Relative humidity is the amount of water vapor in the air compared with the maximum it could hold at the same temperature, expressed as a percentage: 50 % means the air holds half the vapor it can carry before it saturates. Because warm air can hold much more vapor than cold air, relative humidity changes through the day even when the actual amount of water stays the same. Indoors, most guidance puts the comfortable range somewhere between 30 % and 60 %.
Key takeaways
- Relative humidity (RH) compares the water vapor in the air with the maximum possible at that temperature, and is given in %.
- Absolute humidity measures the actual vapor, in grams per cubic meter: saturated air at 20 °C (68 °F) holds about 17 g/m³.
- Outdoors, relative humidity usually peaks around sunrise and is lowest in early afternoon, the reverse of the temperature curve.
- The US Environmental Protection Agency (EPA) advises keeping indoor humidity below 60 %, ideally between 30 % and 50 %, to limit mold.
- Above 60–70 % for long periods, condensation, mold and dust mites increase; below 30 %, skin, eyes and airways dry out.
What is relative humidity in simple words?
In simple words, relative humidity tells you how close the air is to being completely full of water vapor at its current temperature. At 100 % the air is saturated: any extra vapor condenses into droplets, giving fog, dew or steamed-up windows.
The key word is "relative". The same air, with exactly the same amount of water, can show 40 % at midday and 90 % before dawn, only because it has cooled. So relative humidity tells you how near the air is to saturation, not how much water it carries.
Relative humidity formula
The relative humidity formula is RH = (e / es) × 100, where e is the actual vapor pressure and es the saturation vapor pressure at the air temperature. Saturation vapor pressure is usually calculated with the Magnus formula, which the WMO includes in its guide to meteorological instruments: es = 6.112 × exp(17.62 × T / (243.12 + T)), in hectopascals with T in °C.
Hygrometers and weather stations calculate it automatically, giving relative humidity in percent.
Relative humidity vs absolute humidity: what is the difference?
The difference is that absolute humidity measures the actual mass of water vapor, in grams per cubic meter of air, while relative humidity measures what share of the air's maximum capacity that vapor fills. Absolute humidity does not depend on temperature; relative humidity does.
This chart shows how much vapor one cubic meter of saturated air can hold at each temperature, calculated with the Magnus formula:
| Temperature | Maximum vapor (RH 100 %) | At RH 50 % |
|---|---|---|
| -10 °C (14 °F) | 2.4 g/m³ | 1.2 g/m³ |
| 0 °C (32 °F) | 4.8 g/m³ | 2.4 g/m³ |
| 10 °C (50 °F) | 9.4 g/m³ | 4.7 g/m³ |
| 20 °C (68 °F) | 17.3 g/m³ | 8.6 g/m³ |
| 25 °C (77 °F) | 23.0 g/m³ | 11.5 g/m³ |
| 30 °C (86 °F) | 30.3 g/m³ | 15.1 g/m³ |
| 35 °C (95 °F) | 39.5 g/m³ | 19.7 g/m³ |
The air's capacity nearly doubles every 10 °C (18 °F). A practical example: living room air at 20 °C (68 °F) and 50 % holds about 8.6 g/m³. If that same air drifts into an unheated bedroom at 10 °C (50 °F), its relative humidity climbs above 90 % without a single drop of water being added.
That also explains a common paradox: a foggy January morning in London at 100 % humidity has less water in the air than a summer afternoon in Phoenix at 30 %.
Relative vs specific humidity
Specific humidity is a close cousin of absolute humidity: the mass of water vapor per kilogram of air, in g/kg. Meteorologists prefer it because it does not change when air expands or compresses as it rises. For everyday purposes, the most useful measure of how much water the air really carries is the dew point, which does not depend on the current temperature.
What is relative humidity in weather forecasts?
In a weather forecast, relative humidity tells you how damp the air is near the ground and how likely fog, dew or frost are. It is measured between 1.25 and 2 meters (4 to 6.5 feet) above the ground in a ventilated shelter, like the temperature.
On a clear day, humidity draws a curve opposite to the thermometer: highest just before sunrise, when the air is coldest, and lowest in early afternoon, when the day's maximum temperature arrives. The vapor content barely changes between afternoon and night; what changes is the air's capacity to hold it. If humidity reaches 100 % overnight, dew, frost or fog forms, as the guide on fog vs mist explains.
Near the sea the daily swing is smaller, because water damps the temperature changes. Inland and in dry climates, it is normal to go from 90 % at dawn to 30 % in the afternoon. For comfort in summer, a high humidity matters because sweat evaporates more slowly, which is what the heat index captures; see our guide to the feels like temperature.
What is the ideal indoor humidity level?
