Humidity

Humidity is the amount of water vapor in the air. Water vapor is the invisible gas form of water. It tells us how likely it is to see rain, dew, or fog.

The level of humidity changes with temperature and pressure. Cool air can hold less water vapor than warm air. For example, air at 8 °C (46 °F) may have 8 grams of water in each cubic meter, while air at 30 °C (86 °F) can hold up to 28 grams in the same space.

Global distribution of relative humidity at the surface averaged over the years 1981–2010 from the CHELSA-BIOCLIM+ data set

We measure humidity in three main ways: absolute, relative, and specific. Absolute humidity is how much water vapor is in a certain amount of air. Relative humidity shows how close the air is to holding the most water vapor it can at a given temperature, usually shown as a percentage. Specific humidity is the amount of water vapor compared to the total weight of the air.

Humidity is very important for living things. When animals sweat to stay cool, high humidity makes it harder for the sweat to evaporate, making us feel hotter. This effect can be found using a heat index or humidex table.

Definitions

Humidity is the amount of water vapor, or gaseous water, in the air. Water vapor is usually invisible to us. Humidity helps us understand when we might see rain, dew, or fog.

Absolute humidity

Absolute humidity measures the total amount of water vapor in a certain space of air. It looks at how much water vapor is there, no matter the temperature. Absolute humidity changes with temperature and pressure, making it less useful for some scientific work.

Relative humidity

Paranal Observatory on Cerro Paranal in the Atacama Desert is one of the driest places on Earth.

Relative humidity tells us how close the air is to holding the maximum water vapor it can at a certain temperature. If the air holds more water vapor, the relative humidity is higher. When relative humidity reaches 100%, the air is saturated, and water may start to condense into dew or fog.

Specific humidity

Specific humidity is the ratio of water vapor weight to the total weight of the air. It’s often used in heating and cooling system designs.

Related concepts

Relative humidity is special to water vapor in air. For other types of gases and air mixtures, we use the term relative saturation instead.

Measurement

A hygrothermograph for humidity and temperature recording

A tool used to check how much moisture is in the air is called a psychrometer or hygrometer. A humidistat is a switch that turns on on a humid day, and it is often used to control a humidifier or a dehumidifier.

We can find out how much water vapor is in the air by using special charts if we know the dry bulb temperature (T) and the wet bulb temperature (T_w) of the air. These temperatures can be found easily by using a sling psychrometer.

Hygrometer for domestic use, wet/dry psychrometer type

There are many ways to figure out how much water vapor can be in the air depending on the temperature. The Antoine equation is one of the simpler ways, with only three numbers to use. Other ways, like the Goff–Gratch equation and the Magnus–Tetens approximation, are more detailed but give better results.

People also use special tools to measure and control humidity. Very careful measurements can be made with tools like the gravimetric hygrometer, chilled mirror hygrometer, and electrolytic hygrometer. For quick and good measurements, many tools today use capacitance to check how humid it is.

We can also measure humidity from far away using satellites. These satellites can see how much water is in the lower part of the atmosphere, between 4 and 12 kilometers up. They use sensors that pick up infrared radiation, which is a type of light that water vapor gives off. This helps scientists watch changes in the weather and make weather forecasts.

Air density and volume

Main articles: Volume (thermodynamics), Density of air, and Ideal gas law

Humidity is connected to how water changes from liquid to gas and back again, which mostly depends on temperature. When we squeeze a gas that already has a lot of water in it, the space it takes up gets smaller, but some water turns back into liquid. This brings the humidity level back close to what it was before, so the total space the gas takes up is different from what we might expect.

When the temperature drops, some water in the air also turns into liquid, which changes the space the gas takes up too. Because of this, we can talk about the space the air would take up without the water vapor, or the space it would take up if it had as much water vapor as possible.

Air with water vapor is lighter than dry air. This is because a water molecule is lighter than molecules of nitrogen or oxygen, which make up most of the air. When we add water vapor to air, the number of air molecules goes down if we keep the temperature and pressure the same. This makes the air less dense, meaning there is less weight in each bit of space.

Pressure dependence

The amount of water vapor in the air, called humidity, depends on both temperature and pressure. When we heat air without changing its pressure, the humidity drops because warmer air can hold more water vapor. But if we squeeze the air without changing its temperature, the humidity goes up because the water vapor gets packed more tightly together.

Scientists use something called an "enhancement factor" to adjust for how air molecules interact. This factor helps them understand how much more water vapor real air can hold compared to pure water. In most cases, this factor is just a little bit more than one, meaning real air holds just a tiny bit more moisture than we might expect.

Effects

Climate control means keeping the temperature and how much water is in the air just right in buildings, cars, and other places we use. This helps everyone stay comfortable and safe, and it also helps machines and special items work properly.

Climate

Humidity is part of what we call climate, and it also changes other parts of the climate. The air’s humidity is affected by wind and rain.

