Especially with the advent of summer months, the phrase "the humidity is very high" begins to be frequently heard, and people complain as much about humidity as they do of heat. This is particularly a common occurrence in coastal regions. So, "What is humidity?"; and why do we say "The heat is fine, but the humidity is awful".
In short, humidity is the term used for water vapor suspended in the air. There are 3 separate humidity measurements in the HVAC industry(or terminology) which denote different concepts. These are:
Absolute humidity denotes the mass of water vapor per unit volume of air. Measured with the aid of devices called hygrometers, absolute humidity is a value which changes with climate and geography. Therefore it decreases as one moves from the equator to the poles, and from coastal regions to inland and highland areas.
Specific humidity denotes the ratio of the mass of water vapor in the air to the total mass of air. In other words, it is a simple mixture ratio. It is generally used to indicate how many grams of water vapor is contained by 1 kg of air. In the psychrometric chart shown below, the figures on the left indicate specific humidity.
Relative humidity is a ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that the air could hold at a temperature. 100% relative humidity means that the air is fully saturated with water and that it cannot hold any more water vapor. That is, any more water vapor added to the air condenses into a liquid form or is suspended in the air in the form of fog. Based on this, it can also be said that the odds of rainfall is higher with high relative humidity. The value that is sometimes expressed as humidity in weather forecasts is, in fact, relative humidity. In contrast with specific and absolute humidity, we people can sense relative humidity, which directly influences our comfort.
In the psychrometric chart above, the figures on the bottom represent dry bulb temperature, while the figures on the left represent specific humidity, and the blue lines relative humidity. As can be easily seen from the chart, keeping specific humidity, i.e. the mass of water vapor in the air constant, relative humidity drops as temperature rises. This is because, as mentioned above, the amount of vapor that can be held by air increases with increasing temperature.
The figure above also shows the same quantity of water vapor produces different relative humidity values within the same room at varying temperatures.
After briefly mentioning what humidity is, let us come to the effect of humidity on human comfort. Both high and low humidity have an unfavourable effect on human life. Temperature between 20-25°C and relative humidity between 40-60% can be said to be suitable conditions for people to live comfortably. Human life is adversely affected by air conditions that are outside these limits.
The human body is capable of reflexive heat rejection through sweating as the temperature rises. Sweat expelled from the pores in the body evaporates and mixes with the air. As it evaporates it pulls heat from the human body, which cools us down. However during summer months, and especially in coastal areas, since relative humidity is high, the air already holds enough water vapor that it becomes difficult for it to hold any more. The sweat issued by the human body to cool down evaporates more slowly than is needed, or does not evaporate at all. The heat that cannot be expelled leads to an increase in the temperature sensed by the human body.
In addition to this effect of high humidity, it is known to trigger illnesses such as asthma, heart failure, loss of sodium, COPD etc. Nevertheless, not only high humidity but also low humidity has unfavourable effects on human life. The low levels of moisture that can be held by the air (specific humidity) in cold weather can lead to dry atmospheric conditions. Dry air is known to cause dryness in skin, respiratory illnesses, nosebleeds and many other conditions.
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