See the notes at Matthew 16:2-3.
South wind - To the south and southwest of Judea were situated Arabia, Egypt, and Ethiopia, all warm or hot regions, and consequently the air that came from those quarters was greatly heated.
How is it that ye do not discern this time! - You see a cloud rise, and predict a shower; a south wind, and expect heat. These are regular events. So you see my miracles; you hear my preaching; you have the predictions of me in the prophets; why do you not, in like manner, infer that “this is the time” when the Messiah should appear?