Relative Humidity, Dewpoint, Mixing Ratio… The Many Faces of Water Vapor WWWater is known by differferferent names in differferferent states. It can be measured in many ways and described with various terms. The following article explains the behavior of water vapor in air and clarifies the terminology used to describe it.
It is said that a beloved child volume of dry air is oxygen and pressure, so the tea water would this air enters a building it is has many names. This also around 1% is typically argon. boil at well below 70 °C. heated to +21 °C, but the applies to water, including amount of water remains water in gaseous form, which is Water vapor pressure Pw Relative humidity RH (%) constant – no water is removed the source of all life on our (hPa, PSI, Pa, mbar, mmHg, Relative humidity is the most or added to the air in normal planet. Most of us have heard of inHg, mmH20 or inH2O) commonly used humidity unit. ventilation systems. Because of relative humidity and dewpoint The air temperature dictates ‘Relatively’ few people, however, this heating, the saturation temperature, but there are the maximum partial water understand what it means. pressure of the water vapor rises many other ways to measure vapor pressure in air, in other The ‘relative’ in relative (i.e. the maximum possible the presence of water. Partial words, the water vapor humidity expresses the relation amount of water vapor in the water vapor pressure, absolute saturation pressure. The ability between the amount of water air), but the partial pressure of humidity, frostpoint, mixing of water to be gaseous form is vapor present and the the water vapor is unchanged. In ratio, wet bulb temperature and strongly dependent on its maximum amount that is this case, the relative humidity even enthalpy all describe the temperature (See Figure 1: physically possible at that will drop to 5%, which is usually humidity of a gas. Water vapor saturation pressure temperature. In other words, considered too dry for comfort. When the term humidity is curve). The higher the relative humidity, expressed in Temperature changes also used, we usually mean water temperature, the higher the per cent, is the partial water explain why we can sometimes vapor in a gas, typically air. partial pressure of the water vapor pressure in relation to the ‘see our breath’ outdoors. Moisture, on the other hand, is vapor. The partial water vapor saturation pressure. Consider what happens when used for liquids and solid pressure in the immediate we stand outside on a cool % RH= 100% * (p w / p ws ) materials. The term moisture presence of liquid water equals spring morning, at +7 °C and where: also applies to extremely dry the saturation pressure at that 80% RH. As we exhale air at P w = partial water vapor gases, when water vapor is specific temperature. about +32 °C and 90% RH, it pressure considered an impurity. cools rapidly, reaching the Pws = water vapor’s Figure 1. Water vapor saturation saturation point at around +30 saturation pressure Properties of gas mixtures pressure curve °C. As the cooling continues, A full understanding of the If the maximum amount of excess water vapor condenses various terms for humidity and water vapor has been reached into tiny water droplets, which moisture require some basic and more water is introduced we see as steam or mist. knowledge about the properties into the air, an equal amount of of gas mixtures. water must transform back to Dewpoint temperature
In a gas mixture such as liquid or solid form through Td (°C or °F) air, the total pressure (same as condensation. At this point, the This brings us to another widely atmospheric or barometric air is said to be saturated with used humidity unit: dewpoint pressure) of the gas is the sum water, and the relative humidity temperature (°C or °F). Dewpoint of all the individual pressures of At 100 °C, the boiling point is 100%. On the other end of the is the temperature where its gas components. The of water, the water vapor scale, when there is no water condensation begins, or where atmospheric pressure, usually pressure surpasses normal vapor in the air, the relative the relative humidity would be around 1000 hPa, is the total of atmospheric pressure. In this humidity is 0% whatever the 100% if the air was cooled. This the partial gas pressure of light, the boiling point of a temperature, In other words, is readily apparent from the nitrogen (~775 hPa), oxygen liquid is dependent not only on relative humidity always lies diagram for water vapor, given (~10 hPa), argon (~10 hPa), the physical properties of the between 0 and 100%. that dewpoint is just a more carbon dioxide (~0.4 hPa) and a liquid, but also on the As mentioned, the ability intelligible way to express partial number of other gases with surrounding atmospheric of air to hold water vapor is water vapor pressure (see Figure lower partial pressures. All pressure. If the mountain strongly dependent on 2: Dewpoint of gas at 42% RH). gases produce the same climber made himself a cup of temperature. This means that Even though dewpoint is pressure and volume with the tea on top of Mount Everest, the relative humidity is also expressed as a temperature, it same number of molecules, so taste would probably leave strongly temperature correlates with the amount of the partial pressures also something to be desired. The dependent. Let’s imagine that water vapor in the air, and is represent the proportion by atmospheric pressure at an the outside temperature on a therefore not dependent on volume of the various gases. altitude of 8,800 meters is only crisp winter day is -14 °C and ambient temperature. Dewpoint On this basis, 21% of the total about one-third the sea level the relative humidity is 60%. As temperature is always less than or equal to the actual glass. These small water processes the pressure must be Water