John Gill Commentary Matthew 16:2

"But he answered and said unto them, When it is evening, ye say, [It will be] fair weather: for the heaven is red." — Matthew 16:2 (ASV)

He answered and said to them
Knowing full well their views, and having wrought sufficient miracles to confirm his Messiahship, he thought fit to give them no other answer than this:

when it is evening, you say, it will be fair weather, for the sky is
red ;
when the sun is setting, it is a common thing for you to say, looking up to the heavens, and observing the face and colour of them, that it is likely to be fair weather; no rain, that night, nor perhaps the next day, for the sky is red like fire, through the rays of the sun; which show the clouds to be very thin, and so will soon waste away, and consequently, fine weather must follow.