Relative humidity is a useful number, but it's easy to misread. 70% RH at 18 °C feels different from 70% RH at 30 °C, same RH, dramatically different moisture in the air. The number that captures absolute moisture is the dew point: the temperature at which the current air would become saturated and water would start to condense.
The NWS dew-point comfort scale: below 13 °C dew point, air feels dry. 13–18 °C is comfortable. 18–21 °C is noticeable. Above 21 °C is uncomfortable; above 24 °C is oppressive. These are remarkably consistent across people and climates: a New Englander and a Floridian both feel a 24 °C dew point as "sticky".
For Terrestream, the sensor reads RH and T directly; dew point is computed from them via the Magnus-Tetens approximation. The dashboard displays both: RH for the immediate "is my air dry or wet" question, dew point for the comparison question ("muggier than yesterday?") and for the dew-condensation prediction (cold surfaces below the dew point will collect water, the basis of the mold-risk inference).
Outdoor dew point is one of the Open-Meteo forecast fields the dashboard pulls in. Comparing indoor to outdoor dew point answers a question users frequently ask: should I run the dehumidifier or just open windows? If outdoor dew point is lower than indoor, ventilation lowers indoor moisture. If outdoor is higher, ventilation adds moisture, only mechanical dehumidification can lower it.
References
- NOAA NWS - Dew point vs relative humidity www.weather.gov
- ASHRAE Standard 55 - Thermal Environmental Conditions www.ashrae.org
- Open-Meteo - Forecast API documentation open-meteo.com
- EPA - Mold course: water & moisture www.epa.gov
