UPPER-AIR WEATHER MAPS

Objectives:

Weather as reported on surface weather maps provides us primarily with a two-dimensional view of the state of the atmosphere, that is, weather conditions observed at Earth’s surface. Atmospheric conditions reported on upper-air weather maps provide the third dimension, that is, conditions at various altitudes or pressure levels above Earth’s surface. For a more complete understanding of the weather, we need to consult both surface and upper-air weather maps.

After completing this investigation, you should be able to:

• Describe the topography of upper-air constant-pressure surfaces based on height contours, including the identification of topographical Highs, Lows, ridges, and troughs.
• Identify the general relationship between height contours and the temperature of the underlying atmosphere.
• Describe the relationship between the height contours and wind direction on upper-air weather maps.

Introduction:

Upper-air weather maps differ from surface weather maps in several ways. Whereas surface weather conditions are plotted on a map of constant altitude (normally sea level) from observations that are collected at least hourly, upper-air weather conditions are commonly plotted on maps of constant air pressure from rawindsonde observations made twice a day. The altitude at which the particular pressure occurred is reported on these maps. An upper air observation is made by releasing a balloon-borne instrument package to the atmosphere. As the balloon rises, the air pressure decreases. Among the uses of the data collected is the construction of upper-level constant pressure maps. For example, an 850-mb constant-pressure map is based on observations collected at 850 mb at numerous locations, enabling a depiction of an 850-mb constant-pressure surface. Every 12 hours, upper-air maps are routinely drawn for various pressure levels including 850 mb, 700 mb, 500 mb, and 300 mb.

Plotted on upper-air maps are temperature (in °C), dewpoint (in °C), wind speed (in knots), wind direction, and height of the pressure surface above sea-level (coded). Become familiar with the upper-air station model depicted in Figure 1 below. The upper air station is located at the end of the wind shaft opposite the speed “barbs” and/or “flags”.

Figure 1. Upper air station model legend (500 mb).

While the AMS course website provides upper air maps utilizing the station model above with the dewpoint shown directly, it should be noted that many NOAA and other web maps display the dewpoint depression instead. The dewpoint depression is the number of Celsius degrees that the dewpoint is below the plotted temperature. To prevent confusion as to which (dewpoint or dewpoint depression) is being reported when interpreting upper air maps from different sources, keep in mind that the dewpoint can never be greater than the air temperature, so if dewpoint depression is reported, it must always be a positive number or 0.

Whether the dewpoint or dewpoint depression is displayed, meteorologists generally assume that clouds are present (at the station or within the region) when the dewpoint is within 5 Celsius degrees of the air temperature, i.e. the dewpoint depression is 5 or less.

The plotted altitude of the pressure level of the map is a coded value. That is, only the three most significant digits of the value are plotted. To decode the plotted value the following table shows the Standard Atmosphere altitude of the pressure surface, and the missing digits needed to decode the three plotted numbers (xxx).

The 700-mb level may lie below 3000 m necessitating placing a 2 in front of the plotted values to make the meaningful choice in the decode.

Figure 2 is the 500-mb constant-pressure map for 12Z 06 NOV 2011. Meteorologists frequently refer to 500-mb maps because winds at that level generally steer weather systems across Earth’s surface. Hence, the so-called steering winds at 500 mb can be used to predict the track of a low-pressure system.

Figure 2. 500-mb constant-pressure map for 12Z 06 NOV 2011.

1. Solid lines on the 500-mb map join locations where the 500-mb pressure level is at the same altitude. These lines, called contours of height, are drawn at intervals of 60 m. The coded height values on the map are in tens of meters. On the map, contour values are in whole meters. The highest reported 500-mb height at an individual station on the Figure 1 map was [(5730)(5750)(5800)(5850)] m.

2. On the Figure 2 map, the lowest height reported at an individual station for a pressure reading of 500-mb was [(5210)(5250)(5350)(5420)] m.

3. The 500-mb map and other constant-pressure upper-air maps are actually topographic maps that give form or shape to an imaginary surface on which the air pressure is everywhere the same. That is, the contour pattern reveals the “hills” and “valleys” of the constant-pressure surface. The contour pattern of the Figure 2 map indicates that, in general, the 500-mb surface (the surface where the air pressure is everywhere 500 mb) is at a [(higher)(lower)] altitude in southern Canada than in the southern U.S.

