1B - Investigation 1B: SURFACE AIR PRESSURE PATTERNS
Objectives:
Whenever and wherever the air pressure changes from one place to another (producing an air
pressure gradient, i.e., a change in air pressure over distance), the wind tends to blow from
where the air pressure is relatively high to where the air pressure is relatively low. Knowing
the patterns of pressure across the country is basic to understanding what the weather is and
what it is likely to be wherever you live. By analyzing the reported values of air pressure
across the country, the locations of weather map highs and lows can be found.
After completing this investigation, you should be able to:
• Draw lines of equal pressure (isobars) to show the pattern of surface air pressures across
the nation at map time.
• Locate regions of relatively high and low air pressures on the same surface map.
Investigation:
Air pressure at any point on Earth’s surface or in the atmosphere is equal to the weight per
unit area of the atmosphere above that point. This means that air pressure decreases with
increasing altitude. Hence, the higher the elevation of Earth’s surface, the lower the air
pressure.
Air pressures routinely reported on surface weather maps are values “corrected” to sea-level.
That is, air pressure readings are adjusted to what they would be if the reporting station
were actually located at sea-level. Adjustment of air pressure readings to the same elevation
(sea-level) removes the influence of Earth’s relief (topography) on air pressure readings.
This adjustment allows comparisons of horizontal pressure differences that can lead to the
recognition of pressure patterns. These patterns reveal existing broad-scale pressure areas
that have a major influence on the weather.
Horizontal air pressure patterns on a weather map are revealed by drawing lines representing
equal pressure. These lines are called isobars because every point on the same line has the
same air pressure value. Each isobar separates stations reporting pressure values higher than
that of the isobar from stations reporting pressure values lower than that of the isobar.
The Figure 1 surface map segment which follows shows air pressure in millibar (mb) units
at various locations. [One millibar is equal to one hectopascal (hPa).] (Average midlatitude,
sea-level air pressure is 1013.25 mb.) The reported pressure values were observed at the
marked locations just above the numbers. The 1008-mb isobar has been drawn. Complete
the pressure analysis by drawing the 1012-, 1016- and 1020-mb isobars. Label each
isobar by writing the appropriate pressure value at its end points as shown.
Weather Studies: Investigations Manual 2006-2007
1B - 1. By U.S. convention, isobars on surface weather maps are usually drawn using the same
interval (the difference in air pressure value from one isobar to the next) as that used on
the map segment. The isobar interval is _____ mb. The isobar interval is selected so as
to provide what is generally the most useful resolution of the field of data; too small an
interval would clutter the map with too many lines and too great an interval would mean
too few lines to adequately define the pattern.
2. On the Figure 1 map, the direction North is up. All of the numbers on the map segment
to the east, southeast and south of the 1008-mb isobar are [(less than) (equal to) (greater
than)] 1008 mb.
3. Also by U.S. convention, isobars drawn on surface weather maps follow a sequence of
values which produce whole numbers when divided by 4 (e.g., 1000 ÷ 4 = 250) . The
sequence can be found by adding or subtracting 4 from 1000, then adding or subtracting
another 4 from the resulting numbers, and so on. Which of the following numbers do
not fit such a sequence of isobar values: 992, 994, 996, 1000, 1002, 1004, 1008, 1010,
1012? ____________________________
4. On surface weather maps, the strongest pressure gradients are oriented perpendicular to
the isobars. And, the closer isobars appear on a map, the stronger the pressure gradients.
In Figure 1, A and B are shown in the southwest and northeast portions of the map,
respectively. The horizontal pressure gradient is stronger, i.e. pressure changes more
rapidly with distance, in region [(A) (B)].
Tips on Drawing Isobars:
a. Always draw an isobar so that air pressure readings greater than the isobar’s value are
consistently on one side of the isobar and lower values are on the other side.
b. When positioning isobars, assume a steady pressure change between neighboring
stations. For example, a 1012-mb isobar would be drawn between 1013 and 1010
about one-third the way from 1013.
c. Adjacent isobars tend to look alike. The isobar you are drawing will generally
parallel the curves of its neighbors because horizontal changes in air pressure from
place to place are usually gradual.
d. Continue drawing an isobar until it reaches the boundary of the plotted data or
“closes” to form a loop by making its way to its starting point.
e. Isobars never stop or end within a data field, and they never fork, touch or cross one another.
f. Isobars cannot be skipped if their values fall within the range of air pressures reported
on the map. Isobars must always appear in sequence; for example, there must always
be a 1000-mb isobar between the 996-mb and 1004-mb isobars.
Weather Studies: Investigations Manual 2006-2007
1B - g. Always label all isobars.
Investigation 1A dealt with the hand-twist model for relating wind directions to centers of
high or low air pressure. This investigation demonstrates how those air pressure patterns can
be determined and the high and low pressure centers found.
5. The Figure 2 map, originating from the course website and labeled at the top center
as “Pressures”, displays atmospheric pressure values in whole millibars at the time of
______ Z on __________________ (date).
6. The lowest reported air pressure on the map at St. Johns, New Brunswick, in eastern
Canada, is [(977) (988) (995)] mb.
