Humidity

Humidity is the amount of water vapor in the air. If there is a lot of water vapor in the air, the humidity will be high. The higher the humidity, the wetter it feels outside (1). The higher the temperature, the more water vapor the air can hold (2).

Where Does Humidity Come From?

Humidity is a natural part of our atmosphere, it comes from the amount of water vapor in the air. Water vapor enters the atmosphere by evaporating from the large bodies of water on the Earth’s surface including lakes, oceans, and seas. It is an integral part of the water cycle, as water vapor is continuously generated by evaporation and removed by condensation. When the temperature is higher, the air can hold more water vapor, meaning that the warmer the climate, the higher the humidity level can be (2).

 Types of Humidity

1. Absolute humidity

Absolute humidity is the amount of water vapor divided by the amount of dry air in a certain volume of air at a particular temperature. The hotter the air is, the more water vapor it can hold (3).

2. Relative humidity

Relative humidity (expressed as a percent) is the ratio of the current absolute humidity to the highest possible absolute humidity, which will depend upon the current air temperature. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a higher relative humidity if the air is cooler, and a lower relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air. Relative humidity is the term weather forecasters use most often. A relative humidity of 100% means that the air can't hold any more water vapor. It's totally saturated. When this occurs, it can rain. In fact, the relative humidity must be 100% where clouds are forming for it to rain. However, at ground level where the rain lands, the relative humidity can be less than 100% (3, 4).

3. Specific Humidity

Specific humidity definition is the ratio of the total mass of the water vapor in the atmosphere to the total mass of the air parcel. In other words, it refers to the weight (amount) of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of air). The specific humidity is an extremely useful quantity in meteorology. For example, the rate of evaporation of water from any surface is directly proportional to the specific humidity difference between the surface and the adjoining air. Furthermore, the specific humidity does not vary as the temperature or pressure of a body of air changes, as long as moisture is not added to or taken away from it. (Certain other indices of humidity are sensitive to the pressure and temperature.) This stability of the specific humidity makes it useful as an identifying property of a moving air mass. The specific humidity of saturated air (i.e., that in equilibrium with a vapor source, either water or ice) increases rapidly with increasing temperature (4, 5).

How Temperature Affects Indoor Humidity Levels

Colder climates often have lower humidity levels than warmer climates as colder air holds less moisture than warm air. In winter, humidity levels tend to be typically lower. Whereas in summer, humidity levels will be higher, as air can hold more water vapor at a higher temperature (2).

Humidity and Human Health Impacts

Too much or too little humidity can be dangerous. For example, high humidity combined with hot temperatures is a combination that can be a health risk, especially for the very young and the very old (3).

Humans are sensitive to changes in humidity, because our skin uses the air around us to get rid of moisture in the form of sweat. If the relative humidity is very high, the air is already saturated with water vapor and our sweat won't evaporate. When this happens, we feel hotter than the actual temperature. Likewise, very low humidity can make us feel cooler than the actual temperature. This happens because the dry air helps sweat evaporate more quickly than usual. High humidity can have a number of adverse effects on the human body. It can contribute to feelings of low energy and lethargy. In addition, high humidity can cause hyperthermia—over-heating as a result of our body’s inability to effectively let out heat (3, 6). Hyperthermia can cause (6):

• Dehydration
• Fatigue
• Muscle cramps
• Heat exhaustion
• Fainting
• Heat stroke

What is an uncomfortable level of humidity?

Most people are comfortable with a relative humidity level of 30-50 percent. Higher levels are uncomfortable because there's too much moisture in the air, making people sweaty and unable to cool down (7).

References

1. https://education.nationalgeographic.org/resource/humidity
2. https://www.airthings.com/en/what-is-humidity
3. https://wonderopolis.org/wonder/what-is-humidity
4. https://www.weather.gov/lmk/humidity
5. https://www.britannica.com/science/specific-humidity
6. https://www.nbcnews.com/health/health-news/it-s-not-just-heat-it-really-humidity-know-risks-n629486
7. https://science.howstuffworks.com/nature/climate-weather/atmospheric/question651.htm