4.2: Relative Humidity Diurnal Cycle
- Page ID
- 41832
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\( \newcommand{\dsum}{\displaystyle\sum\limits} \)
\( \newcommand{\dint}{\displaystyle\int\limits} \)
\( \newcommand{\dlim}{\displaystyle\lim\limits} \)
\( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)
( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\id}{\mathrm{id}}\)
\( \newcommand{\Span}{\mathrm{span}}\)
\( \newcommand{\kernel}{\mathrm{null}\,}\)
\( \newcommand{\range}{\mathrm{range}\,}\)
\( \newcommand{\RealPart}{\mathrm{Re}}\)
\( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)
\( \newcommand{\Argument}{\mathrm{Arg}}\)
\( \newcommand{\norm}[1]{\| #1 \|}\)
\( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)
\( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)
\( \newcommand{\vectorA}[1]{\vec{#1}} % arrow\)
\( \newcommand{\vectorAt}[1]{\vec{\text{#1}}} % arrow\)
\( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\( \newcommand{\vectorC}[1]{\textbf{#1}} \)
\( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)
\( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)
\( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)
\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)
\(\newcommand{\longvect}{\overrightarrow}\)
\( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)
\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Daily Changes in Relative Humidity
Because temperature affects saturation vapor pressure, there can be a daily change in relative humidity, even if the actual amount of moisture in the atmosphere doesn’t change. As temperatures warm, saturation vapor pressure increases, causing relative humidity to decline. Meanwhile, at night, the air temperature cools, lowering the saturation vapor pressure and increasing relative humidity. Let’s investigate this more in-depth: Figure \(\PageIndex{1}\) is a plot of air temperature taken over a 24-hour period at the De Anza College weather station on January 24, 2019.
- The high temperature on January 24, 2019, was approximately 63°F at:
- 12 pm
- 2 pm
- 4 pm
- 6 pm
- The low temperature on January 24, 2019, was approximately 43°F at:
- 12 am
- 3 am
- 5 am
- 9 am
- At the warmest time of the day, the saturation vapor pressure would be (based on the saturation vapor pressure table):
- 17.7 mb
- 20.3 mb
- 18.9 mb
- 19.6 mb
- At the coldest time of the day, the saturation vapor pressure would be (based on the saturation vapor pressure table):
- 9.4 mb
- 10.2 mb
- 8.7 mb
- 11.8 mb
As previously mentioned, relative humidity is the RATIO of the amount of moisture in the atmosphere (vapor pressure) to the amount of moisture that the air can “hold” (saturation vapor pressure). A simple equation for relative humidity is:
\[\ Relative Humidity = \frac{Vapor Pressure (mb)}{Saturation Vapor Pressure (mb)} * 100\]
Let’s assume that the vapor pressure is 9 mb, and there was no change in the amount of moisture (vapor pressure) in the atmosphere during the period from 12 am on January 24, 2019, to 12 am on January 25, 2019. Reminder: You can obtain the saturation vapor pressure using Table 4.1.1 or the online table.
- The relative humidity at De Anza College would be ___________ when the temperature is highest. (Hint: Use the saturation vapor pressure table.)
- 40%
- 46%
- 48%
- 53%
- The relative humidity at De Anza College would be _________when the temperature is the lowest:
- 96%
- 89%
- 100%
- 92%
- Just using the results from questions 9 and 10, we can say that relative humidity ________ as air temperature increases.
- increases
- decreases.
- Therefore, during the day, the relative humidity will be the highest when:
- The temperature is the highest.
- The temperature is the lowest.
- And the relative humidity will be the lowest when:
- The temperature is the highest.
- The temperature is the lowest.
We can confirm this result by examining the plot of Temperature, Dew Point Temperature AND Relative Humidity for that day (Figure 4.2.2). This data isn’t directly available for the De Anza College weather station, but is available for nearby Moffett Field, in Mountain View, CA.
- Assuming little change in moisture, the temperature (red line) and relative humidity (green line) on Figure 4.2.2 are ________ related:
- directly (when one increases, the other increases too)
- inversely (when one increases, the other decreases).
- This _______ the conclusion we made in question 12 and 13.
- confirms
- cannot confirm.
Dew Point Temperature
In Figure 4.2.2, there is a third line that we haven’t discussed yet – the blue line represents Dew Point Temperature, which is the temperature that the air needs to be cooled to in order for the atmosphere to become saturated. The Dew Point temperature is directly related to the amount of moisture in the air – a high dew point means the air is close to saturation, so it doesn’t need to cool much. Whereas a low dew point means that the air is significantly unsaturated, and thus needs to be cooled substantially to become saturated.