Where does the water go, and how does it come back?
A wet towel dries overnight. Nothing dripped away and nobody wrung it out—the water turned into vapour and slipped into the invisible air. That process is evaporation, and it happens at every water surface, all the time.
Zoom in on the surface: countless molecules dash about at different speeds. The few that happen to sit near the top, head upward, and move fast enough to break free of their neighbours’ pull fly off first. So evaporation takes the fastest molecules; the ones left behind are slower on average, and the water cools—why wet hands feel chill.
There is a limit to how much vapour air can hold, and warmer air holds more. When humid air meets the icy wall of a glass, it cools, its “backpack” shrinks, and the surplus vapour is surrendered as beads on the outside wall—that is condensation. Evaporation and condensation are the same road travelled in two directions: water leaves; water returns.
Two bathroom experiments
- Three ways to hang a towel. Wring one towel to the same wetness each time: first leave it bunched on a chair, then shake it out and hang it, finally hang it by an open window. Feel it (or weigh it) every ten minutes: spread out dries faster than folded, breezy faster than still. What changed is the evaporating area and how quickly humid air is swept away—the towel itself did nothing different.
- Breathe on the mirror. Before a hot shower, breathe on the mirror: it fogs instantly. Touch it with the back of your hand—it is cold. Your breath carries vapour; meeting a mirror below the dew point, the surplus is handed over at once. Wait, and the fog disappears as those droplets evaporate back into the air.
Guess first: the same wet towel on a summer day and a winter day—which dries sooner? Then ask why winter windows fog on the inside face.
The Magnus formula, the dew point and evaporative cooling
Open: how big is the air’s backpack?
Textbooks estimate the saturation vapour pressure above water with the Magnus formula (in kPa):
Converted to grams of vapour per cubic metre: about 4.8 g/m³ at 0 °C, 17.3 g/m³ at 20 °C, 30.3 g/m³ at 30 °C—every 10 °C roughly doubles the capacity. The experiment’s rate follows the same logic:
Both vapor concentrations are evaluated at the same surface temperature, keeping the sign consistent with the dew point. The bracket measures the difference from equilibrium: a large gap and molecules leave eagerly; zero gap and evaporation balances condensation; a negative gap and vapour condenses back onto the surface instead.
Open: the dew point, running the formula backwards
Given the air’s temperature and relative humidity, you can compute the vapour pressure it actually carries. Now run Magnus backwards: at what temperature does the curve fall to exactly that value? That temperature is the dew point—the moment the backpack is exactly full. One degree lower, the surplus must condense: dew on grass, a fogged bathroom mirror, beads on an iced drink—moments when the dew point is crossed. In the experiment, drop the surface below the dew point and watch condensation begin.
Open: why does evaporation cool what is left behind?
The molecules that escape rank among the fastest, carrying away more than the average energy; the remainder’s average kinetic energy drops, and so does its temperature. That is why wet hands and alcohol swabs feel cold, and why sweating cools the body. The staged film uses a teaching value of a few degrees; a real wet surface settles near the meteorologist’s wet-bulb temperature, set by the gap between air temperature and dew point—the drier the air, the wider the gap.
Open: how is boiling different?
Evaporation works only at the surface, quietly, at any temperature. Boiling allows vapor bubbles to form within the liquid: only near the boiling point (100 °C at standard pressure) can bubbles form throughout the water and roll out of it. Drying laundry, fogged windows and clouds are evaporation and condensation; a rolling kettle is boiling—evaporation and boiling are both vaporization, while condensation is the reverse change, from vapor to liquid.
These simplifications and figures follow OpenStax College Physics on phase changes (latent heat, evaporation versus boiling) and the USGS Water Science School water cycle (the global evaporation–condensation–precipitation loop).