4.2: Relative Humidity Diurnal Cycle
- Page ID
- 41832
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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 up, saturation vapor pressure becomes larger, which causes relative humidity to decline. Meanwhile, at night, air temperature cools down, causing saturation vapor pressure to decline, which causes relative humidity to increase. 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:
- 12pm
- 2pm
- 4pm
- 6pm
- The low temperature on January 24, 2019 was approximately 43°F at:
- 12am
- 3am
- 5am
- 9am
- At the warmest time of the day, the Saturation Vapor Pressure would be:
- 17.7mb
- 20.3mb
- 18.9mb
- 19.6mb
- At the coldest time of the day, the Saturation Vapor Pressure would be:
- 9.4mb
- 10.2mb
- 8.7mb
- 11.8mb
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:
\[\ Relative Humidity = \frac{Vapor Pressure (mb)}{Saturation Vapor Pressure (mb)} * 100\]
Reminder: You figure out Saturation Vapor Pressure by using the table found here: svp.pdf
- Let’s assume that the the Vapor Pressure is 9mb, and there was no change in the amount of moisture (vapor pressure) in the atmosphere during the period from 12am on January 24, 2019 to 12am on January 25, 2019. The relative humidity at De Anza College would be __________________ when the temperature is highest (Hint: Use the Saturation Vapor Pressure Table from the previous page).
- 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, relative humidity should be highest when:
- Temperature is Highest
- Temperature is Lowest.
We can actually confirm this result by looking at a 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 is highest, the other is highest too)
- Inversely (when one is highest, the other is lowest).
- This _________________ confirm the conclusion we made in Question 12.
- Can
- Cannot.