Heat makes air spread out and get lighter than the cooler air around it, so it floats upward — the same reason a hot-air balloon climbs.
What's actually happening
Heat a parcel of air and its molecules move faster, hit each other harder, and shove their neighbours further apart. The parcel swells. Same number of molecules, more volume — so the warm parcel is now less dense than the cool air around it.
Here is the part the phrase "hot air rises" hides: the hot air is not climbing, it is being pushed. Gravity pulls the surrounding denser, cooler air down harder than it pulls the light warm parcel. The cold air slides down and underneath, and the warm parcel gets squeezed upward like a beach ball released at the bottom of a pool. Buoyancy is just gravity playing favourites with density.
Set this loop running continuously and you have convection: warm air rises, cools as it expands higher up, gets dense, sinks, warms again. That single loop, at different scales, is your radiator heating a bedroom, a gliding hawk circling in a thermal, a sea breeze arriving on a summer afternoon, and a thunderstorm building its tower. Weather is mostly this one idea with bigger budgets.
- 1Cut a paper spiral (a snail shape) about the size of your palm and tie a thread to its centre.
- 2Dangle it 30 cm above a warm radiator or a recently boiled kettle — never a flame.
- 3The spiral spins steadily in the rising current. No wind, no motor: you are watching invisible convection made visible.