The ideal indoor humidity is between about 30 % and 50 %, and below 60 % at all times, according to the US EPA's guidance on mold. Many health and building sources quote a slightly wider comfort band of 40 % to 60 %. Within that range most people feel comfortable, while mold, dust mites and dry air are all kept in check.
| Indoor humidity | What happens |
|---|---|
| Below 30 % | Dry skin, eyes and throat, static shocks, cracking wood |
| 30–50 % | Ideal range for most homes |
| 50–60 % | Acceptable; watch cold walls and windows |
| Above 60 % | Mold, dust mites and condensation become likely |
| Above 70 % for days | Mold growth and musty smells almost certain |
Ideal indoor humidity in winter
In winter, aim for the lower part of the range, around 30–40 %, especially in cold climates. The colder the outside, the colder your windows and outside walls, and the sooner indoor moisture condenses on them. That is why window condensation is such a common problem in UK, Irish and Canadian homes in winter.
Ideal indoor humidity in summer and with air conditioning
In summer, keep indoor humidity below 50–60 %. Air conditioning helps, because it removes moisture as it cools the air. In very humid climates, such as Houston or Miami, a dehumidifier may still be needed in basements, closets and rooms with poor airflow. Basements are prone to dampness everywhere, so keep them below 60 % too.
What happens if humidity in your house is too high or too low?
If humidity is too high, you get condensation and mold; if it is too low, your skin, eyes and airways dry out. These are the usual signs.
Humidity too high (above 60–70 % for long periods):
- Condensation on windows, frames and cold walls, especially on winter mornings.
- Mold spots in corners, behind wardrobes and in the bathroom.
- More dust mites, which thrive in damp air and worsen allergies.
- A clammy, cold feeling in winter and a sticky, heavy one in summer.
Humidity too low (below 30 %):
- Dry skin, lips, eyes and throat, and discomfort for contact lens wearers.
- Irritated nose and throat, common in winter.
- Static shocks from door handles and sweaters.
- Wooden furniture, floors and musical instruments that crack.
How to measure and fix humidity at home
To measure indoor humidity, you only need a hygrometer, a cheap device often combined with a thermometer. Place it away from windows, radiators, the kitchen and the bathroom, and check it at different times over several days before drawing conclusions.
To test whether your hygrometer is accurate, use the salt test: put it in a sealed container with a small pile of damp table salt, not dissolved. After a few hours the air inside settles at about 75 %; the difference from the reading is your device's error.
To lower humidity:
- Ventilate every day for about 10 minutes, ideally with windows open on opposite sides.
- Use extractor fans when cooking and showering, and keep the bathroom door closed.
- Dry laundry outdoors or in a ventilated room.
- Run a dehumidifier in the dampest rooms if the problem persists.
- Keep temperatures steady: very cold walls encourage condensation.
To raise humidity:
- Use a humidifier and clean it often so it does not grow bacteria.
- Do not overheat: the warmer you heat the air, the lower its relative humidity.
- Add houseplants or bowls of water near radiators as a small extra help.
How to check relative humidity with Clima de Hoy
On Clima de Hoy you can see current relative humidity and dew point on each city page, and how humidity changes hour by hour in the hourly view, next to the temperature. Compare the hourly forecast for London, where humidity stays high most of the day in winter, with the hourly forecast for Madrid, where the gap between dawn and afternoon is much larger.
It is also a handy way to pick the best time to air your home: in winter, the middle of the day usually has the driest outside air.
Frequently asked questions
What is a normal humidity level?
Outdoors it depends on place and time: on the coast it often stays high all day, while inland it drops a lot in the afternoon. Indoors, a level between 30 % and 50 % is considered healthy, and the EPA advises staying below 60 %. Readings of 100 % only occur with fog, rain or dew.
Is 70 % humidity too high in a house?
Yes, 70 % for days at a time is too high for a home. It encourages condensation on windows, mold and dust mites. Ventilate more, use extractor fans and, if that is not enough, a dehumidifier until the level falls below 60 %.
Why is humidity higher when it is cold?
Because cold air can hold much less water vapor than warm air, so the same amount of water brings it closer to saturation. Saturated air at 0 °C (32 °F) holds only about 4.8 g/m³, compared with about 17.3 g/m³ at 20 °C (68 °F). That is why relative humidity rises in winter and overnight even when the air is fairly dry.
Is relative humidity or dew point better for knowing if it will feel muggy?
Dew point is the better guide to mugginess, because it measures directly how much water the air holds. Humidity of 50 % at 35 °C (95 °F) feels oppressive, while 90 % at 10 °C (50 °F) does not. Once the dew point passes about 18 °C (65 °F), most people find the air sticky.
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