pressure has to be lower to maintain the lift.  At times the temperature drop is sufficient for the water vapour to condense.

The same thing happens in a high-g turn.

There's a good article on the subject here:  https://tinyurl.com/y8pye6mb