The places with the most humidity are usually close to the equator and near water. Cities like Bangkok, Ho Chi Minh City, Kuala Lumpur, Hong Kong, Manila, Jakarta, Naha, Singapore, Kaohsiung and Taipei stay very humid because they’re near water and close to the equator.

Some places get extremely humid when it rains a lot and it’s warm, which can feel like being in a warm bath. Examples include Kolkata, Chennai and Kochi in India, and Lahore in Pakistan. The city of Sukkur in Pakistan, located on the Indus River, often has very uncomfortable humidity during the rainy season.

Hygrostat set to 50% relative humidity

When it’s very hot and humid, it can feel much hotter than the actual temperature. Places like Darwin, Houston, Miami, Osaka, Shanghai, Shenzhen and Tokyo can feel very hot and sticky in the summer.

Global climate

Animal and plant life

Humidity is very important for where animals and plants can live. It helps decide which plants grow in a forest or a desert, and which animals can survive there.

Our bodies cool down by sweating. When the air is humid, our sweat doesn’t evaporate as quickly, so we feel hotter. If the air is as warm as our skin, our bodies can’t cool down well, and we can get very tired quickly.

Plants and some pets need the right amount of humidity to stay healthy, especially when they are kept inside homes.

Human comfort

Even though humidity matters for how comfortable we feel, people notice changes in temperature more than changes in humidity. Humidity matters more when it’s hot outside.

Our bodies cool down best when we sweat and the sweat evaporates. When the air is humid, our sweat doesn’t evaporate as fast, so we feel hotter.

People can feel comfortable in a range of humidity levels, but when it’s very hot, lower humidity helps us feel better. For example, at higher temperatures, we need lower humidity to stay comfortable.

Some people have trouble breathing when the air is humid. This can be because of health issues or just how they feel. Very dry air can also make people uncomfortable and can hurt their noses and eyes. Using a humidifier at home can help keep the air at a good level.

Air conditioning not only cools the air but also lowers humidity, which helps people feel better. Heating cold air can make the indoor air very dry.

Human health

Too much moisture in buildings can create problems. It can lead to growth that can make it harder for some people to breathe. Keeping the air at the right humidity level is important for health.

Building construction

Average humidity around Australia year-round at 9 am 80–90% 30–40%

When building homes and other places, it’s important to keep the inside at the right temperature and humidity. Modern buildings are often sealed tightly, which can sometimes cause problems with moisture building up inside walls.

For buildings that heat and cool the air, like offices and homes, it’s important to keep the humidity at a comfortable level—not too high and not too low.

When it’s hot and dry outside, things like clothes dry quickly, and wood can shrink. When it’s cold and very humid, water can collect on surfaces and cause problems like mold.

Some places, like factories and hospitals, need special humidity levels to make sure everything works right.

Vehicles

The same ideas about keeping buildings comfortable also apply to cars and other vehicles. High humidity inside a car can cause problems like fog on the windows and damage to electrical parts.

Aviation

Airplanes fly with very dry air inside because the air they use is very cold and dry from high up in the sky. This can make people feel uncomfortable, but airplane designers can’t add more moisture because it would be too heavy.

When airplanes fly into warmer air, the air inside can become more humid. This can cause water to collect on the airplane’s surface.

Tillandsia usneoides in Tropical house, Royal Botanic Gardens, Kew. It is growing where the climate is warm enough and has a relatively high average humidity.

Pilots need to think about humidity when they plan how far they can fly and how much fuel they need.

Electronics

Electronic devices work best in certain humidity levels. Too much moisture can cause problems, and too little can make materials break. When electronic devices are moved from a cold place to a warm, humid place, water can collect inside and cause damage.

It’s best to let electronic devices adjust to the new air for a few hours before turning them on. In some cases, the device can tell you when it’s safe to use it.

Very dry air can cause static electricity, which can damage electronic devices. Places that keep lots of computers, like data centers, watch the humidity very carefully.

Industry

In factories and plants that use furnaces, high humidity can make it harder for them to work properly. This is because humid air has less oxygen, so machines need to take in more air to work the same way.

Baking

When baking, high humidity can make the air hold more moisture, which changes how fast food cooks. Too much humidity can make baked goods cook too fast or burn, while too little humidity can slow down baking.

Other important facts

When the air is full of water vapor (at 100% relative humidity), it reaches its dew point. This means the air can't evaporate more water or make more ice form.

Sometimes, the air can hold more than 100% water vapor, called supersaturated air. This is needed for clouds to form. Tiny particles help clouds and fog form. When air is warmer, it can hold more water vapor. This is why humid air feels heavier in the summer and why indoor air can feel dry in winter. Humidity affects how we feel temperature, making it feel hotter when it's humid. Tools like hygrometers measure humidity.

