The first drop changes everyone’s attention before it changes the ground.

Attention moves ahead of the event. One drop lands and a whole street reorganises itself: heads tilt, bags are gathered, someone starts walking faster, and nothing on the pavement is even wet yet. What people respond to is the prediction rather than the rain. Much of what looks like reacting to a change is really reacting to the announcement of one.

When you notice a room shifting mood, ask what has actually happened and how much is anticipation. Anticipation is useful when it makes you fetch a coat and costly when it makes you spend the afternoon braced. Work out which is going on before you join in.