SURFACE AIR PRESSURE PATTERNS

Objectives:

Many of our weather experiences involve air in motion from gentle breezes to hazardous gales. Wind is the motion of air relative to Earth’s surface. Differences in the pressure exerted by the air over a distance are what cause the air to move. These air movements, winds, carry temperature changes, produce clouds, and lead to precipitation. Across a horizontal surface such as Earth at sea level, variations in air pressure lead wind to blow from locations where the pressure is relatively high toward locations where the pressure is relatively low. Determining these air pressure patterns with their differences results in understanding air motions and predicting what weather is likely to be. We analyze the reported values of air pressure across the country to determine the pressure pattern which then shows locations of centers of high and low pressure on weather maps. The centers also typically coincide with fair and stormy weather systems, respectively.

After completing this investigation, you should be able to:

• Show the patterns of surface air pressures across the nation at map time by drawing lines of equal pressure (isobars).
• Locate regions of relatively high and low air pressures on the same surface map.

Introduction:

Air pressure at any point on Earth’s surface or in the atmosphere is given by the weight of the atmosphere above that point pressing on a unit area. This means that air pressure decreases as altitude increases. Hence, the higher the elevation of Earth’s surface, the lower the surface air pressure at that location. Consequently, locating centers of high and low atmospheric pressure, which helps to identify weather systems, requires analysis of air pressure values determined at numerous locations at the same elevation.

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 all the reporting stations were actually located at sea level. Adjustment of air pressure readings to the same elevation eliminates the variations of air pressure based on Earth’s topographical relief. This adjustment allows determinations of horizontal pressure differences and recognition of pressure patterns due to the weather systems. These patterns reveal existing broad-scale areas of high and low pressure that are a major influence on the weather we experience.

Horizontal air pressure patterns on a weather map are revealed by drawing lines representing points where equal pressure is shown or would exist. These lines are called isobars because every point on the line has the same air pressure (barometric) value. Each isobar separates stations reporting pressures higher than that of the isobar from stations with pressures lower than its value.

The Figure 1 surface map shows air pressure in millibar (mb) units at various locations. [One millibar (the pressure unit traditionally used for atmospheric pressure) is equal to one hectopascal (hPa), that is, one hundred Pascals. A Pascal is one Newton per square meter, named for Blaise Pascal, a French mathematician and physicist.] (Average midlatitude, sea-level air pressure is 1013.25 mb.) On the map, consider each pressure value to have been observed at the center of the plotted number.

Figure 1. Surface weather map with pressures reported in whole millibar units. The 1016- and 1020-mb isobars have been drawn and labeled.

1. On the Figure 1 map, the highest plotted pressure is 1023 mb and the lowest plotted value is [(997)(1003)(1010)] mb.

The 1016- and 1020-mb isobars have been drawn. Complete the pressure analysis by drawing the 1012-mb, 1008-mb, 1004-mb and 1000-mb isobars. Label each completed isobar by writing the appropriate pressure value at a break in the curve if it is closed (as shown) or at its ends as shown.

Please note that Figure 1 and all other Investigations Manual figures are also available via the Learning Files section of the course website. To view these images, click on the “Investigations Manual Images” link on the website, go to the row containing the appropriate investigation name, and then select the appropriate figure within that row. For example, to view Figure 1 online, go to the row labeled “1A” and then select “Fig. 1”. This will provide an easy way to format and print an image to work on or examine in more detail.

Tips on Drawing Isobars: Keep the following “rules” about drawing isobars in mind whenever you are analyzing air pressure values reported on a surface weather map.

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 uniform pressure change between neighboring stations. For example, a 1012-mb isobar would be drawn between 1010 and 1013 about two-thirds the way from 1010.

c. Adjacent isobars tend to be shaped alike. The isobar you are drawing will generally align with 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 even if no values between 996 mb and 1004 mb are plotted on the map.

Always label all isobars.

2. By U.S. convention, isobars on surface weather maps are usually drawn using the same interval (the difference in air pressure between adjacent isobars) as that described for the Figure 1 map. The isobar interval is [(2)(3)(4)(5)] 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 (for example, 1 mb) would clutter the map with too many lines and too great an interval (for example, 10 mb) would ordinarily mean too few lines to adequately define the pattern.

3. Also by U.S. convention, isobars drawn on surface weather maps are a series of values that, when divided by 4, produce whole numbers (e.g., 1000 ÷ 4 = 250). The progression of isobaric values can be found by adding 4 sequentially to 1000 and/or subtracting 4 sequentially from 1000 until the full range of pressures reported on the map can be evaluated. Which of the following numbers would not fit such a sequence of isobar values: [(1000)(1004)(1006)(1008)(1012)(1016)]?

4. The change of pressure over a given distance is called the pressure gradient. On surface weather maps, the directions of the pressure gradients (greatest pressure change over distance) are always oriented perpendicular to the isobars. And, the closer together the isobars appear on a map, the stronger the pressure gradients. From the isobar pattern you have shown to exist on the Figure 1 map, the horizontal pressure gradient is stronger across [(North and South Dakota)(east Texas)].

