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First of all, air has weight and takes up space. At sea level, the air pressure is 14.7 PSI due to the atmosphere, which consists of nitrogen, oxygen, carbon dioxide, argon, some more trace elements, and water vapor. The amount of water vapor in the atmosphere is variable based on different environmental conditions, and it depends a lot on temperature.
Relative humidity refers to the amount of humidity there currently is compared to how much humidity the air could possibly hold (100% RH or saturation). The point of 100% RH or saturation is also known as the dew point temperature. Once the air reaches saturation or the dew point temperature, it can no longer hold any more moisture. We keep evaporator coils cold so that the air reaches the dew point and has to drop some moisture as it cools; that moisture then drains out as liquid water.
It’s also worth noting that water vapor is lighter than air and travels up in the atmosphere; from there, it hits the dew point and forms a cloud.
Warmer air can hold more moisture before saturation is reached. Air can hold less moisture on a cold day, so you could be at 100% RH and still have less total moisture in the air than on a hot day. You can think about tea or coffee: high amounts of sugar won’t dissolve in cold tea or coffee, but more will dissolve when you heat the tea or coffee.
Humidity is a huge driver of comfort or indoor air quality. Higher humidity levels make it harder for us to dissipate heat through sweat; our bodies aren’t able to dissipate heat via evaporation as easily. The target relative humidity range from a comfort standpoint is typically between 30 and 60%. From an IAQ standpoint, it’s typically good to stay between 40 and 50%. Moisture buildup due to conversation in homes with high relative humidity can lead to microbial growth and present a health hazard.