activity aw temperature, with the extremes droplets are called dew. known in order to calculate Water activity can be defined for normal outdoor air being – If the temperature of the mixing ratio from other as the free moisture available in 30°C and +30°C. Dryer and drink is above the dewpoint humidity variables. material as opposed to the wetter gases can be found in temperature of air, the relative Mixing ratio is mainly used chemically bound moisture. In industrial environments, for humidity of the air surrounding for calculating water content simple terms, water activity is example, where dewpoints the glass will be higher than the when the mass flow of air is the equilibrium relative between –100 °C and +100 °C ambient humidity, but no visible known, for example, in humidity created by a sample are sometimes measured. condensation will occur. ventilation systems. of material in a sealed air space. Theoretically, the dewpoint Water activity is used in temperature can be as low as – Frostpoint Tf (C° or °F) Wet bulb temperature connection with moisture in oil
273 °C (absolute zero), but at a If the dewpoint temperature is Tw (°C or °F) measurements. It indicates normal atmospheric pressure it below the freezing point, the As water evaporates, it directly if there is a risk of free can never exceed 100 °C. When term frostpoint is sometimes consumes heat. This cooling water formation. With a relative the dewpoint is 100 °C, the air used. The water vapor effect depends on the ambient scale from 0 (no water present) only contains water vapor and saturation pressure of ice is temperature and the difference to 1 (the oil is saturated with no other gas, so the amount of slightly lower than that of water, between the water vapor water) it gives a reliable water cannot be raised without which must be taken into pressure of the ambient air and indication of how close the oil is increasing the density of the account when calculating the saturation pressure at that to the saturation point. The vapor, and hence the pressure. frostpoint. When frost actually temperature. By measuring the advantage of aw is that the forms on a surface, it always cooling effect, it is therefore measurement is independent Figure 2: occurs at the frostpoint, and not possible to determine the of oil type, age and Dewpoint of gas at 42% RH at the dewpoint temperature. ambient relative humidity. The temperature. cooling effect is measured with Absolute Humidity a a psychrometer, an instrument Available as calculated (g/m 3 or gr/ft3) with two thermometers, one of variable Absolute humidity refers to the which is covered by a wet cloth. Vaisala has a choice of products weight of water in a certain The reading of this for measuring relative humidity, volume of gas. For example, on thermometer is called the wet temperature and dewpoint. a typical summer day (+23°, bulb temperature. Wet bulb Some products also have built- 55% RH), there are 11.3 grams temperature can also be in calculation options to give of water vapor per cubic meter. calculated from the outputs in terms of other The density of air varies with temperature, pressure and humidity variables mentioned The water vapor saturation pressure, so the absolute relative humidity. in this article. For example pressure at different humidity depends quite Vaisala HUMICAP ® Humidity temperatures is a known strongly on the gas pressure. In Enthalpy h and Temperature Transmitter variable, so the dewpoint can be pressurized processes, the (kJ/kg or Btu/lb) Series HMT330 provides the calculated from the relative pressure must be known in Enthalpy is a unit expressing most flexible measurement humidity and temperature. order to calculate absolute the energy content of a gas. quantities for humidity. Conversely, if the dewpoint and humidity from the other Strictly speaking, it should not In addition to standard temperature or relative humidity humidity variables. be included in this article of relative humidity and are known, the missing variable Because absolute humidity humidity units. But as water temperature, the HMT330 can be calculated. provides a reliable idea of the vapor has a very high specific (depending on the model) A glass of beer or any cold amount of water present, it is heat capacity and can be outputs dewpoint temperature, drink provides a practical fairly widely used. present in air in widely different mixing ratio, absolute humidity, example of dewpoint. Since the concentrations, it has a very wet bulb temperature, enthalpy, glass conducts heat fairly well Mixing Ratio x strong influence on enthalpy. and water vapor pressure. compared to air, it cools to (g/kg or gr/lb) Enthalpy represents the almost the same temperature as Mixing ratio defines the weight amount of energy needed to Juhana Hakkanen, M.Cc. (Tech.) the drink. The air surrounding of water in the volume occupied bring a gas to its current from a Channel Manager the glass is then cooled, creating by one kilogram of dry gas. In dry gas at a temperature of 0 °C. Vaisala Tokyo a thin layer of air at nearly the other words, 9.6 grams of water Energy is consumed to vaporize Japan same temperature as the glass. would have to be vaporized into the water and raise the If the temperature of the drink is a kilogram of air to obtain the temperature of the humid gas. Paivi T. Laitinen, B. Sc. below the dewpoint same summer air as described Enthalpy is most commonly (Env. Eng.) temperature of the surrounding in the previous section. The used when comparing the heat Product Marketing Specialist air, the air around the glass will density of air varies with content of gases in air Vaisala Helsinki become saturated with water pressure, so the mixing ratio is conditioning and other systems. Finland and the excess water will also dependent on the pressure condense on the surface of the of the gas. In pressurized