‘But he answered and said to them, “When it is evening, you say, ‘It will be fair weather, for the heaven is red'. And in the morning, ‘It will be foul weather today, for the heaven is red and lowering.' You know how to discern the face of the heaven, but you cannot discern the signs of the times.”
Jesus first replies by pointing out that like all Jews they are able to discern weather signs. A clear red heaven in the evening indicates to them fine weather. A sky that is red and lowering in the morning indicates to them foul weather. Thus they are adept at interpreting such signs. There is perhaps some sarcasm here. They can tell whether the sky is cloudy or not, but they cannot spot the cloudiness in their own thinking.
‘But you cannot discern the signs of the times.' Those who should be able to recognise the coming of the Messiah in the works that He has done and the words that He has spoken are unable to do so because their minds are clouded.
(We should possibly note that these two verses are omitted in some very important manuscripts (Aleph B f13 etc). They are supported by D W Theta f1 etc. Their omission would not affect the sense in any way, but a possible reason for their omission is that these weather signs were not applicable in such major Christian centres as Alexandria in Egypt where copying often took place. On the other hand there is no really good explanation as to why the words were quite unnecessarily introduced here from an unknown source if they were not genuine. Usually interpolations are explicable in terms of being introduced in order to conform with other passages, or as explanatory comments which were later accidentally incorporated in the text).