Measurements · Meteorology
Relative Humidity
Also known as RH, air moisture, sample rh
Relative humidity of air.
What it is
What is relative humidity?
Relative humidity (RH) is the amount of water vapor in the air expressed as a percentage of the maximum the air could hold at that temperature. Because warm air can hold far more moisture than cold air, RH is temperature-dependent: the same quantity of water vapor gives a low RH in a warm room and a high RH in a cold one. The vapor itself comes from evaporation from water, soil, and vegetation, from human activity such as cooking, washing, and breathing, and from industrial processes. RH is closely tied to two related parameters: dew point, the temperature at which that vapor would condense, and the heat index, which combines RH with temperature.
Health and operational effects
Humidity has a pronounced comfort and health range. Air that is too dry, below roughly 30% RH, dries out skin, eyes, and airways, increases static electricity, and can help some airborne viruses survive and spread. Air that is too humid, above roughly 60–70%, feels clammy, hampers the body's evaporative cooling, and, above all, promotes mold growth, dust mites, and condensation. The band in between is where comfort, respiratory health, and building durability all sit.
Operationally, RH governs condensation and mold risk on cold surfaces, corrosion, the stability of stored materials from paper and timber to food and pharmaceuticals, and electrostatic discharge in electronics. It is a core control variable wherever moisture matters, which is almost everywhere.
Limits
Relative Humidity Limit Values
The published values, by averaging period; each links to the standard that sets it.
Design range, operating room
Maximum, most clinical spaces
Maximum, compounding areas
Maximum
- ASHRAE TC 9.9 60 %: to protect circuit boards from corrosion
Relative humidity is reported as a percentage (%). Guidance generally frames a comfortable, healthy indoor band rather than a single target, and the ranges in the limit table on this page reflect that; interpret RH against temperature, since the same percentage means different absolute moisture at different temperatures. The most useful reading is often not the room RH but the implied risk at the coldest surface. As a surface cools, local RH there rises toward 100%, and condensation and mold become likely even when the room reading looks moderate.
How it is measured
How Relative Humidity Is Measured
The workhorse technology is the capacitive polymer sensor: a thin hygroscopic film sits between electrodes, and its capacitance changes as it absorbs and releases water vapor. Capacitive sensors are compact, accurate across the mid-range, and almost universal in building and portable instruments; they are packaged with a temperature element, since RH cannot be interpreted without temperature. Their weaknesses are slow drift, saturation, and contamination in permanently damp or polluted air. Older resistive sensors and, for laboratory reference, chilled-mirror hygrometers (which measure dew point directly) also exist, but capacitive sensing dominates monitoring.
Built into Aethair PRO and Aethair IAQ2
Measured by the monitor's own sensors, calibrated before it ships: nothing to add.
An instrument on Thiamis12
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing relative humidity sensors
Mount the sensor in the occupied or monitored zone, away from direct moisture sources such as kettles, showers, and humidifiers, away from supply air, and away from cold external walls where a local cold spot would read artificially high. Because RH tracks temperature, the sensor should sit where it experiences the air people actually occupy, not tucked against glazing or in a still corner. For condensation and mold investigations, readings taken near the cold surface of concern are more revealing than room-center values.
Every device and option that measures Relative Humidity 14
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
Aethair PRO built-in sensors | Built-in sensors | Aethair PRO | |
 Aethair IAQ built-in sensors | Built-in sensors | Aethair IAQ | |
 Airmar · 110WX, 110WXS, 120WX… | Weather Stations | Thiamis | |
 Thermo Scientific · pDR-1500, ADR-1500 | Particulate & Dust Monitors | Thiamis | |
 Davis · Davis Wizard III, Davis Wizard II, Davis Monitor… | Weather Stations | Thiamis | |
 Met One · E-BAM, E-BAM Plus | Particulate & Dust Monitors | Thiamis | |
 Fluke Calibration · 1620A-H, 1620A-S | Heat Stress & Comfort | Thiamis | Relative HumidityAir Temperature |
 Lufft · WS100, WS200, WS300… | Weather Stations | Thiamis | |
 Gill Instruments · MaxiMet-GMX100, MaxiMet-GMX200, MaxiMet-GMX240… | Weather Stations | Thiamis | |
 AethLabs · MA200, MA300, MA350… | Specialty Air Analyzers | Thiamis | |
 Particles Plus · Particles Plus 5301, Particles Plus 5301-AQM1, Particles Plus 5301-AQM2… | Particulate & Dust Monitors | Thiamis | |
 TSI · QuestTemp-32, QuestTemp-34, QuestTemp-36… | Heat Stress & Comfort | Thiamis | |
 Durridge · RAD7 | Specialty Air Analyzers | Thiamis | |
 R.M. Young · 93000, 93500, 92000… | Weather Stations | Thiamis |
Where it is monitored
Solutions That Monitor Relative Humidity
Each one links to the solution page showing what that industry measures it for.
