1.1: What is Weather?
- Page ID
- 40729
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Geographical Location
The condition of the atmosphere at a given time and place defines weather. It is important to note that weather conditions can vary greatly depending on geographical location and time of day. The weather at the location where you are currently reading this can be vastly different than the weather conditions at a location a few miles away. Figure \(\PageIndex{1}\) shows temperatures across the San Francisco Bay Area on Thursday, May 1, 2025. The term "Weather" is often used interchangeably with "Climate" during casual conversations. But the two aren’t the same. In future investigations, we’ll cover Climate more in-depth, but Climate is defined as the “average” conditions of the atmosphere at a location. But for now, let's focus on the weather and try to answer the following question.
- In Figure \(\PageIndex{1}\), we notice that the temperatures across the San Francisco Bay Area at this time:
- were very similar everywhere.
- varied greatly from location to location, with areas further inland being colder than areas near the coast.
- varied greatly from location to location, with areas further inland being warmer than areas near the coast.
Time of day
In addition to weather conditions varying from place to place, they also vary greatly depending on the time of day. Open a weather app on your phone. It won't just tell you the ambient air temperature at that time, but also other weather quantities, such as Relative Humidity and Dew Point Temperature. These quantities indicate the amount of moisture or water vapor in the air, indicating whether the air is dry or humid. We'll learn more about these terms in Investigation 4, but for now, let's understand how these quantities can vary during the day. Figure \(\PageIndex{2}\) shows weather conditions at San Jose International Airport from July 21 to July 23, 2025.
| Date | Time | Temperature (°F) | Dew Point (°F) | Relative Humidity (%) |
|---|---|---|---|---|
| July 21 | 12:00 AM | 60 | 54 | 75 |
| July 21 | 6:00 AM | 58 | 54 | 85 |
| July 21 | 12:00 PM | 72 | 55 | 55 |
| July 21 | 6:00 PM | 68 | 55 | 65 |
| July 22 | 12:00 AM | 61 | 54 | 80 |
| July 22 | 6:00 AM | 58 | 54 | 88 |
| July 22 | 12:00 PM | 70 | 55 | 62 |
| July 22 | 6:00 PM | 67 | 55 | 70 |
| July 23 | 12:00 AM | 60 | 54 | 78 |
| July 23 | 6:00 AM | 61 | 54 | 78 |
Let's try to answer the following questions based on the information provided earlier.
- According to the observations in Figure \(\PageIndex{2}\) and Table \(\PageIndex{1}\), all three weather conditions ______ over the 3 days.
- did not change much
- changed quite a bit
- In general, air temperature and relative humidity:
- were directly related (when one went up, the other went up as well)
- were inversely related (when one went up, the other went down)
- had no relationship with one another
You may have already noticed, as you go about your daily life, that the ambient air temperature is often cool in the morning and at night, while it is warmer in the afternoon. Figure \(\PageIndex{2}\) confirms your suspicions. It shows that the weather conditions change depending on the time of day.