Reading the Sky

Why it matters

Torricelli flipped a tube of mercury into a dish and the column froze at 76 cm — held up by nothing but the weight of the sky. Carry it up a mountain and the sky gets visibly lighter.

The idea in plain words

One tube, three instruments. Tap the terms.

P(atm) = h(mercury) ρ g

The column reads 67 cm. Roughly how high are you?

≈ 1000 m above sea level

• the choice: why mercury — 13.6× denser than water → a 76 cm column instead of a 10.3 m water tower; doesn't wet glass, low vapour
• use 1: altimeter — steady fall of ~0.9 cm per 100 m of climb → aircraft altimeters
• use 2: weather glass — SUDDEN fall = storm coming (low-pressure system); slow rise = fair weather
• the upgrade: aneroid — the no-liquid barometer — a flexing vacuum box, portable and dry
• fall = 76 − 67 = 9 cm
• 9 ÷ 0.9 × 100 m

Predict first

You carry a barometer up a mountain. The mercury column…

76 cm is SEA-LEVEL's number. Climb, and you leave air below you — roughly 0.9 cm of mercury per 100 m. A barometer is an altimeter in disguise.

• falls — less atmosphere above you, less pressure — correct
• rises — mountains are windy
• stays at 76 cm everywhere

What you do

Climb until the column reads 67 cm and hold it there to name your altitude — then flick the weather and read the warning.

Check yourself

Water is a poor barometer liquid because…

P/ρg with water's small ρ gives 10.3 m — a barometer the size of a building.

A SUDDEN fall in the barometer usually means…

Storms are violent low-pressure systems — the mercury dips before the rain arrives.

Mountaineers' nosebleeds happen because at altitude…

The pressure balance we never notice at sea level breaks — from the inside out.

• the column would need to be over 10 m tall — correct
• water is opaque
• water is too heavy
• a storm is approaching — correct
• fine clear weather ahead
• the barometer is broken
• outside pressure drops below the body's internal blood pressure — correct
• the cold freezes the nose
• thin air is toxic