Every surface has a mirror score between 0 and 1 called its albedo, $\alpha$ — the Greek letter alpha. The sunlight it bounces away is $G_r = \alpha G$ (read aloud: G-r equals alpha G), where $G$ is the sunshine landing on each square metre, in W/m², and $G_r$ — the r is for reflected — is the share sent straight back. Everything not reflected, $(1-\alpha)G$, stays to warm the ground. The scores tell a climate story in one line: fresh snow reflects about 0.9 of its sunlight; grass about 0.25; open water a mere 0.06. Melt sea ice and a 0.9 mirror becomes a 0.06 sponge — the ocean drinks the sunshine the ice used to refuse, warms, and melts more ice. That loop is one of the planet's most important pieces of arithmetic.

Second tool: climb a mountain and the air cools at a steady clip called the lapse rate, $\Gamma = \dfrac{\Delta T}{\Delta z}$ — Gamma equals delta-T over delta-z; $\Gamma$ is capital gamma, and $\Delta$, delta, means “the change in” — so $\Delta T$ is the temperature drop in C° and $\Delta z$ the height gained in kilometres. When two levels get named, subscript 1 is the lower one you start from and 2 the higher one you finish at: $T_1$ is the valley reading, $T_2$ the summit reading you are solving for. Ordinary air runs 5 to 8 C° per kilometre — which is why a 20 °C valley can hold up its own snow-capped peak.