Water-vapor content of air

In theory, since the absolute water vapor content of air reduces as the temperature sinks or the altitude rises, the volume of precipitation should decline. Yet the opposite is the case, particularly in mountainous regions in the temperate latitudes here the annual volumes of precipitation generally tend to rise with the altitude. This is the result, on the one hand, of higher wind speeds at higher altitudes, which cause a relatively large shift in humid air masses. On the other hand, precipitation occurs more frequently and often at a much greater intensity. 

The percent water vapor content of air at saturation versus representative temperatures and pressure altitudes is given in Table 5. 

TABLE 5. PERCENT WATER VAPOR CONTENT OF AIR AT SATURATION VERSUS REPRESENTATIVE TEMPERATURES AND PRESSURE ALTITUDES... 

Percent relative humidity is the best known ami perhaps the most widely used method for expressing the water vapor content of air. Percent relative humidity is defined as the ratio of the prevailing water vapor pressure c . to the water vapor pressure if the air were saturated, t, multiplied by 100 ... 

We have already encountered many aspects of temperature — indeed temperature affects essentially all processes in plants. For instance, when introducing the special properties of water in Chapter 2, we noted that physiological processes generally take place within a fairly narrow temperature range, that the water vapor content of air at saturation is very temperature dependent (also see Appendix I), and that biochemical reactions... 

As we can see from relations such as Equation 8.2 (J = gjAcj = ACjlrj), the conductances or the resistances of the various parts of the pathway determine the drop in concentration across each component when the flux density is constant. Here we will apply this condition to a consideration of water vapor concentration and mole fraction in a leaf, and we will also consider water vapor partial pressures. In addition we will discuss the important effect of temperature on the water vapor content of air (also considered in Chapter 2, Section 2.4C). 

The accompanying figure is useful in determining the water-vapor content of air at high pressure in contact with liquid water. 

Atmospheric water can be present as vapor, hqirid, or ice. The water vapor content of air can be expressed as humidity. Relative humidity, expressed as a percentage, describes the amount of water vapor in the air as a ratio of the maximum amount that the air can hold at that temperature. Air with a given relative humidity can undergo any of several processes to reach the saturation point at which water vapor condenses in the form of rain or snow. For this condensation to happen, air must be cooled below a temperature called the dew point, and condensation nuclei must be present. These nuclei are hygroscopic substances such as salts, sulfriric acid droplets, and some organic materials, including bacterial cells. Air pollution in some forms is an important source of condensation nuclei. 

The large value of Le results in a very strong dependence of vapor pressure on temperature. As a result, the water vapor content of the air is extremely variable, from parts per million by volume in the coldest parts of the atmosphere to several percent in the warmest and wettest... 

Mixing dry and water vapor-saturated air in defined proportions also can be used to generate constant relative humidity. Control of flow rates and the water vapor content of the dry and saturated air are essential [27,28],... 

A hand-held field PID consists of an air uptake pump, an ultraviolet (UV) ionization lamp, a photo multiplier, and a readout device. The air drawn in by the pump passes along the lamp, where organic compounds are ionized with UV light. The resultant current is converted into a signal proportional to the number of ionized molecules. The UV lamp is calibrated with a standard (isobutylene in air), and the readout device provides the vapor concentrations in parts per million-volume (ppm-v). UV lamps are sensitive to the presence of moisture high water vapor content in air will suppress their ionizing action. Some PID models have a moistureabsorbing filter that can be attached to the instrument s inlet. 

Since pa r depends on water vapor content of laboratory air, one must first measure the barometric pressure and the relative humidity in the balance room, to determine pn r using tabulated data. 

Some of the earliest proposed descriptions of stratospheric transport were based on observations of chemical species. The first observations of stratospheric water vapor densities showed that the stratosphere was extremely dry, exhibiting mixing ratios of the order of a few parts per million by volume, in marked contrast to the troposphere, where water vapor abundance reaches a few percent. Brewer (1949) suggested that the dryness of the stratosphere was determined primarily by condensation and that the water vapor content of an air parcel rising from the troposphere to the stratosphere would therefore be determined by the lowest temperature experienced by the parcel, which would normally correspond to the tropopause. He also noted that the tropopause temperatures in the tropics were low enough to yield stratospheric water vapor densities as low as those observed, while the... 

