Mould does not grow because a room is humid. It grows on the one surface that is colder than the rest — a corner, a window reveal, the wall behind a wardrobe. The air touching that surface can sit at 97% while the middle of the room reads a comfortable 55%.
Hygrometer works out what is happening on that surface, and tells you the humidity to keep the room under.
WATCH A ROOM
• Add the cold surface you are worried about: a corner, a window reveal, an uninsulated wall, single or double glazing
• See the humidity at that surface, not just in the room
• Get the limit for the room, worked out from the surface temperature and the weather outside
• That limit moves with the forecast — the same wall tolerates 73% on a mild day and 41% in a cold snap
THREE WAYS TO GET THE NUMBERS
• A HomeKit humidity sensor, read automatically — Eve Room, Aqara, SwitchBot, Netatmo and others
• Typed in from your own hygrometer, whenever you check it
• Or neither: Hygrometer estimates the likely range from the outdoor conditions and how the room is used, using the EN ISO 13788 humidity classes. Shown as a range, never as a measurement of your room
WARNINGS THAT ARRIVE IN TIME
• Mould risk when the surface reaches 80% humidity, which is long before anything looks wet
• Condensation when the surface falls below the dew point
• And before either: a colder forecast tonight pulls the limit down, so the warning comes before the surface turns, not after
SHOULD YOU OPEN THE WINDOW?
The answer relative humidity gets backwards. Airing out only dries a room when the outdoor air carries less water in absolute terms. In winter it nearly always does, even at 90% outside. On a warm humid day it often does not, and opening a cool room makes it damper. Hygrometer compares both sides in g/m³ and names the driest hour of the day.
THE OUTDOOR INSTRUMENT
• Outdoor relative humidity on a 0–100% analog dial with comfort bands
• Dew point, absolute humidity in g/m³, mixing ratio, pressure and conditions
• 24-hour humidity chart
• Home Screen and Lock Screen widgets, which fetch their own reading rather than waiting for you to open the app
ALSO
• Follow your location, or search any city worldwide
• °C / °F throughout
• Eight languages
• No account, no tracking, no ads
Outdoor readings come from Apple Weather. Indoor readings come from your own sensor or from you — iPhone has no humidity sensor, and this app never pretends otherwise.
Weather data provided by Apple Weather.
