Exercises (b)
1. Establish compliance between parts of sentences:
1. moisture is continually entering 1. as vapour, liquid or solid
the atmosphere by evaporation 2. the initial humidity of the air,
and transpiration wind force and the heat
2. several expressions are used 3. and leaving it in the form of
for describing precipitation
3. moisture exists in all three stages 4. phase changes
of matter in the atmosphere 5. the amount of moisture in the air:
4. changes from one state to other absolute, specific and relative
state are known as humidity
5. three principal factors
favor evaporation:
2. Speak about absolute and relative humidity using material of the unit and data of table:
Table 1
Absolute humidity |
Relative humidity |
Absolute humidity is the total mass of water vapor present in a given volume of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C.
Absolute
humidity is the mass of the water vapor ( The absolute humidity changes as air temperature or pressure changes. This makes it unsuitable for chemical engineering calculations, e.g. for clothes dryers, where temperature can vary considerably. As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air, also known as the mass mixing ratio (see "specific humidity" below), which is better suited for heat and mass balance calculations. Mass of water per unit volume as in the equation above is also defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 (revised 2002) suggests avoiding the term "absolute humidity". Units should always be carefully checked. Many humidity charts are given in g/kg or kg/kg, but any mass units may be used. The field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named psychrometrics.
|
The
relative humidity Relative humidity is normally expressed as a percentage and is calculated by using the following equation:
Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summerweather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F (28.7 °C ±0.7 °C) at ~44% relative humidity.
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),
divided by the volume of the air and water vapor mixture (
),
which can be expressed as:
of
an air-water mixture is defined as the ratio of the partial
pressure of
water vapor (H2O) 
in
the mixture to the saturated vapor
pressure of
water 
at
a given temperature. Thus the relative humidity of air is a
function of both water content and temperature.