Water vapor saturation mixing ratio

The mixing ratio of water vapor q is defined by q = Pw/P, where /0 is the density of water vapor and p is the density of dry air. When the mixing ratio exceeds the saturation mixing ratio Qs, the excess water vapor over Qs ordinarily gives rise to condensation in the form of rain or to subhmation in the form of snow. Latent heat of condensation or sublimation is liberated in the phase transition and becomes available to heat the air. The opposite of condensation is the evaporation that acts to cool the air. The rate of heating or cooling from this process is designated by Qc inEq. (17). 

In the troposphere, the mixing ratio of water vapor decreases with height due to the lowering of the saturation vapor pressure with decreasing temperature and condensation of the excess. The coldest temperatures between... 

FIGURE 1.3 U.S. Standard Atmosphere temperature and saturation water vapor mixing ratio at 45°N and at the equator. 

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-... 

The water vapor in air is a result of vaporization of water from the earth s surface. We can consider liquid water to be condensed gas. At any given time, a eertain number of molecules can escape the liquid from the surfaee to the surrounding air (we call it evaporation). Because of air motion (turbulent mixing and adveetion) there is no equilibrium, i. e., transfer of water moleeules from the air baek to the surface (we call it condensation) in the same flux as evaporation. Such equilibrium can only be reached in a elosed undisturbed ehamber. Hence the vapor pressure dependency shown in Fig. 2.37 is theoretical and caimot be directly apphed to the atmosphere. If the equilibrium between eondensed and vaporous water is reaehed, the pressure is called saturation pressure p°°. Sueh eonditions are important for cloud formation but are also frequently observed in the tropics. The relative humidity RH is the ratio of the vapor pressure e) at temperature T to the saturation vapor pressure at the same temperature expressed as a percentage. 

Two factors account for the observed decrease in the concentration of water vapor (termed the water vapor mixing ratio, grams of water vapor per gram of air) with height. First, the earth s surface is the primary somce of water vapor. Second, the air temperature decreases with height in the troposphere. Since the maximum possible mixing ratio, termed saturation, decreases with decreasing temperature, water is squeezed out as parcels ascend in the atmosphere. 

Humidity is a parameter that helps describe the water content in a gas and water mixture. There are many different methods to express humidity absolute humidity, RH, dew point temperature, or mixing ratio. In PEM fuel cell technology, RH is commonly used. The RH is a measure of the amount of water vapor in the air relative to the maximum amount of water vapor that air can hold at a specific temperature, and is usually expressed as a percentage value. The RH at temperature T (RH(T)) is defined as the ratio of the partial pressure and the saturated vapor pressure of water, and can be expressed as in Eqn (8.1) [12] ... 

A lower bound on final pressure can be found by balancing the chemical reaction for the two tanks considered as one system. That is, the contents of bodi tanks are assumed to be placed in one vessel, fully mixed, and burned. The final pressure is found by multiplying the initial pressure by the ratio of die final number of mols in the system to the initial number. The water in the products is at first assumed to be fuUy condensed, so only nitrogen, hydrogen, and possible oxygen appear in the products. After this calculation, die saturation pressure of water at 303 K (about 0.04 bar) is added to represent the watra diat still exists as vapor. These values are listed in Table 2, along with the actual final system pressures for the tests run. 

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