Absolute humidity of Earth's atmosphere at sea level in g/m³ (oz/cu. yd)
Temperature	Relative humidity
Temperature	0%	10%	20%	30%	40%	50%	60%	70%	80%	90%	100%
50 °C (122 °F)	0 (0)	8.3 (0.22)	16.6 (0.45)	24.9 (0.67)	33.2 (0.90)	41.5 (1.12)	49.8 (1.34)	58.1 (1.57)	66.4 (1.79)	74.7 (2.01)	83.0 (2.24)
45 °C (113 °F)	0 (0)	6.5 (0.18)	13.1 (0.35)	19.6 (0.53)	26.2 (0.71)	32.7 (0.88)	39.3 (1.06)	45.8 (1.24)	52.4 (1.41)	58.9 (1.59)	65.4 (1.76)
40 °C (104 °F)	0 (0)	5.1 (0.14)	10.2 (0.28)	15.3 (0.41)	20.5 (0.55)	25.6 (0.69)	30.7 (0.83)	35.8 (0.97)	40.9 (1.10)	46.0 (1.24)	51.1 (1.38)
35 °C (95 °F)	0 (0)	4.0 (0.11)	7.9 (0.21)	11.9 (0.32)	15.8 (0.43)	19.8 (0.53)	23.8 (0.64)	27.7 (0.75)	31.7 (0.85)	35.6 (0.96)	39.6 (1.07)
30 °C (86 °F)	0 (0)	3.0 (0.081)	6.1 (0.16)	9.1 (0.25)	12.1 (0.33)	15.2 (0.41)	18.2 (0.49)	21.3 (0.57)	24.3 (0.66)	27.3 (0.74)	30.4 (0.82)
25 °C (77 °F)	0 (0)	2.3 (0.062)	4.6 (0.12)	6.9 (0.19)	9.2 (0.25)	11.5 (0.31)	13.8 (0.37)	16.1 (0.43)	18.4 (0.50)	20.7 (0.56)	23.0 (0.62)
20 °C (68 °F)	0 (0)	1.7 (0.046)	3.5 (0.094)	5.2 (0.14)	6.9 (0.19)	8.7 (0.23)	10.4 (0.28)	12.1 (0.33)	13.8 (0.37)	15.6 (0.42)	17.3 (0.47)
15 °C (59 °F)	0 (0)	1.3 (0.035)	2.6 (0.070)	3.9 (0.11)	5.1 (0.14)	6.4 (0.17)	7.7 (0.21)	9.0 (0.24)	10.3 (0.28)	11.5 (0.31)	12.8 (0.35)
10 °C (50 °F)	0 (0)	0.9 (0.024)	1.9 (0.051)	2.8 (0.076)	3.8 (0.10)	4.7 (0.13)	5.6 (0.15)	6.6 (0.18)	7.5 (0.20)	8.5 (0.23)	9.4 (0.25)
5 °C (41 °F)	0 (0)	0.7 (0.019)	1.4 (0.038)	2.0 (0.054)	2.7 (0.073)	3.4 (0.092)	4.1 (0.11)	4.8 (0.13)	5.4 (0.15)	6.1 (0.16)	6.8 (0.18)
0 °C (32 °F)	0 (0)	0.5 (0.013)	1.0 (0.027)	1.5 (0.040)	1.9 (0.051)	2.4 (0.065)	2.9 (0.078)	3.4 (0.092)	3.9 (0.11)	4.4 (0.12)	4.8 (0.13)
−5 °C (23 °F)	0 (0)	0.3 (0.0081)	0.7 (0.019)	1.0 (0.027)	1.4 (0.038)	1.7 (0.046)	2.1 (0.057)	2.4 (0.065)	2.7 (0.073)	3.1 (0.084)	3.4 (0.092)
−10 °C (14 °F)	0 (0)	0.2 (0.0054)	0.5 (0.013)	0.7 (0.019)	0.9 (0.024)	1.2 (0.032)	1.4 (0.038)	1.6 (0.043)	1.9 (0.051)	2.1 (0.057)	2.3 (0.062)
−15 °C (5 °F)	0 (0)	0.2 (0.0054)	0.3 (0.0081)	0.5 (0.013)	0.6 (0.016)	0.8 (0.022)	1.0 (0.027)	1.1 (0.030)	1.3 (0.035)	1.5 (0.040)	1.6 (0.043)
−20 °C (−4 °F)	0 (0)	0.1 (0.0027)	0.2 (0.0054)	0.3 (0.0081)	0.4 (0.011)	0.4 (0.011)	0.5 (0.013)	0.6 (0.016)	0.7 (0.019)	0.8 (0.022)	0.9 (0.024)
−25 °C (−13 °F)	0 (0)	0.1 (0.0027)	0.1 (0.0027)	0.2 (0.0054)	0.2 (0.0054)	0.3 (0.0081)	0.3 (0.0081)	0.4 (0.011)	0.4 (0.011)	0.5 (0.013)	0.6 (0.016)