4.1: What is Relative Humidity?
- Page ID
- 41831
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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 term for the amount of moisture (in the form of Water Vapor) in the air. Unlike the amount of Oxygen and Nitrogen, the humidity of the air can vary greatly from place-to-place, and from day-to-day. This explains why some locations feel bone-dry while other locations feel muggy and gross. There are actually a number of variables meteorologists use to quantify humidity, but the most commonly used one by the public is Relative Humidity.
If you have a smartphone (Apple iPhone, any kind of Android Phone, etc), open up the phone’s weather app, and look at your current conditions (if you don’t have a smartphone, click on this link to view weather information from the National Weather Service (NWS) for San Jose Airport. Scroll until you find Humidity.
- On these apps, and the NWS weather page, humidity is given as a:
- Mass (grams, kilograms)
- Weight (pounds, ounces)
- Temperature (°C, °F)
- Percentage (%)
But what does that number mean? How does it change? That’s the goal of the first part of this investigation; to uncover Relative Humidity.
Relative Humidity is the ratio of the amount of moisture in the atmosphere to the amount that the atmosphere can “hold” (let me clarify that the atmosphere doesn’t “hold” moisture… but instead the atmosphere can have a certain amount of moisture in the air before the air becomes saturated). To calculate Relative Humidity, there are two terms we need to learn:
- Vapor Pressure: The Mass of the moisture currently in the atmosphere.
- Saturation Vapor Pressure: The Mass of moisture that would be present in a saturated atmosphere.
In short, Vapor Pressure is the amount of moisture in the atmosphere, whereas Saturation Vapor Pressure is the amount of moisture that the air can “hold.” (NOTE: It's terrible terminology to say that the air "holds" moisture, because the vapor pressure is determined by rates of evaporation and condensation, as opposed to physical space in the atmosphere available for moisture).
Vapor pressure is simply determined by how much moisture is currently in the atmosphere at a given time. So if there is more moisture in the atmosphere, Vapor pressure would be high, and if there was less moisture in the atmosphere, Vapor pressure would be low.
On the other hand, Saturation Vapor pressure, the amount of moisture present in a saturated atmosphere, is dependent on the balance between evaporation and condensation. A cooler atmosphere favors less evaporation and more condensation, thus decreasing the amount of moisture in the atmosphere. On the other hand, a warmer atmosphere favors more evaporation and less condensation, thus increasing the amount of moisture in the atmosphere.
Vapor pressure (the amount of moisture in the atmosphere) can only be determined by actual observation. However, there is an equation, called the Clasisus-Clayperon Equation that models Saturation Vapor Pressure. While the equation, its derivation and plot are beyond the scope of this class, there is a nifty table that we will use to determine Saturation Vapor Pressure for a given temperature, which can be found here.
Go to that link and take a look at the table. The following three questions below are based on the table.
- A temperature of 55°F would yield a Saturation Vapor Pressure of _________________.
- 15.3mb
- 14.8mb
- 14.4mb
- 55mb
- On the other hand, a temperature of 75ºF would yield a Saturation Vapor Pressure of:
- 23.2mb
- 31.3mb
- 29.6mb
- 35.0mb
- Written Question: What is the relationship between Temperature and Saturation Vapor Pressure? For help, look at your answers to the previous two questions.