7. The highest reported pressure is _____ mb at Arcata in northern California.
8. The isobars in the conventional series that will be needed to complete the pressure
analysis between those lowest and highest values on this map are: 992, _____, _____,
_____, _____, _____, _____, _____, _____, and _____. More than one isobar of the
same value may need to be drawn if pressure values located in separate sections of the
map area require it.
Using a pencil, follow the steps below to draw the indicated isobars on this map to
determine the pressure pattern that existed at the time the observations were made.
Consider each pressure value to be located at the center of the reported number. The 1016mb and 1020-mb isobars have already been drawn in the eastern portion of the map.
Let us arbitrarily choose to complete the analysis of pressures by first drawing the 1012-mb
isobar adjacent to the western 1016-mb isobar already on the map. The 1012-mb isobar
enters the map data region between “1007” and “1016” in central Canada and across Lake
Superior. It passes through the center of “1012” in Lower Michigan, between numbers
heading southward to finally pass through the 1012s along the south Texas Gulf coast. Label
the isobar value by printing 1012 at both ends of the isobar line you drew. Note, this isobar
generally follows the curve of its neighboring 1016-mb isobar.
Moving westward from the 1012-mb isobar towards lower pressures, next draw the 1008-mb
isobar. Arbitrarily starting in Canada near the 1012-mb isobar, the 1008-mb isobar curves
southeastward and around to central Texas before heading back northward along the AZ-NM
border, finally ending at the data edge back in central Canada. Label the isobar at the ends
with its value, 1008. (It is customary to end an isobar at the edge of the plotted values unless
the plotted data indicate otherwise. Connecting the nearby ends of the 1008-mb isobar could
be done and labeled in a break of the line, see below.)
Complete the pattern of lower-valued isobars in the central U.S. These isobars, 1004 and
1000, will circle around to meet within the data field. Label each isobar line with its value
at a break along the line rather than at the end. (Although an isobar value, 996 represents a
Weather Studies: Investigations Manual 2006-2007
1B - single point, so no line needs to be drawn as separation of values below that.) Place a bold L
in South Dakota where the lowest pressure value is plotted.
Complete the pressure analysis over the western quarter of the map for the appropriate
isobar values listed in the sequence above (#8). Beginning with 1012, the isobars increase
westward again in value. Also, complete the isobars across the northeastern U.S. and
Canada, to the east of the eastern 1016-mb isobar where the pressure values decrease from
1012 to 992 mb. (Again, 988 represents only a single point, hence needing no line.) Be sure
to label each isobar with its value.
9. Figure 3 is the “Isobars, Fronts, Radar & Data” map for 00Z 16 JAN 2006. This map
[(is) (is not)] the same time and date as the Figure 2 map of pressures you have just
analyzed.
10. The time/date of the Figure 3 map [(is) (is not)] the same time as the U.S. Data map of
national weather conditions you used for the hand-twist patterns in Investigation 1A.
11. Compare your hand-drawn pressure analysis on the Figure 2 map with the analysis made
by computer on the Figure 3 map. Some of the differences between your analysis and
that shown are because the computer analysis is based on data from a larger number
of stations. Note the position of the L in east-central South Dakota and the H near the
California - Oregon border on the Figure 3 “Isobars, Fronts, Radar & Data” map. These
positions [(are) (are not)] near the approximate locations of the L and H you positioned
in the Investigation 1A map used for the hand twists.
12. These L and H positions [(are) (are not)] also the approximate locations of locally
lowest and highest plotted air pressure values you showed with your isobar analysis of
the Figure 2 “Pressures” map.
The map you have just analyzed represents atmospheric conditions across the country at the
time of those observations. Meteorologists (or their computer systems) analyze pressure
patterns as you have to locate centers of storminess and fair weather. In Investigation 2A we
will look at the more complete set of weather conditions reported in the station models.
Suggestions for further activities: The course website routinely delivers unanalyzed
(“Pressures”) and analyzed (“Isobars & Pressures”) surface pressure maps (under Surface
section on the website). Practice drawing isobars by calling up and printing out the
unanalyzed version. Use the analyzed map as your “answer key”. You might try this
“detective scheme” of analyzing surface air pressures to find storms or broad-scale air masses
that are mentioned in the news.
You may want to practice more on drawing isopleths (lines of a constant value) in fields of
numbers beginning with simple patterns. If so, go to: http://cimss.ssec.wisc.edu/wxwise/
contour/. This page also examines the set of lines in addition to isobars that are used in
weather map analysis.
Weather Studies: Investigations Manual 2006-2007
1B - Figure 1. Map segment with surface pressures at points, in whole millibars.
Weather Studies: Investigations Manual 2006-2007
1B - Weather Studies: Investigations Manual 2006-2007
Figure 2. Pressures map for 00Z 16 JAN 2006.
1B - Weather Studies: Investigations Manual 2006-2007
1B - Weather Studies: Investigations Manual 2006-2007
Figure 3. Surface (Isobars, Fronts, Radar & Data) weather map for 00Z 16 JAN 2006.
1B - Weather Studies: Investigations Manual 2006-2007
1B - 10
Weather Studies: Investigations Manual 2006-2007