Air holds water as invisible vapour, and how much it CAN hold depends on temperature: warm air is a bigger sponge. The ceiling is called the saturation vapour pressure, $p_{sat}$ — say it p-sat — and it climbs fast with warmth: about 2.5 kPa at 21 °C, 3.0 kPa at 24 °C, 4.0 kPa at 29 °C. The vapour actually present has its own pressure, $p_v$ — p-vee — and it can never beat the ceiling.

Relative humidity is simply how full the sponge is: $\mathrm{RH} = \dfrac{p_v}{p_{sat}}$ (read aloud: R-H equals p-v over p-sat — then dress the fraction as a percentage. A ratio of pressure over pressure means every unit cancels: RH travels as a naked percent. And notice the sneaky consequence: warm the air without adding a single drop and RH FALLS, because the ceiling rose while the water stayed put. That one idea explains dry winter classrooms and the next lesson's fog.