Wet-bulb temperature: the ceiling on evaporative cooling

A wet-bulb temperature is what a thermometer reads with a damp wick around its bulb and air passing over it. Water leaving that wick carries heat away with it, so the instrument settles below the air it is sitting in. Put a dry thermometer beside it and the pair disagree for as long as the wick holds water.

How far the wetted one falls is decided by the air, not by the instrument. Dry air takes up vapour readily, evaporation runs hard, and the reading drops a long way. Air already close to full takes up very little, evaporation nearly stops, and the wetted bulb sits close to the dry one. The instrument is reporting the air's appetite for water.

The National Weather Service defines this figure as the lowest temperature obtainable by evaporating water into the air. Read that as a specification limit rather than a curiosity. Nothing that cools by evaporating water gets past it, however well built it is and whatever it cost.

That is why the figure earns a homeowner's attention. It is not a comfort score, and no remote has a button for it. It is a boundary the air draws around one method of cooling, and it moves with the weather rather than with anything anyone owns.

Dew point is the other reading taken from this same air, and it settles something else entirely. It marks the temperature at which water starts appearing on a surface, which is the condensation story. The wetted reading marks how far evaporation is still able to carry a temperature downward. The dew point guide covers its half. This page covers the other.

One wetted reading on its own says very little. Held against the dry-bulb figure taken beside it, it says a great deal. The difference between the two carries its own name, wet-bulb depression, and that difference is what predicts behaviour.

A wide gap means evaporation has somewhere to go. Sweat clears from skin, washing on a rack finishes, a mopped floor dries, and any machine built on evaporation performs near its rating. The air is doing half the work by accepting what gets offered to it.

A narrow gap means the reverse, and it means it for everything simultaneously. The afternoon that leaves laundry damp on the rack is the afternoon sweat stays put and a water-filled cooler barely shifts the room. Those are not separate observations. They are one property of the air, surfacing in several places at once.

Indoor air is not stuck with the outdoor figure, which is the useful piece of news here. A coil that condenses water pulls both readings down inside a shut room, so a bedroom held cold for hours usually offers more headroom than the rest of the flat. That advantage lasts exactly as long as the room stays shut. A shower, a wok, a rack of wet washing or a propped door all hand the gap straight back.

Anything that proposes to cool a room by adding water to it deserves care in this climate. Misting fans, water-filled boxes and damp-cloth arrangements all narrow the gap while they run. The air comes down a little and gives up part of what made it able to dry anything at all, a person included.

A fan helps less because the air it delivers is no better at absorbing moisture than the air it pushed aside. Moving air restarts evaporation that had stalled in a still layer against a surface. It cannot raise the amount of water that air will accept, so the relief a fan buys is capped by the same figure throughout.

That cap explains why the identical fan reads strong one week and weak the next. Drive a lot of air across a damp surface when the gap is generous and the surface cools noticeably. Do it when the gap has closed and almost nothing happens. Whether pairing a fan with the aircon pays for itself on the bill is a different matter, and there is a guide for it.

Skin is the version everyone has already felt, and a separate guide takes that on properly. The part belonging here is the shared ceiling. A body, a portable cooler and a rack of washing all run into one figure, and on a bad afternoon all three report it at once.

Portable air coolers are the ones worth being careful about. These are evaporative devices: a fan, a wetted pad, and a tank the owner refills. The definition further up is their entire specification. They cannot deliver air below the wet-bulb of the room they stand in, and they return part of the water they use to that room, closing the gap for everything else in it.

Which climate a design assumes rarely makes it into the marketing. Evaporative cooling grew up where air is dry and the gap is generous. Move the same box into wet air and it does far less for the same electricity. NEA publishes hourly wet-bulb readings from the Changi climate station, running back to 1982, so where local air sits is on public record rather than open to argument.

Where a building rejects its heat through a cooling tower on the roof, the identical principle runs at a far larger scale. That is why tower performance gets quoted against a wetted reading instead of against an air temperature. Buildings cooled that way are covered in the guide on chilled-water systems. A split unit in a flat is not an evaporative machine at all, and that difference carries the next section.

A split unit is not an evaporative machine, so this reading sets no floor under it. Refrigerant chills the coil, and the coil can sit well below anything evaporation would reach. What a rising figure changes is not how cold the machine can get. It is the size of the job handed to it.

Air arriving at the coil brings a temperature and a quantity of water, and the coil has to deal with both. Water leaves that air only by condensing onto the fins before it drains away, and turning vapour into liquid consumes genuine capacity. Two rooms reading alike on a wall thermometer, with different wet-bulbs, hand the same machine two different workloads.

How a room's total load divides between the dry part and the moisture part is worked through in the guide on heat load. The reading matters here because it is what tells the two apart. Dry-bulb cannot: it describes one half and stays silent on the other, which is how two rooms look identical on paper and behave nothing alike.

Rated capacity already allows for this, and the allowance is written down. The international test standard, ISO 5151, rates a room unit with indoor air entering at 27 °C dry-bulb and 19 °C wet-bulb, against outdoor air at 35 °C dry-bulb and 24 °C wet-bulb. The moisture condition is stated because the capacity figure would mean nothing without it.

A nameplate is therefore a result under a named condition, not a promise about a bedroom. Feed the same machine air with a higher wet-bulb and more of its output goes into water and less into temperature, with nothing faulty anywhere. Why a rated figure and an observed one diverge is treated at length elsewhere. This page supplies one of the reasons they do.

Hold on to that when a system gets called undersized after one sticky week. Undersizing is real and does get sold past people. What separates it from a wet spell is whether the room also gave up on days the air was drier, and that is a question about the pattern rather than about the worst day.

A household that changed nothing can find the same setting stops satisfying for a run of weeks. No part of the machine has to move for that to happen. What changed was the air being fed to it, and the part that changed is the part no display reports.

These reports have a recognisable shape. The room reaches its number and still sits heavy. The unit runs longer than it used to for the same result. Washing hung out overnight is damp in the morning. A wardrobe starts smelling faintly of nothing in particular. They arrive together because one property of the air is behind all of them.

What separates this from a fault is what happens when the weather turns back. A machine that has genuinely lost capacity disappoints on dry days too. Loaded air stops being a problem the moment the air stops being loaded, with nobody touching the unit. Noticing which of those two happened costs a household nothing and is the most useful thing it can contribute.

Push back on a gas top-up quoted from a description alone during a wet spell. A room that cannot shed moisture and a circuit that has lost gas land as one indistinguishable complaint. Instruments on the system are what separate them. Ask which readings were taken and what they showed. A quote resting on the weather having been humid has separated nothing.

The limitation runs in the other direction as well. A unit can measure as intact while the household stays uncomfortable. Capacity is what the reading confirms, and the demand the air places on that capacity is a separate matter. Both findings can hold in one visit, and saying so plainly beats picking whichever is easier to sell.