“It’s not the heat – it’s the humidity!” How many times have you heard (or even said) that? The fact is that humidity does affect the perceived temperature, which can ultimately affect cooling costs.
Heat, Humidity and the Human Body
The human body controls its temperature using thermal radiation and perspiration. As perspiration evaporates, the body cools. When the humidity levels are high, the body’s sweat production and evaporation are severely reduced. This reduction of sweat and its evaporation make the body more hot at the same ambient temperature with a lower humidity.
What is Humidity?
Humidity is the measure of the amount of water vapor content of air. Absolute humidity is the water content of the mixture of all of air’s constituent parts. Relative humidity (expressed in %) refers to the current humidity relative to the maximum possible humidity. The humidity is used to calculate the heat index by meteorologists; taking into account the reduced ability for the body to sweat. Reduced sweating, and evaporation of that sweat, impairs the body’s ability to self-cool.
> Absolute Humidity Calculation
The formula for determining the absolute humidity (AH) in the air is:
mw = the mass of dissolved water vapor
Vnet = volume of total moist air
The units of humidity are expressed as:
grams/cubic meter , and the maximum quantity is equal to 30 grams/cubic meter.
It is not unusual to use any mass:volume units (pounds/cubic foot in US). The % absolute humidity is the quantity as measured divided by the maximum possible of 30 grams/cubic meter.
> Relative Humidity Calculation
Used for weather forecasts and reporting, Relative Humidity (RH or ) is the measure of the water vapor present in the air at a given temperature relative to the amount of vapor the air is able to hold at that same temperature without condensing:
AH = p(H2O) ÷ p*(H2O) x 100
p(H2O) =partial pressure of water vapor in the mixture (at a given temperature)
p*(H2O) = saturation pressure of water at a temperature equal to the temperature of the gas mixture
On a global scale, humidity is measured by satellites, that detect water concentrations at altitudes between 4 and 12 kilometers (troposphere). Because water vapor absorbs and re-radiates the radiation in this band, these satellites sense the infrared radiation in the troposphere. Producing a water vapor image, the satellites are a key component of weather forecasting as climate changes are monitored.
Effects of High Humidity
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) recommends keeping relative humidity between 30% – 60% (ASHRAE, 1992; Thermal Environmental Conditions for Human Occupancy). At relative humidity greater than 60%, the ability to cool (through sweating, evaporation, and thermal conduction) is decreased, which makes us feel hotter than at lower humidity levels. When the relative humidity is below 30%, mucosal membranes in the sinuses dry out, increasing the potential for nosebleeds.
Seattle Mitsubishi Mr. Slim Split System Heat Pump reduces the relative humidity in your home in summer. In winter, the heating system adds moisture to the air in your home as necessary, to assure relative humidity levels that are most comfortable for your family.