4. Contour lines on constant-pressure upper-air maps separate regions that have higher altitudes from those areas that have lower altitudes than the value of that contour line. In Figure 2, the area to the south of the 5820-m contour across the Southeast U.S. is where 500-mb altitudes are the [(lowest)(highest)] on the map. Conversely, on the same map, the area within the loop of the 5400-m contour line including Montana and North Dakota is a region where 500-mb altitudes are among the lowest.

5. The wave pattern of most of the contour lines on the Figure 1 map consists of topographic ridges and troughs, that is, elongated crests and depressions, respectively. A broad [(trough)(ridge)] appears on the Figure 2 map over the eastern U.S.

6. On the same map, there is evidence of a [(trough)(ridge)] across the northern Rocky Mountains.

As demonstrated in Investigation 5B, air pressure drops more rapidly with altitude in a column of cold air than in a column of warm air. Hence, the height of the 500-mb surface is lower where the underlying air is relatively cold. Conversely the 500-mb surface is higher where the underlying air is relatively warm.

7. Therefore, the air below the 500-mb region of lowest heights in Figure 2 must be [(colder)(warmer)] than the air below the surrounding higher 500-mb surfaces.

8. The upper air station model also gives the air temperature at 500 mb. The plotted station data show that, as latitude increases (i.e. moving poleward), the general decline of 500-mb temperatures are accompanied by a(n) [(increase)(decrease)] in the altitude of the 500-mb surface.

9. Suppose that at 12Z 6 NOV 2011 you board an airplane and fly non-stop directly from Great Falls, Montana to Miami in southern FL. En route, the plane cruises along the 500-mb surface. Flying from Great Falls to Miami, the aircraft’s cruising altitude [(increases)(decreases)(does not change)].

10. At the same time, the air temperature outside the aircraft [(rises)(falls)].

11. A relationship exists between the orientation of height contours and wind direction on 500-mb maps, especially at higher wind speeds. As seen in Figure 2 across the central portion of the U.S., wind direction is generally [(perpendicular)(parallel)] to nearby height contour lines. This is because the frictional forces acting on moving air at and near Earth’s surface diminish rapidly with height and are essentially absent in determining middle and upper atmosphere motions.

Investigation 8A examined forces that arise from horizontal pressure differences that put air into motion. Once in motion, Coriolis and friction forces enter the mix, and the combination leads to the circulations around Lows and Highs as seen on surface weather maps. In this investigation we extend our examination to the three-dimensional pattern of weather systems by looking at upper air maps. We will compare a surface weather map that we are familiar with to the display of atmospheric conditions on a mid-tropospheric map.

Conditions at 500-mb are those in the middle troposphere associated with surface conditions, including storm systems and shown on a surface map at the same time. Weather systems extend well into the troposphere and a three-dimensional understanding of them is necessary for making accurate weather forecasts.

Maps of upper-atmospheric conditions are made twice each day at 00Z and 12Z from data gathered by rawinsonde soundings. Those upper air maps and Stüve diagrams displayed on the course website are created from the radiosonde instruments launched from about 70 stations across the continental U.S. and Canada/Mexico. On an upper-air map, the temperature, dewpoint, height and wind data from a station’s rawinsonde report at a specific pressure are plotted around each station location (at the forward end of the wind arrow) in an upper-air station model format, as discussed in the introductory portion of Investigation 8B and the User’s Guide, linked from the Extras section of the course website.

Figure 3 is the surface weather map for 12Z 29 OCT 2013. At this time, high pressure over the eastern U.S. was bisected by a stationary front dividing cool air to the north of the front from warmer and more humid air to the south. Portions of the central U.S. were experiencing stormy conditions associated with a developing storm system that formed along the stationary front. And another storm system was beginning to develop in the Rockies.

Figure 3. Surface weather map for 12Z 29 OCT 2013.

12. The Dallas, TX station model had one of the highest surface wind speeds at map time. The wind speed was shown with one and a half feathers indicating a speed of about [(15)(25)(45)] knots.

13. Dallas’ wind direction was oriented [(parallel)(at an angle)] to the nearby 1016-mb isobar. This wind pattern was a result of the combination of horizontal forces acting on air near the surface, including the pressure gradient force directed toward the northwest.

Figure 4 is the 500-mb constant-pressure map for 12Z 29 OCT 2013. These were the upper-air conditions over the coterminous U.S. and adjacent areas of Canada and Mexico at the same time as the conditions shown on the Figure 3 surface weather map.

