5.1.3: Humidity
- Page ID
- 21684
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Humidity is a measure of the water vapor content of the air. The amount of water vapor in the air depends on the controls over evaporation and transpiration discussed earlier. There are several ways to express the humidity of the air. Each humidity measure is controlled to some degree by air temperature.
Absolute humidity is the weight of water vapor per unit volume of air, usually measured in units of grams of water vapor per cubic meter of air. Absolute humidity is not often used to express the moisture content of air because it is sensitive to changes in both the temperature of the air and atmospheric pressure. For instance, let's say that a 1 cubic meter parcel of air at the surface has 2 grams of water in it. Now lift the parcel of air upwards into the atmosphere. As the air rises upward the decrease in atmospheric pressure on the parcel allows it to expand outward occupying more space. Let's say that the parcel doubles in volume as a result of uplift. Before rising, the absolute humidity was 2 gm/m3. As the air doubles in volume the new absolute humidity is 1 gm/m3. In actuality the parcel still has the same weight of water in it, 2 grams. But given the way absolute humidity is calculated it appears the amount of water in the air has decreased.
Instead of absolute humidity, we can use a measure that is not sensitive to volume changes in the air. Specific humidity is measured as the weight of water vapor in the air per unit weight of air, which includes the weight of water vapor. The units of measurement are grams of water vapor per kilogram of air. Given that weight is not significantly influenced by temperature or atmospheric pressure, specific humidity is much more useful as a measure of humidity.
Humidity is not only measured as a weight, but also by the pressure it creates. Vapor pressure is the partial pressure created by water vapor. Vapor pressure, like atmospheric pressure, is measured in millibars and is relatively insensitive to volumetric expansion or temperature. The saturation vapor pressure is simply the pressure that water vapor creates when the air is fully saturated.
The saturation level of the air is directly related to the air's temperature. As air temperature increases, more water can remain in a gas phase. As temperature decreases, water molecules slow down and there is a greater chance for them to condense on to surfaces. The graph below shows the relationship between air temperature and relative humidity.
Dew point temperature is the temperature at which condensation takes place and is used as a measure of moisture content. The dew point temperature depends on the amount of moisture in the air, the more moisture in the air, the higher the dew point temperature. It gets its name "dew point" because dew will form on surfaces when the air reaches saturation.
Have you ever noticed that even though it's 100% relative humidity out, it feels a lot drier during the winter than the summer? To see why, we have to examine relative humidity. Relative humidity is the ratio of the amount of water vapor in the air to its saturation point. Figure \(\PageIndex{1}\) shows that the saturation level of the air with respect to water vapor depends strongly on the air's temperature. We know that as air temperature increases, the ability for the air to keep water in its vapor state is easier. That is, as the air temperature increases it can keep more water in the vapor state. So saturated cold air feels drier than saturated warm air because the specific humidity of cold air (say 0o C) is about 5g/kg , whereas for warm air (say 40o C) it is 50g/kg!