Others parameters affecting losses at the chimney are the air humidity content, the air and fuel inlet temperatures, and the unburned hydrocarbons. Vapor content of air influences chimney losses in two ways it takes away part of the heat given by the fuel and modifies the dew point temperature as already seen. Not only does the combustion reaction produce water vapor as one of the outputs, but also a—relatively small—fraction of the water vapor... 

I In this extreme case, F = 0.945 x 102 Km-1 or 9.45Kkm 1. Therefore the difference between Iv and F is always less than 3% and the water vapor content of a cloud free air parcel can be neglected for all practical purposes. 

Figure 20-23. Dependence of electric conductivity of perovskite-type oxides on the water vapor content in air at 400 °C ...

When a volume of air at a given temperature holds the maximum amount of water vapor, the air is said to be saturated. Hence, relative humidity, sometimes called humidity ratio or degree of saturation denoted RH, is a dimensionless quantity which corresponds to the water-vapor content of the air relative to its maximum content at saturation at the same dry-bulb temperature. Relative humidity can also be regarded as the ratio of the mole fraction of water vapor in moist air to the mole fraction in moist air at saturation at the same temperature, and pressure. By use of the ideal gas law, this can be expressed as the ratio of the partial pressure of water vapor to the water vapor pressure at the same temperature. Therefore, relative humidity is the ratio of absolute humidity to absolute humidity at saturation, or the ratio of partial water vapor pressure, and vapor pressure of water at a given temperature ... 

Acetylene burns in air with an intensely hot, luminous, and smoky flame. The ignition temperatures of acetylene and of acetylene-air and acetylene-oxygen mixtures vary according to composition, initial pressure, initial temperature, and water vapor content. As a typical example, an air mixture containing 30 percent acetylene by volume at atmospheric pressure can be ignited at about 581 °F (305°C). The flammable limits of acetylene-air and acetylene-oxygen mixtures similarly depend on initial pressure, temperature, and water vapor content. In air at atmospheric pressure, the upper flammable limit is about 80 percent acetylene by volume and the lower limit is 2.5 percent acetylene. Some references list the upper flammable limit as 100 percent, which is due to the de-... 

Figure 11-5. Water vapor content of saturated air. After Lam faum etal. (1X5 ...

Some hydrogen detectors may remove the water vapor content of the hydrogen air steam mixture. In this case the measured hydrogen mole fraction (of the dry hydrogen air mixture) will be larger than the value in the original mixture. The correction required to recover the original value is... 

Here the two y, are slightly different from each other, because the water vapor content of the air is only 1/5 as large at 5 atm as at 1 atm. If we simplify by ignoring that and taking the ynitrogen 0-79 (but see Problem 3.8 ), we can solve approximately ... 

Rework Example 3.3 for air and water, including the carbon dioxide in the air. How many variables are there How many equations In atmospheric air the carbon dioxide concentration changes slightly from place to place and time to time, but on the average, for dry air, the concentration is about 390 ppm. Assume that the ratio of mole fractions of CO2 to N2 is always 0.000,390/0.79, independent of the water vapor content of the air. 

Since a perfect gas behavior was assumed in the derivation of Eq, (6.76), caution is advised in the use of this equation when the pressure of the gas mixture deviates appreciably from this assumption. For example, experimental measurements have shown that the actual water vapor content in air will be over four times that predicted by ideal gas behavior at a temperature of — 227K and a pressure of 20.2 MPa. Familiarity with these deviations is necessary if problems are to be avoided with this method of impurity removal. Data of this type are available as enhancement factors, defined as the ratio of the actual molar concentration to the ideal molar concentration of a specific impurity in a given gas. 

Many commercial gases are generated by burning hydrocarbons (qv) eg, natural gas or propanes, in air (see Gas, natural Liquified petroleum gas). The combustion process, especially the amount of air used, determines the gas composition. For a given fuel-to-air ratio, the gas composition can be used to determine the water vapor content required to achieve a desired equiUbrium carbon content of the austenite (see Combustiontechnology). 

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