Figure 4. 500 mb constant pressure map for 12Z 29 OCT 2013.

14. On the Figure 4, 500-mb map, the plotted report for Flagstaff, in northern Arizona, shows that at the 500-mb pressure level over the station, the temperature was [(–8)(–19)(–23)] °C.

15. The dewpoint at 500 mb over Flagstaff was [(–11)(–20)(–25)] °C.

16. Recalling that the heights plotted at individual stations on 500-mb maps are in tens of meters (add a 0 to the three plotted digits), the height at which 500 mb occurred over Flagstaff was [(5610)(5760)(5820)] meters above sea level.

17. The wind at Flagstaff was generally from the southwest at about [(20)(35)(70)] knots. [Note: When winds of 50 knots or greater are reported, as on the map in the Great Lakes and Southwest, a pennant is used on the station’s wind shaft for a 50-kt increment along with the appropriate number of long and short “feathers”.]

The following data were from a rawinsonde report at another station’s 500-mb level at 12Z 29 OCT 2013 (note that temperature, dewpoint and wind speed values are rounded for plotting) –

Height (m): 5860, temperature (°C): –9.1, dewpoint (°C): –18.1,wind direction (deg. clockwise from N): 235 (i.e. SW is 270°), wind speed (kts): 32.

18. Examining the stations plotted on the 500-mb map in Figure 4 shows this station to be [(Wilmington, OH)(Dallas, TX)]. Current rawinsonde reports of upper air data can be found from the Upper Air section, “Upper Air Data - Text” on the course website.

The pattern of 500-mb heights (heights above sea level where the air pressure is 500 mb as found by radiosondes at that time) can be shown by contour lines. To better visualize the contour pattern plotted by the computer on the Image 2 map, highlight the blue 5820-m contour by tracing over it. [The 5820-m contour is labeled along the northern Georgia border. It enters the U.S. in western Texas and extends eastward to exit in South Carolina.] But also, note the closed 5520-m contour over NV-UT-ID. This contour encloses a topographic Low in the pressure surface at that location.

19. The contour pattern of the 500-mb map has [(a trough in the central U.S.)(generally straight west-to-east flow across the U.S.)(a ridge in the central U.S.)].

20. Comparing the wind speeds in general for the 500-mb level with those for the surface level at 12Z 29 OCT 2013, shows that, as altitude in the atmosphere increases, wind speeds generally [(decrease)(remain the same)(increase)]. This relationship results partially from the absence of friction in the middle and upper atmosphere.

21. The Figure 4, 500-mb map also shows that, where contour lines are relatively close, such as the Southwest region, wind speeds are relatively [(low)(high)] compared to where contour lines are more widely spaced. This principle corresponds to that of the spacing of isobars and wind speeds on surface maps.

22. At upper levels, the wind directions are also related to the contours. That is, especially where winds are relatively fast, the winds are generally [(“parallel” to the contours)(directed across the contours at large angles)].

The absence of friction at upper levels means that flow is controlled mainly by the pressure gradient and Coriolis forces. Therefore, winds are generally along the contours on upper level maps as opposed to the inward circulations with Lows and outward with Highs seen on surface maps. Compare these 500-mb flows to those seen on the Monday surface map.

23. Using the 500-mb station values as well as the contour pattern, compare the height at Wilmington, in southern Ohio, with that at Elko, in northeastern Nevada, stations at approximately equal latitudes. The 500-mb height was [(lower)(higher)] over Wilmington compared to that over Elko.

24. Also compare the temperatures for Wilmington and Elko. The 500-mb temperatures are [(lower)(higher)] over Wilmington compared to Elko. As you recall, the relation of column temperatures to heights of pressure surfaces was examined using conceptual “pressure blocks” in Investigation 5B.

Middle and upper tropospheric conditions are inextricably linked with surface weather features. They are involved in the development and movement of weather systems over the Earth. We will consider these relationships along with upper tropospheric maps and conditions in Investigation 9A.

Suggestions for further activities: You might try making a height-contour analysis by printing an unanalyzed 500-mb map (“500 mb – Data”) from the website. You can then compare your hand-analyzed pattern to the computer-analyzed map with contours. Also, compare upper-air map patterns to surface weather maps and the weather conditions you experience locally. See if you can link upper-air troughs and ridges with surface Lows and Highs.