What is Humidity ? Its Types and Effect on Temperature

Aug 10, 2022

Humidity

What is humidity? The word “humidity” is very commonly used by people on hot days, especially in coastal areas. Humidity definition is the amount of moisture or water vapor in the air. It has three types— relative humidity, absolute humidity, and specific humidity.

What is high humidity? Ever wondered why 30°C feels like 36°C in coastal areas? Well, because high humidity in the air prevents the human body from perspiring freely, because of which, it can not dissipate heat effectively. This makes you feel extremely hot even when the temperature might not be that high. This discussion aims to focus on the physics behind humidity, its applications, what causes humidity, and the factors governing it.

Types of Humidity

Absolute Humidity: Absolute humidity definition is the content of water in the air and is expressed in terms of grams per cubic meter or grams per kilogram. Its value in the atmosphere ranges from almost zero to around 30 gm per cubic meter. In mathematical terms, absolute humidity is expressed as the mass of water vapor in the atmosphere divided by the total volume of the air and the mixture of water. The value of absolute humidity changes relative to the temperature and pressure of air if the volume is not constant. In the equation, the water vapor mass per unit volume is also called volumetric humidity.

Relative Humidity: It is defined as the ratio of the water vapor’s partial pressure in the air-water mixture to the water’s equilibrium vapor pressure over a pure water’s flat surface at a particular temperature. Normally, it is expressed in terms of percentage. The higher the percentage, the more humid the air-water mixture.

The concept of relative humidity can be employed to determine the content of water vapor in the atmosphere. It is defined as the ratio of the amount of moisture in the atmosphere at a certain temperature to the maximum moisture content withstood by the atmosphere at the same temperature. During rainy seasons, relative humidity is equal to 100%.

Relative Humidity = actual vapor density / saturation vapor density * 100%

The relative humidity is an important metric employed in weather forecasting because it is an indicator of dew, fog, or precipitation. What is high humidity? On hot summer days, relative humidity is high, which increases the temperature. This is because the process of evaporation of perspiration from the skin is hindered due to the humidity.

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.

Effects of Humidity

Animals and Plant Life

Whether an animal or a plant can thrive in a particular environment is determined by its humidity. What causes humidity? When the moisture levels of the air are high, perspiration is hindered. High humidity makes our skin feel hot and sticky. In such situations, the temperature of the body depends on increased blood circulation, excessive sweating, and respiration.

Electronics

Electronic devices are made to function under a particular humidity level. When the humidity is high, it may increase the conductivity in permeable insulators, causing them to malfunction. If the humidity is too low, the materials may become brittle. Regardless of the stated humidity range, condensation is another significant factor leading to damage to electronic devices. If an electronic device is moved from a cold temperature area to a high-temperature area, its circuit boards and other insulating materials undergo condensation. This results in a short circuit. Low humidity also causes the build-up of static electricity, causing the computers to shut down with discharge.

Industry

If the humidity is high, the capacity of chemical plants is affected adversely as furnaces are used during certain processes in these plants.

Unit of Humidity

Humidity does not have an official SI unit for measurement. Depending on certain factors, it is expressed in terms of different units.

Origin of Water Vapor

Consider a closed chamber filled with water, as illustrated in the figure above. The average kinetic energy and the impulse of the water molecules depends on the temperature. Certain water particles reveal higher energy and are able to exit the liquid by the process of evaporation. After some time, an equilibrium point occurs at which the number of water molecules exiting the water surface is the same as the number of molecules rejoining it. When this point is reached, the total number of water particles in the gaseous phase remains constant.

Saturated Vapor Pressure

The wall of the closed chamber is bombarded by the gaseous water molecules, applying pressure on it. At the equilibrium point, this pressure is called saturated vapor pressure.

The figure above shows the saturated vapor pressure above the surface of the water. The Magnus formula is used to calculate the saturated vapor pressure above ice ei (t). The parameters given in the table below are used in the calculation:

Condition	Temperature range in •C	á (hPa)	â	ã (•C)
Above water	-40 to – 60	6.112	17.62	243.12
Above ice	-80 to – 0.01	6.112	22.46	272.62

Here, t is the temperature in •C and the rest of the parameters are given in the table above.

Between the temperature range of -45°C and 60°C, there is an uncertainty below ±0.6% at a 95% confidence level in the Magnus formula. If we need more precision, absolute functions result from the equations for saturated vapor pressure that can only be calculated numerically.

Water Vapor Enhancement Factor

Air often contains additional gasses or mixtures of gasses. Therefore, the total barometric pressure is the sum of the individual partial pressures. Since the different gas particles in air interact, a pressure-dependent correction may be required to change the vapor pressure. The total saturated vapor pressure is given by,

e’w = ew ⋅ fw or e’i = ei ⋅ fi

where the f, the water enhancement factor can be given by,

F_w(p) = 1.0016 + 3.15.10^-6.p – 0.074 . 1/p

The barometric pressure or p can vary between 3kPa and 110kPa. It can be assumed that at standard barometric pressure, f = 1. The enhancement factor at 6bar is 1.02.

Heat Index

The heat index stipulates how the human body perceives temperature. If there is high relative humidity, the rate of evaporation from the human body is decreased because of which heat is not radiated from the body as easily as it does when the air is dry. In case of low relative humidity, the rate of perspiration is high, so the body dissipates heat and cools itself. Depending on the subjective measurements, the heat index is only significant if it is above 24•C and 40% RH. The heat index is given by:

HI = c00 + c10 t + c01 Uw + c11 tUw+ c20 t2 + c02U2w + c21t2Uw + c12tU2w + c22t2U2w

The heat index depends upon the humidity and relative temperature, as depicted by the picture above. The significance of the values is given below:

< 30°C: not uncomfortable
30 – 40°C: little uncomfortable
40 – 45°C: very uncomfortable
> 45°C: life-threatening
> 54°C: impending stroke

Applications

Psychrometric Chart

Also called Mollier diagrams, psychrometric charts reveal aequi-potential enthalpy curves corresponding to the relative humidity on charts of mixing ratio and temperature. They help in navigating systems in which humidity and temperature play a big role, for instance, a tumble dryer.

Wet and dry bulb temperatures

The figure above shows a wet and dry bulb hygrometer or a psychrometer. This instrument is used to calculate lower temperatures than those calculated by the dry bulb thermometer because of the evaporation at its bulb. As discussed already, the rate of evaporation depends on relative humidity. A porous medium encloses one of the thermometers, which is kept wet by a wet sock, whose temperature decreases on the evaporation of water. As the evaporation rate is higher when the air is dry as compared to when it is wet, the temperature difference is a measure of relative humidity.

Conclusion

This discussion focused on the physics behind what is humidity and what causes humidity, describing its most important definitions and mechanisms about it. All the important equations related to the concept have also been mentioned. Hope it provides some physical framework to help you calculate most physical values pertaining to water vapor.

Frequently Asked Questions (FAQ’s)

1. What is pressure?

Pressure can be defined as the force applied at right angles to the surface of an object or a physical body per unit area over which the force is subjected.

2. What is hydrostatic pressure?

Hydrostatic pressure can be defined as the pressure exerted by water or any other liquid at any point of equilibrium because of the gravitational force. It is directly proportional to the depth of the liquid measured from its surface as the weight of the liquid goes up when a force acting downwards is exerted on it.

3.What is the dew point temperature?

Dew point temperature can be defined as the temperature at which an amount of air is cooled down so that condensation takes place at a constant pressure.