Dew point, not humidity, tells you when to open
Relative humidity is a percentage. Dew point is the temperature that makes air feel sticky. Use it to decide between a short opening and throwing every window wide.
The number people mean when they say the air feels sticky is dew point, not relative humidity.
Relative humidity is a percentage of how full the air is compared with how much moisture it could hold at that temperature. It swings when the thermometer swings. Dew point is the temperature where water in the air would start to condense. It stays put as the day warms, so it is a better read of how muggy the air actually is.
High dew point means muggy air that does not cool you well, even if the outdoor thermometer is a little lower than the room.
What the numbers feel like
These are rules of thumb, not medical thresholds:
- Below about 10 °C: dry
- 10–16 °C: comfortable
- 16–20 °C: slightly heavy
- Around 20 °C and above: muggy
- Past about 22 °C: oppressive
When outdoor dew point is high, or close to your indoor temperature, a short opening is usually better than throwing every window wide. You still dump some heat. You do not fill the flat with air that will not evaporate sweat.
The dew point checklist
- Trust dew point more than the humidity percentage
- Open wide when dew point is comfortable and outdoor air is cooler
- Prefer a burst of air when it feels heavy or sticky
Ventilate Pro uses outdoor dew point in the window advice. Temperature still leads. Dew point only narrows a wide-open call into a shorter one. See when cooler air is not worth letting in.
This is not medical advice. If heat is making you unwell, follow your local health agency.