Agriculture & Irrigation
Core parameterRelative humidity is central to plant-disease and frost forecasting, since fungal pathogens and dew formation depend on it.
Cleanrooms, Labs & Manufacturing
Core parameterRelative humidity is tightly controlled in cleanrooms and laboratories to protect products, prevent static and condensation, and keep processes reproducible.
Healthcare Air Monitoring
Core parameterRelative humidity is tightly controlled in healthcare because too little raises static and infection risk in operating rooms, while too much promotes mold and microbial growth in patient rooms and storage areas.
HVAC & Building Management
Core parameterRelative humidity is a core HVAC performance and health parameter, governing comfort, condensation risk on cold surfaces, and the mold that follows poor moisture control.
Indoor Air Quality
Core parameterRelative humidity shapes both comfort and health indoors: too low dries airways and raises static and infection risk, while too high drives mold, dust mites, and condensation.
School Air Monitoring
Core parameterRelative humidity shapes both comfort and health in schools: dry winter air irritates airways, while damp rooms invite mold in aging buildings.
Weather Stations & Meteorology
Core parameterRelative humidity is a core weather-station parameter, governing dew, fog, comfort, and the moisture available for precipitation.
Heat Stress & Outdoor Work
Often addedWBGT for worksites, athletics, and events
Outdoor Monitoring
Often addedContinuous outdoor air quality networks that sit alongside reference monitors
Article Library
Reading on Relative Humidity
ASHRAE 170 Requirements and Which Edition Applies to You
ASHRAE 170 requirements explained: what the standard sets, which edition your facility is held to, what changed in 2025, and how each value gets verified.
Hospital Air Quality Monitoring: Standards by Space
Hospital air quality standards from ASHRAE 170 and The Joint Commission explained, plus what to measure in operating rooms, isolation rooms, and labs.
Workplace Air Quality Monitoring for Offices and Employers
How to monitor workplace air quality in offices, what to measure for employee health and focus, and how continuous connected monitoring documents the result.
Indoor Air Quality in Schools: Standards & Monitoring 2026
Indoor air quality in schools: which standards apply, what to monitor, and how CO2 and ventilation affect student attendance, health, and test scores.
IAQ Monitoring Parameters: PM2.5, CO2, VOCs & Formaldehyde
Indoor air quality monitoring parameters explained: PM2.5, CO2, VOCs, and formaldehyde, plus the sensors, thresholds, and OSHA, ASHRAE 62.1, and WELL limits.
Commercial Building IAQ Monitoring: ASHRAE 62.1 & WELL
Indoor air quality monitoring for commercial buildings: what to measure, which standards apply, and how property managers build a program tenants trust.
Related
More in Meteorology
Questions
Relative Humidity FAQ
What is a healthy indoor humidity level?
Most guidance points to a mid-range band that avoids both the dryness problems below it and the mold and condensation problems above it, rather than a single ideal figure. Read the band in the limit table on this page together with temperature, and remember the comfortable range shifts a little with the season.
Why does high humidity cause mold and condensation?
When humid air meets a surface cooler than its dew point, water vapor condenses onto that surface, and sustained dampness lets mold, mildew, and dust mites take hold. High room humidity raises the surface temperature at which this happens, so cold spots, thermal bridges, and poorly ventilated corners are affected first.
Does relative humidity depend on temperature?
Yes. It is defined relative to how much moisture the air could hold at its current temperature, and that capacity rises steeply with warmth. So the same water-vapor content reads as a lower RH when the air is heated and a higher RH when it is cooled, which is why dew point is used when an absolute moisture measure is needed.
How do I monitor humidity?
The Aethair family (Aethair PRO and Aethair IAQ) has a built-in capacitive humidity sensor paired with temperature, so it reports RH, dew point, and heat index together for indoor comfort and mold-risk monitoring. Use the configurator to combine humidity with temperature, CO₂, and particulates in a full healthy-building configuration.
Monitor Relative Humidity on the Record
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