Why it's the difference between sleeping comfortable or waking up with a dry nose!
HUMIDITY BY DEFINITION
The relative humidity (RH or ϕ) of an air–water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the equilibrium vapor pressure of water over a flat surface of pure water at a given temperature
Relative humidity is normally expressed as a percentage; a higher percentage means that the air–water mixture is more humid.
Humans are sensitive to high humidity because the human body uses evaporative cooling, enabled by perspiration, as the primary mechanism to rid itself of waste heat. Perspiration evaporates from the skin more slowly under humid conditions than under arid. Because humans perceive a low rate of heat transfer from the body to be equivalent to a higher air temperature, the body experiences greater distress of waste heat burden at high humidity than at lower humidity, given equal temperatures.
For example, if the air temperature is 24 °C (75 °F) and the relative humidity is 0%, then the air temperature feels like 21 °C (69 °F). If the relative humidity is 100% at the same air temperature, then it feels like 27 °C (80 °F). In other words, if the air is 24 °C (75 °F) and contains saturated water vapor, then the human body cools itself at the same rate as it would if it were 27 °C (80 °F) and at 20% relative humidity (an unstated baseline used in the heat index). The heat index and the humidex are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body.
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Low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of Rhinovirus cold viruses. Low humidity is a common cause of nosebleeds. The use of a humidifier in homes, especially bedrooms, can help with these symptoms.
Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out.
Humans can be comfortable within a wide range of humidities depending on the temperature—from 30% to 70%—but ideally between 50% and 60%. Very low humidity can create discomfort, respiratory problems, and aggravate allergies in some individuals. In the winter, it is advisable to maintain relative humidity at 30% or above. Extremely low (below 20%) relative humidities may also cause eye irritation.