Weather is a story about air pushing on things, so this chapter opens with the idea of pressure: a force, shared out over the area that carries it. $P = \dfrac{F}{A}$ (read aloud: P equals F over A). $F$ is the force in newtons, $A$ is the area in square metres that carries it, and $P$ — the pressure these questions ask for — is what you get by sharing one over the other. The unit is the pascal: one newton spread over one square metre. A pascal is tiny — a single sheet of paper lying flat presses on the table with about 1 Pa. The whole atmosphere, stacked kilometres deep above your head, presses with about 100,000 Pa — 100 kPa — and you don't notice only because it pushes from every side at once.

The formula explains snowshoes in one line: your weight $F$ is fixed, so the only way to press GENTLER on deep snow is to grow $A$. Same force, four times the area, a quarter of the pressure — you float where boots would sink. Skates run the trick backwards: shrink $A$ to a thin blade and the same person presses hard enough to glide. One formula, both tricks.