Know which way the wind blows
The old principal was a relaxed person who liked to be informal. But now there is a new principal and she is very conscious of image and style. You have never seen your history teacher, Mr Wong, in a suit before, but today he shows up for class wearing a new suit, pressed shirt, tie and shiny shoes.
You might say: 'I see which way the wind blows', or 'Mr Wong certainly knows which way the wind blows'.
The idiom means to be aware of changing circumstances and to adapt to them. It is often said with a knowing look, showing that you understand clearly what exactly is going on.
The idiom may have come from ships. When you are on a sailing ship, you need to be very sensitive to changes in wind direction and adjust your sails as the wind turns.
The idiom could also come from hunting - a very common activity among our ancestors. The hunter needs to be upwind of his prey so that his scent is not carried to the animal. If necessary, he will circle around. Knowing which way the wind blows is an essential skill for success. If the prey gets wind of (discovers) what is happening, it will run off to safety. The police raid was a failure. Somehow the criminals had got wind of their plan.
Turn a blind eye
You forget to wear your school badge, or perhaps you are eating a snack during class. You are usually a law-abiding student and the rule you are breaking is a very minor one, so the teacher pretends not to see. She turns a blind eye. She doesn't want to see what you have done, so she turns away and you escape punishment.
The expression comes from a remarkable true story involving Britain's great sea hero, Admiral Lord Nelson. Last year were the 200th anniversary celebrations of the Battle of Trafalgar, during which Nelson died defeating the naval forces of France and Spain in the time of Napoleon. His statue is on top of the column in the centre of London's Trafalgar Square. In the course of fighting for his country, Nelson was injured in various battles. He lost an eye during one clash and after that always wore a black eye patch. In 1801, Nelson was in charge of part of a fleet that went to Copenhagen to stop Napoleon from cutting off British naval supplies. He went in on the attack, determined to win, but the senior commander thought the situation was getting out of control. In those days, the only way to send messages was to use small flags that followed a code. Orders came in this way. An officer told Nelson the commander was signalling. Nelson did not want to see the orders, so he put a telescope to his blind eye and said he could not see any signal. He fought on and won.