This is the boss. No calculator — every number is chosen to fit in your head, and the whole chapter reports for duty: force over area for the instrument plate, the vapour ratio for the humidity, the forecaster's dew-point rule (1 °C below air temperature for every 5 points of humidity under 100), and 3 seconds per kilometre for the storm itself. Every answer feeds the next — the humidity you find sets the dew point you call — so write each line down as you go; one early slip rolls all the way down the log.

Mental-math kit: dividing by 0.25 is multiplying by 4; dividing by 0.5 is doubling; 1.5 out of 3.0 is a half. Everything from the chapter rides in this one afternoon. 5★ under par here is the chapter crown. Eyes on the sky — clock in.