6.2: High and Low Pressure Systems
- Page ID
- 44402
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Areas of high and low pressure
Once isobars are plotted on a weather map, they can be used to identify pressure patterns and the locations of high and low-pressure systems. For example, if the isobars form a "bull's-eye" pattern, the center of that bull's-eye is a pressure system. If isobar values decrease as you approach the center of the pressure system, it represents a low-pressure system, while a bull's-eye with increasing values as you approach the center represents an area of high pressure. Figure \(\PageIndex{1}\) is a weather map of computer-generated high- and low-pressure systems and isobars. This map is for 0100 UTC on July 20, 2020, and is a more realistic representation of air pressure than Figure 6.1.1, which was simplified.
- While isobars are plotted every 4 mb, recorded sea-level air pressures are also displayed for values between two isobars. Such a location is San Francisco, which is reporting an air pressure of:
- 1013 mb
- 1015 mb
- 1017 mb
- 1019 mb.
We learned in Investigation 5 that when air parcels rise, they often cool and condense, forming clouds. When air rises from the surface, it often leaves behind a low-pressure region. Thus, low pressure is often associated with cloudier, stormier weather, while areas of high pressure are associated with fair-to-clear weather.
- From Figure \(\PageIndex{1}\), which of these regions should expect stormy weather?
- The Pacific Northwest (Oregon and Washington)
- The New Mexico/Colorado state Line
- The California/Arizona State Line
- All of these regions
Figure \(\PageIndex{2}\) is a full weather analysis of the continental U.S. at the same time as Figure \(\PageIndex{1}\). This includes radar precipitation data along with previously shown isobars, fronts, and station data.
- In Figure \(\PageIndex{2}\), do you see large green or orange blobs from the radar that indicate precipitation over the regions you expected stormy weather in question 11?
- Yes
- No
One of the big caveats of using only high- and low-pressure systems to diagnose weather conditions is that they are only one factor that can influence stormy weather. Other factors, such as moisture availability (in the case of the thermal low pressure over the CA/AZ border), Frontal Systems (such as the long cold front extending across Indiana, Illinois, and Missouri), troughs (the dashed brown lines), mountains (such as over New Mexico and Colorado), and even the time of year can greatly influence the ability of a pressure system to produce stormy weather. We will investigate these large-scale weather patterns in Investigations 7 and 8.