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 defined as the amount of water vapor in the air. Unlike the concentrations of Oxygen and Nitrogen in our atmosphere, which are fairly constant, humidity can vary greatly depending on location and time of day. This explains why some locations feel bone-dry while other locations feel muggy and gross. There are several variables meteorologists use to quantify humidity, but the one most commonly used by the public is Relative Humidity.
If you have a smartphone (Apple iPhone, any Android Phone, etc.), open the phone’s weather app and check your current conditions. If you don’t have a smartphone, you can view current weather information from the National Weather Service (NWS) for San Jose Airport. Scroll until you find Humidity.
- On these apps, and/or 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? Relative Humidity is the ratio of the amount of moisture in the atmosphere to the amount that the atmosphere could potentially contain at a given temperature and pressure. Some definitions refer to it as the ratio of existing water vapor to the maximum water vapor the air can "hold". It is important to know that the atmosphere doesn’t really “hold” moisture. Instead, the atmosphere can contain a certain amount of moisture before it 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 pressure exerted by the current amount of moisture in the atmosphere, whereas Saturation Vapor Pressure is the pressure exerted by the maximum amount of moisture that the air can contain. Vapor pressure is determined by the rates of evaporation and condensation, rather than by the physical space in the atmosphere available for moisture. If there is more moisture in the air, the vapor pressure would be high; if there were less moisture, it would be low.
On the other hand, saturation vapor pressure occurs when the rate of evaporation balances the rate of condensation. In this case, temperature is the primary factor that determines this pressure value. A cooler atmosphere favors less evaporation and more condensation, thus decreasing the amount of moisture in the atmosphere and the saturation vapor pressure. On the other hand, a warmer atmosphere favors greater evaporation and less condensation, thereby increasing atmospheric moisture and the saturation vapor pressure.
Vapor pressure (the amount of moisture in the atmosphere) can only be determined by actual observation. However, there is an equation, called the Clausius-Clapeyron equation, that models the 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. Part of it is shown in Table 4.1.1
|
Temperature (deg F) |
Temperature (deg C) |
Vapor Pressure (mb) |
|---|---|---|
|
40 |
4.4 |
8.4 |
|
41 |
5.0 |
8.7 |
|
42 |
5.6 |
9.1 |
|
43 |
6.1 |
9.4 |
|
44 |
6.7 |
9.8 |
|
45 |
7.2 |
10.2 |
|
46 |
7.8 |
10.6 |
|
47 |
8.3 |
11.0 |
|
48 |
8.9 |
11.4 |
|
49 |
9.4 |
11.8 |
|
50 |
10.0 |
12.3 |
|
51 |
10.6 |
12.7 |
|
52 |
11.1 |
13.2 |
|
53 |
11.7 |
13.7 |
|
54 |
12.2 |
14.2 |
|
55 |
12.8 |
14.8 |
|
56 |
13.3 |
15.3 |
|
57 |
13.9 |
15.9 |
|
58 |
14.4 |
16.4 |
|
59 |
15.0 |
17.0 |
|
60 |
15.6 |
17.7 |
|
61 |
16.1 |
18.3 |
|
62 |
16.7 |
19.0 |
|
63 |
17.2 |
19.6 |
|
64 |
17.8 |
20.3 |
|
65 |
18.3 |
21.1 |
|
66 |
18.9 |
21.8 |
|
67 |
19.4 |
22.6 |
|
68 |
20.0 |
23.4 |
|
69 |
20.6 |
24.2 |
|
70 |
21.1 |
25.0 |
|
71 |
21.7 |
25.9 |
|
72 |
22.2 |
26.8 |
|
73 |
22.8 |
27.7 |
|
74 |
23.3 |
28.7 |
|
75 |
23.9 |
29.6 |
|
76 |
24.4 |
30.6 |
|
77 |
25.0 |
31.7 |
|
78 |
25.6 |
32.7 |
|
79 |
26.1 |
33.8 |
|
80 |
26.7 |
35.0 |
- Based on Table 4.1.1, a temperature of 55°F would yield a saturation vapor pressure of _______.
- 15.3 mb
- 14.8 mb
- 14.4 mb
- 55.0 mb
- On the other hand, a temperature of 75ºF would yield a saturation vapor pressure of:
- 23.2 mb
- 31.3 mb
- 29.6 mb
- 35.0 mb
What is the relationship that you observe between temperature and saturation vapor pressure?
- Saturation vapor pressure ______ when the temperature _____.
- increases, increases
- decreases, decreases
- does not change, increases
- increases, does not change