Water vapor specific humidity

As seen in Fig. 9.15,a, during this month the maxima of the mean air temperature in CC/HBD and ICD occurred at noon and one hour earlier compared with the RD and non urban areas. Moreover, on a diurnal cycle, the maximum difference between air temperatures in mentioned areas was less than 0.8°C. The diurnal cycle of the mean water vapor specific humidity is shown in Fig. 9.15,b. As seen, the CC/HBD cells had the lowest values compared with all other cells. In opposite, for the non urban areas, the values are the highest. 

Figure 9.15 Diurnal cycle (in October) variability of the mean a) air temperature and b) water vapor specific humidity for the city center/high building district (CC/HBD), industrial commercial district (ICD), residential district (RD), and non urban areas of Copenhagen, Denmark (Mahura et al., 2005 [389]).

Humidity is defined as the concentration of water molecules in the atmosphere. In practice, parameters important in the measurement of humidity are partial pressure of water, mixing ratio, specific humidity, absolute humidity, mole fraction of water vapor, relative humidity (r. h.), and dew-point temperature. Of these measurements, relative humidity, which is the ratio of the actual water pressure to the saturated pressure, is widely used. The concentration of water molecules in the air is low and, moreover, the effects of water are very complicated not only chemically but also physically. The measurement of humidity is difficult compared to the measurement of temperature. In many industries from the electronic industry to agriculture, there is a demand for humidity control. For example, a dry atmosphere is required for the pro-... 

Humid volumes are given by the curves entitled Volume mVkg diy air. The volumes are plotted as func tions of absolute humidity and temperature. The difference between dry-air specific volume and humid-air volume at a given temperature is the volume of water vapor. 

Specific humidity, which is the weight of water vapor to the weight of dry air, is given by the following ratio ... 

Figure 28 shows the key features of the humidity chart. The chart consists of the following four parameters plotted as ordinates against temperature on the abscissas (1) Humidity H, as pounds of water per pound of dry air, for air of various relative humidities (2) Specific volume, as cubic feet of dry air per pound of dry air (3) Saturated volume in units of cubic feet of saturated mixture per pound of dry air and (4) latent heat of vaporization (r) in units of Btu per pound of water vaporized. The chart also shows plotted hiunid heat (s) as abscissa versus the humidity (H) as ordinates, and adiabatic humidification curves (i.e., humidity versus temperature). Figure 28 represents mixtures of dry air and water vapor, whereby the total pressure of the mixture is taken as normal barometric. Defining the actual pressure of the water vapor in the mixture as p (in units of mm of mercury), the pressure of the dry air is simply 760 - p. The molal ratio of water vapor to air is p/(760-p), and hence the mass ratio is ... 

In calculations with humid air, when the pressure is not high (usually the atmosphetic pressure of 1 bar), water vapor and dry air can be handled as an ideal gas, as we have already done in Eqs. (4.76) and (4.78). For ideal gases the specific enthalpy is just a function of tempetatute ... 

The dimensionless number Le is called the Lewis number (m Russian literature it is called the Luikov number). The Lewis number incorporates the specific heat capacity of humid air pCp (J/m C), the diffusion factor of water vapor in... 

Temperature rc) Humidity kg HjO/kg dry air) Water vapor partial pressure (kPa) Water v K>r partial density (kg/m ) Water vaporization heat M/kg) Mixture enthalpy (kj/kg dry air) Dry air partial density (lKinematic viscosity (I0< mJ/s) Specific heat (kJ/K kg) Heat conductivity (W/m K) Diffusion factor water air (1 O mJ/s) Temperature rc)... 

Equation (4.137) is almost exactly the same as the approximation equation (4.123) derived for wet bulb temperature. When the partial pressure of water vapor is low compared with the total pressure—in other words when the humidity x is low—the specific heat of humid air per kilogram of humid air, Cp, and the specific heat of humid air per kilogram of dry air, Cp, are al most the same Cp = Cp. Therefore, in a situation where the humidity is low and Le s 1, the thermodynamic wet bulb temperature is very nearly the same as the technical wet bulb temperature dy... 

Humidity, specific The mass of water vapor per unit mass of dry air. 

Specific humidity The mass of water vapor that is present in a unit mass of moist air. 

Specific humidity used in calculations on certain types of compressors is a totally different term. It is the ratio of the weight of water vapor to the weight of dry air and is usually expressed as pounds, or grains, of moisture per pound of dry air. Where pa is the partial air pressure, specific humidity can be calculated as ... 

A deliquescent material takes up moisture freely in an atmosphere with a relative humidity above a specific, well-defined critical point. That point for a given substance is defined as the critical relative humidity (RH0). Relative humidity (RH) is defined as the ratio of water vapor pressure in the atmosphere divided by water vapor pressure over pure water times 100% [RH = (PJP0) X 100%]. Once moisture is taken up by the material, a concentrated aqueous solution of the deliquescent solute is formed. The mathematical models used to describe the rate of moisture uptake involve both heat and mass transport. 

Measurement of relative humidity depends on the system used. Systems employing vacuum are usually evacuated prior to introduction of water vapor [29]. For cases in which there is not a gas-forming reaction occurring, measurement of total pressure in the system can be used as a measure of water vapor pressure. Systems in which air is not evacuated require specific measurement of water vapor pressure. (For the latter type of system, caution should be taken to assure that the relative humidity source is in close proximity to the solid, since the diffusion of water vapor through air to the solid is required to maintain a constant relative humidity in the immediate vicinity of the solid.) A wide variety of pressure measuring instrumentation is commercially available with varying accuracy, precision, and cost. 

Relative Humidity The ratio of the quantity of water vapor present in the air to the quantity which would saturate it at any specific temperature. 

Absolute humidity is usually expressed in grunts of water vapor per cubic meter or. in engineering practice, in grains per cubic foot. Because this measure ol atmospheric humidity is nol conservative with respect in adiabatic expansion or compression, it is noi commonly used hy mcieorologisis. As occasionally used in air-conditioning practice, absolute humidity refers to the number of grains of water vapor per pound of tuoisl air. which is dimensionally identical with the specific humidity t defined below I. 

The ideal humidity instrument would he n linear, wide-range pressure gage, specific to water vapor and employing a primary or fundamental measuring method. Such an instrument, although physically possible, would be cumbersome. Most humidity measurements arc made hy some secondary instrument which is responsive io humidity-related phenomena. 

The balance between conduction and diffusion still operates for a much larger isolated wet object, provided radiation is excluded. This is the basis of the wet bulb thermometer method for measuring humidity. The actual rate of evaporation now is not as simply determined and is influenced by wind. The wet bulb temperature is almost independent of wind condition, owing to a convenient accident. Heat conduction is a diffusion process, and the diffusion coefficient for water vapor in air (0.24 sq. cm./sec.) is numerically close to the diffusion coefficient of temperature in air (thermal conductivity/specific heat = 0.20 sq. cm./sec.). Hence, the exact way in which each molecular diffusion process merges into the more rapid eddy diffusion process is not important because no matter how complex the transition is, it must be quantitatively similar for the two processes. 

Atmospheric GCMs simulate the time evolution of various atmospheric fields (wind speed, temperature, surface pressure, and specific humidity), discretized over the globe, through the integration of the basic physical equations the hydrostatic equation of motion, the thermodynamic equation of state, the mass continuity equation, and a water vapor transport equation. To reproduce the... 

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