RH

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The relative humidity (RH) is the ratio between the partial pressure of water vapor and the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor produces a higher relative humidity in cold air than in warm air. A related parameter is the dew point.

Definition

Relative humidity ${displaystyle (RH}$ or ${displaystyle phi)}$ of a mixture of air and water is defined as the ratio of partial water vapor pressure ${displaystyle (p_{mathrm {H_{2}O} })}$ in mixture to water balance steam pressure ${displaystyle (p_{mathrm {H_{2}O} }^{*})}$ on a flat surface of pure water at a given temperature:

{displaystyle phi ={frac {p_{mathrm {H_{2}O} }}{p_{mathrm {H_{2}O} }^{*}}}.}

Relative humidity is usually expressed as a percentage; a higher percentage means that the air-water mixture is more humid. At 100% relative humidity, the air is saturated and at its dew point.

Meaning

Climate control

Climate control refers to the control of temperature and relative humidity in buildings, vehicles and other enclosed spaces in order to provide human comfort, health and safety, and to meet environmental requirements of machines, sensitive materials (for example, historical) and technical processes.

Relative humidity and thermal comfort

Along with air temperature, mean radiant temperature, air velocity, metabolic rate, and clothing level, relative humidity plays a role in human thermal comfort. In accordance with ASHRAE Standard 55-2017: Thermal Environmental Conditions for Human Occupancy, indoor thermal comfort can be achieved through the PMV method with relative humidities ranging from 0% to 100%, depending on the levels of other factors that contribute to thermal comfort. However, the recommended range of indoor relative humidity in air-conditioned buildings is generally 30-60%.

In general, higher temperatures will require lower relative humidities to achieve thermal comfort compared to lower temperatures, all other factors being held constant. For example, with clothing level = 1, metabolic rate = 1.1, and air velocity 0.1 m/s, a change in air temperature and mean radiant temperature from 20°C to 24°C would lower the relative humidity maximum acceptable from 100% to 65% to maintain thermal comfort conditions. The CBE Thermal Comfort Tool can be used to demonstrate the effect of relative humidity on specific thermal comfort conditions and can be used to demonstrate compliance with ASHRAE Standard 55-2017. When using the adaptive model to predict comfort indoors, relative humidity is not taken into account.

Although relative humidity is an important factor in thermal comfort, humans are more sensitive to variations in temperature than to changes in relative humidity. Relative humidity has a small effect on thermal comfort in the outside when air temperatures are low, a slightly more pronounced effect at moderate air temperatures, and a much greater influence at higher air temperatures.

Human discomfort caused by low relative humidity

In cold climates, the outside temperature causes a lower capacity for water vapor flow. So even though it may be snowing and the relative humidity outside is high, once air enters a building and warms up, its new relative humidity is very low, making the air very dry, which which can cause discomfort. Dry cracked skin can result from dry air.

Low humidity causes nasal passages lining tissue to dry out, crack, and become more susceptible to penetration by rhinovirus cold viruses. Low humidity is a common cause of nosebleeds. Using a humidifier in homes, especially bedrooms, can help with these symptoms.

Indoor relative humidities should be maintained 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, problems respiratory and aggravate allergies in some people. In the winter, it is recommended to keep the relative humidity at 30% or higher. Extremely low relative humidities (below 20%) can also cause eye irritation.

Buildings

For climate control in buildings using HVAC systems, the key is to keep the relative humidity in a comfortable range, low enough to be comfortable but high enough to avoid problems associated with very dry air.

When the temperature is high and the relative humidity is low, the evaporation of water is rapid; the ground dries up, wet clothes are hung on a line or dried quickly, and perspiration easily evaporates from the skin. Wood furniture can shrink, causing the paint that covers these surfaces to crack.

When the temperature is low and the relative humidity is high, the evaporation of water is slow. When relative humidity approaches 100 percent, condensation can form on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration. Condensation can pose a safety risk, as it can promote mold growth and wood rot, as well as potentially freezing emergency exits.

Certain production and technical processes and treatments in factories, laboratories, hospitals, and other facilities require specific levels of relative humidity to be maintained by humidifiers, dehumidifiers, and associated control systems.