Optional: If you are unsure about your isobar-drawing skills or just crave more experiences drawing isopleths (lines of constant value, including isobars), visit http://cimss.ssec.wisc.edu/wxwise/contour/ () before attempting analyses of real-world weather maps. Try it, it’s fun.

Please also note that Internet addresses appearing in this Investigations Manual can also be accessed via the Learning Files section of the course website by clicking on “Investigations Manual Web Addresses”. Then, go to the appropriate investigation and click on the address link. We recommend this approach for its convenience. If a link does not work, click the icon () following it to go to the Web Addresses page. This also enables AMS to update any website addresses that are changed after this Investigations Manual was prepared.

Figure 2 (“Pressures” map) was acquired from the course website and shows reports of surface air pressures (corrected to sea level) rounded to the nearest whole millibar on 27 August 2013 at 13Z. [UTC or Z time is four hours “ahead” of Eastern Daylight Time (EDT), so the 13Z map depicts conditions at local times of 9 AM EDT (8 AM CDT, 7 AM MDT, 6 AM PDT, etc.)]. [Consider each pressure value as being located at a point at the number’s center.]

5. The lowest plotted air pressure on the map was 1007 mb at [(Chicago, IL)(Phoenix, AZ)].

6. The highest reported pressure was [(1019)(1022)(1031)] mb at Birmingham, AL.

7. The isobars in the conventional series that will be needed to complete the pressure analysis between the lowest and highest values on this map are:
[(1007, 1011, 1015, 1019, 1023)(1008, 1012, 1016, 1020)(1010, 1014, 1018, 1022, 1026)].

Using a pencil, follow the steps below to complete the pressure analysis for the map area to determine the pressure pattern that existed at the time the observations were made. For completing the map, refer to the Tips on Drawing Isobars in the introductory portion of this investigation. More than one isobar of the same value may need to be drawn on the map if pressure values located in separate sections of the map area require it.

Figure 2. Partially completed map of sea-level air pressures for 13Z 27 AUG 2013.

Consider each pressure value to be located at the center of the reported number. The isobars have already been drawn in the western U.S. The labels for the isobars have been added at a break in the line where they circle or at their ends where they reach the boundary of the map area having plotted data.

8. While any isobar value of the set that must appear on the map may be chosen to be drawn, let us start by extending the 1016-mb isobar where arrowheads indicate it must be continued. This isobar will continue through locations where the air pressure was 1016. Label this isobar with its value, 1016, at its ends. Then continue with the remaining isobars of the set of isobars in order. The pressure values at surrounding stations inside of the 1016-mb isobar loop are [(less than)(equal to)(greater than)] 1016 mb.

Continue drawing and labeling isobars of the series where they existed within the data pattern over the U.S. Complete all isobars in the range of values necessary to cover the data points. If a single isolated station reporting the value of an isobar is shown but all surrounding pressure values are either all higher or all lower than that value, e.g. the 1012 value in southwestern Idaho, no isobar of that value is needed.

After completing all the isobars, label the area with the lowest values within the 1012-mb isobar in the north-central U.S. area with a bold L (about 1 cm high). Also, place an L over the lowest value in the Southwest The broad area of high pressure in the Southeast should have an H situated over its highest value.

9. Figure 3 is the analyzed surface pressure map from the course website produced at the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Prediction (NCEP) for 12Z 27 AUG 2013. The Figure 3 map shows the locations of isobars, air pressure system centers and fronts one hour prior to those on the Figure 2 map you have analyzed. The overall national-scale isobar patterns on the two maps are generally [(similar)(very different)].

Figure 3. Analyzed NCEP surface weather map for 12Z 27 AUG 2013 showing weather systems and isobars.

The Figure 3 map of isobars is constructed by a computer based on a much more complete set of pressure values. (This may account for some of the variations between your analysis and that by the computer. The computer-based analysis is the source of several additional plotted Ls denoting locally minimally lower pressure centers and Hs for higher pressure areas, respectively.) The low-pressure area in the Southwest is a seasonal feature resulting from intense solar heating of the desert region. In Phoenix every day in August, 2013 except two, the maximum temperature was above 100 degrees! These “heat lows” are not large-scale migratory weather systems that affect broad weather patterns.

By analyzing the pressure values reported on weather maps to find pressure patterns, one can locate the centers of locally highest and lowest pressures. We will see that these pressure centers often mark the midpoints of major weathermakers, either regions of fair weather or stormy conditions, respectively.

As we go through the term, geographic mention of locally understood regions will appear in the Daily Weather Summary and Current Studies. Common NWS terminology for these regions of the country which may be useful to understanding is at: http://www.hpc.ncep.noaa.gov/images/us_bndrys1_print.gif and http://www.hpc.ncep.noaa.gov/images/us_bndrys2_print.gif. ()