Vehicles

The basic principles for buildings, above, also apply to vehicles. Additionally, there may be security considerations. For example, high humidity inside a vehicle can cause condensation problems, such as fogging up windshields and shorting out electrical components. In pressure vessels and vehicles such as pressure aircraft, submersibles, and spacecraft, these considerations can be critical to safety, requiring complex environmental control systems that include pressure-maintaining equipment.

Aviation

Airliners operate with low internal relative humidity, often below 10%, especially on long flights. The low humidity is a consequence of inhaling very cold air with low absolute humidity, which is found at the aircraft's cruising altitudes. Subsequent heating of this air lowers its relative humidity. This causes discomfort such as sore eyes, dry skin, and dry mucosa, but humidifiers are not used to raise it to comfortable mid-range levels because the volume of water needed to carry on board can be a significant penalty. of weight. As airliners descend from cooler altitudes into warmer air (perhaps even flying through clouds a few thousand feet above the ground), the relative humidity of the environment can increase dramatically. A portion of this moist air is generally drawn into the pressurized aircraft cabin and other non-pressurized areas of the aircraft and condenses on the cool aircraft deck. Liquid water can usually be seen running along the skin of the aircraft, both inside and outside the cabin. Due to drastic changes in relative humidity inside the vehicle, components must be rated to operate in those environments. Recommended environmental ratings for most commercial aircraft components are listed in RTCA DO-160.

Cold, moist air can promote icing, which is a hazard to aircraft by affecting the wing profile and increasing weight. Carburetor engines have a greater danger of ice forming inside the carburetor. Aviation weather reports (METARs), therefore, include an indication of relative humidity, usually in the form of dew point.

Pilots should take humidity into account when calculating takeoff distances, as high humidity requires longer runways and will decrease climb performance.

The density altitude is the altitude relative to standard atmospheric conditions (International Standard Atmosphere) at which the air density would be equal to the indicated air density at the place of observation, or, in other words, the height measured in terms of air density rather than distance from the ground. "Altitude density" is the set pressure altitude for a non-standard temperature.

An increase in temperature and, to a much lesser degree, humidity, will cause an increase in density altitude. Therefore, in hot and humid conditions, the density altitude at a particular location can be significantly higher than the actual altitude.

Measurement

A hygrometer is a device used to measure the humidity of the air.

The humidity of a mixture of air and water vapor is determined by the use of psychrometric tables if both the dry bulb temperature (T) and the wet bulb temperature (T_w) of the mixture. These quantities are easily estimated by using a slingshot psychrometer.

There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. Antoine's equation is among the least complex of these, with only three parameters (A, B, and C). Other formulas, such as the Goff-Gratch equation and the Magnus-Tetens approximation, are more complicated but yield higher precision.

The Arden Buck equation is commonly found in the literature on this topic:

{displaystyle e_{w}^{*}=(1.0007+3.46times 10^{-6}P)times 6.1121,e^{17.502T/(240.97+T)},}

where $T$ is the temperature of the dry bulb expressed in degrees Celsius (°C), $P$ is the absolute pressure expressed in millibars, and ${displaystyle e_{w}^{*}}$ is the balance Vapor pressure expressed in millibars. Buck has reported that the maximum relative error is less than 0.20% between −20 °C and +50 °C when this particular form of the generalized formula is used to estimate the water balance steam pressure.

Water vapor is independent of air

The notion that air "holds" water vapor or is "saturated" it is often mentioned in connection with the concept of relative humidity. However, this is tricky: the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. In fact, a vacuum has about the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature. There is a very small difference which is described in "Improvement Factor" below, which can be ignored in many calculations unless high precision is required.

Dependency of pressure

The relative humidity of an air-water system depends not only on the temperature but also on the absolute pressure of the system of interest. This dependence is demonstrated by considering the air-water system shown below. The system is closed (ie, it doesn't matter if you enter or leave the system).

If the system in state A is heated isobarically (heating with no change in system pressure), then the relative humidity of the system decreases because the equilibrium vapor pressure of water increases with increasing temperature. This is shown in state B.

If the system in State A is isothermally compressed (compressed with no change in system temperature), then the relative humidity of the system increases because the partial pressure of water in the system increases with reduction in volume. This is shown in State C. Above 202.64 kPa the RH would be greater than 100% and water could begin to condense.

If the pressure of State A were changed simply by adding more dry air, without changing the volume, the relative humidity would not change.

Therefore, a change in relative humidity can be explained by a change in the temperature of the system, a change in the volume of the system, or a change in both properties of the system.

Improvement Factor

The improvement factor ${displaystyle (f_{w})}$ is defined as the ratio of water saturated steam pressure in wet air ${displaystyle (e'_{w})}$ to the saturated steam pressure of pure water:

{displaystyle f_{W}={frac {e'_{w}}{e_{w}^{*}}}.}

The improvement factor is equal to unity for ideal gas systems. However, in real systems, interaction effects between gas molecules result in a small increase in the equilibrium vapor pressure of water in air relative to the equilibrium vapor pressure of pure water vapor. Therefore, the improvement factor is normally slightly greater than unity for real systems.

The improvement factor is commonly used to correct for the equilibrium vapor pressure of water vapor when using empirical relationships, such as those developed by Wexler, Goff, and Gratch, to estimate the properties of psychrometric systems.

Buck has reported that, at sea level, the vapor pressure of water in saturated moist air is equivalent to an increase of about 0.5% over the equilibrium vapor pressure of pure water.

Related concepts

The term relative humidity is reserved for water vapor systems in the air. The term relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.

Other important facts

A gas in this context is called saturated when the vapor pressure of water in air is at the equilibrium vapor pressure for water vapor at the temperature of the gas-water vapor mixture; Liquid water (and ice, at the right temperature) will not lose mass through evaporation when exposed to saturated air. It can also correspond to the possibility of dew or fog formation, within a space that lacks temperature differences between its portions, for example, in response to the decrease in temperature. Fog consists of very minute liquid droplets, held mainly in the air by isostatic motion (in other words, the droplets fall through the air at maximum velocity, but because they are very small, this terminal velocity is also very small, therefore, they seem suspended).

The statement that relative humidity (RH%) can never be above 100%, while a good guide, is not entirely accurate, without a more sophisticated definition of humidity than what is provided here. Cloud formation, in which aerosol particles are activated to form cloud condensation nuclei, requires supersaturation of a parcel of air at a relative humidity slightly above 100%. A smaller-scale example is found in the Wilson cloud chamber in nuclear physics experiments, in which a state of supersaturation is induced to do its job.

For a given dew point and its corresponding absolute humidity, relative humidity will change inversely, though not linearly, with temperature. This is because the partial pressure of water increases with temperature, the operating principle behind everything from hair dryers to dehumidifiers.

Due to the increase in the potential for higher partial pressure of water vapor at higher air temperatures, the water content at sea level can be as much as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, leading to dry skin, itchy eyes and persistent static electrical charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outdoor air entering the interior increases its moisture capacity, lowering relative humidity and increasing evaporation rates from interior moist surfaces (including human bodies and domestic plants).

Similarly, during the summer in humid climates, a large amount of liquid water condenses from the air cooled in air conditioners. The warmer air cools below its dew point and excess water vapor condenses. This phenomenon is the same one that causes water droplets to form on the outside of a mug containing an ice-cold drink.

A rule of thumb is that the maximum absolute humidity doubles for every 20°F or 10°C rise in temperature. Therefore, relative humidity will decrease by a factor of 2 for every 20°F or 10°C increase in temperature, assuming conservation of absolute humidity. For example, in the normal temperature range, air at 68°F or 20°C and 50% relative humidity will become saturated if cooled to 50°F or 10°C, its dew point, and 41°F or 5 °C Air at 80% relative humidity heated to 68 °F or 20 °C will have a relative humidity of only 29% and feel dry. By comparison, the ASHRAE Thermal Comfort Standard 55 requires systems designed to control humidity to maintain a dew point of 16.8 °C (62.2 °F) although no lower humidity limit is established.

Water vapor is a lighter gas than other gaseous components of air at the same temperature, so moist air tends to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature for humans (and other animals) by preventing the evaporation of perspiration from the skin as relative humidity increases. This effect is calculated as the heat index or humidex.

A device used to measure humidity is called a hygrometer; The one used to regulate it is called a humidistat or sometimes a hygrostat. (These are analogous to a thermometer and a thermostat for temperature, respectively.)

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