6.6: Alternative Text Descriptions for Investigation 6

Figure 6.2.1: Surface Air Pressure for the United States

This is a surface pressure analysis map of the contiguous United States valid at 23Z on July 15, 2025. The map shows labeled sea-level pressure values (in millibars or hPa) plotted at representative points across each state. These values outline the large-scale pressure pattern over the U.S. at this time.

A strong surface high pressure system dominates the northern Great Plains, with central pressure values reaching 1027 hPa over Southern Canada, North Dakota, South Dakota, and Minnesota. This high extends broadly into Montana, Wyoming, and parts of the Upper Midwest, with surrounding values ranging from 1020 to 1024 hPa.

Lower pressure values are found in the western U.S. and desert Southwest, with sea-level pressure near 1006 to 1008 hPa in Arizona, New Mexico, Nevada, and Utah. The lowest pressures across the country are located along the Intermountain West, where parts of Nevada, Utah, and Colorado show values as low as 1004 hPa.

Another relative minimum in pressure is observed in the southern U.S., particularly across Texas, Oklahoma, and Louisiana, where pressures fall between 1007 and 1010 hPa. Broad areas of 1012 to 1016 hPa span the Southeast and Mid-Atlantic states.

The Northeast U.S. shows higher pressures as well, with values from 1016 to 1019 hPa, and similar values extend through parts of the Ohio Valley. A moderate pressure gradient exists across the central U.S., particularly from the northern Plains into the lower Mississippi Valley, suggesting areas of enhanced surface winds.

This map contains no color or contour lines; only numeric pressure values are labeled over each state. Isobar spacing and patterns must be interpreted based on the distribution of these values.

Figure 6.3.1: Isobar Map for the United States at 0100 UTC on July 20, 2020

Surface weather map of the continental United States valid at 01Z on July 20, 2020, showing isobars (lines of equal sea-level pressure) in blue, labeled in millibars (hPa), at 4 mb intervals. The map also marks areas of high pressure ("H") and low pressure ("L") to show the general surface pressure pattern across the country.

Key features:

• High-pressure systems (H) are located in several regions:
  • One over the Pacific Northwest (centered near 1017 mb),
  • Another across the Southern Plains (around Oklahoma, ~1012 mb),
  • One over the Southeast (Florida, ~1018 mb),
  • And a broad high across the Northeast U.S. (~1013 mb).
• A well-defined low-pressure system (L) is centered over southern Arizona and southeastern California, with a central pressure of 1002 mb. This low is likely associated with the Southwest Monsoon pattern. Surrounding isobars form a closed loop, with pressure gradually increasing outward (1003, 1004, 1005 mb, etc.), indicating a tight pressure gradient, which could lead to increased wind or convective activity in the region.
• Another low-pressure area is found in the far northern Great Lakes region, with pressures dropping below 996 mb, and dense isobar spacing suggesting a stronger synoptic system, possibly with organized winds or precipitation.
• Elsewhere, isobars are generally more spaced out, especially across the central and southeastern U.S., indicating weaker pressure gradients and likely calmer surface winds.
• The isobar pattern also shows a trough extending from the low in Arizona northeastward into the central Plains and upper Midwest.

This map helps students identify areas of rising and sinking air, anticipate potential wind direction and speed based on isobar spacing and curvature, and recognize typical summer surface pressure patterns, such as thermal lows in the desert Southwest and broad subtropical highs in the Southeast.

Figure 6.3.2: Surface Analysis for the Continental United States

Surface weather map of the continental United States valid at 01Z on July 20, 2020. This composite map shows multiple features including:

• Isobars (lines every 4 mb) representing sea-level pressure patterns,
• Surface fronts (plotted as of 21Z) including cold fronts (lines with triangles), warm fronts (lines with semicircles), stationary fronts, and troughs,
• Radar reflectivity indicating precipitation intensity,
• Surface observations at individual stations (using standard station models showing temperature, dew point, wind, pressure, and cloud cover).

A few key features include:

• A low-pressure system is centered over the Desert Southwest (southern Arizona), marked as 1002 mb, with counterclockwise flow around it. This is likely a monsoonal thermal low, common in summer over this region, accompanied by radar returns showing scattered convection (thunderstorms).
• Another low is located over the Great Lakes, centered near Michigan, with a strong cold front extending southwest through the Midwest and into the southern Plains. This front is followed by cooler, drier air and is associated with a line of thunderstorms, seen on radar from Illinois to Tennessee.
• A stationary front lies across the central Plains and into the Ohio Valley, where scattered showers and storms are visible on radar, indicating an active boundary with warm, moist air on one side and cooler air on the other.
• Several high-pressure centers ("H") are noted:
  • One over the Pacific Northwest (~1020 mb),
  • Another across the central U.S., around Oklahoma (~1012 mb),
  • A third over the Southeast and Florida (~1018 mb), promoting more stable conditions in those areas.
• Widespread radar echoes, especially in the Midwest and Southeast, indicate areas of active precipitation, some likely convective (thunderstorms), with stronger reflectivity (yellows and reds) indicating heavier rainfall.
• Surface station plots show warm and humid conditions across much of the country, with temperatures commonly in the 70s to 90s °F, and dew points mostly in the 60s to 70s °F, confirming unstable air in many regions.

Figure 6.5.1: Surface Analysis for Southeast United States

Surface station plot for the southeastern United States valid at 19Z (2:00 PM CDT) on August 30, 2017. The map displays surface observations from numerous locations using standard station models, which include:

• Temperature (in degrees Fahrenheit),
• Dew point,
• Sea-level pressure (in milibars with leading "10" or "9" omitted),
• Pressure tendency,
• Cloud cover (represented by the shading of the circle),
• Wind barbs (showing wind direction and speed extending from the circle),
• Present weather symbols (e.g., rain or fog), where applicable.

Across the broader region:

• Temperatures mostly range from the upper 70s to mid-80s °F.
• Dew points are generally high—in the 70s °F—indicating moist and humid surface conditions.
• Winds vary, but southerly to southeasterly surface flow dominates much of the Southeast.

Over Louisiana and parts of Mississippi and eastern Texas, there is a clear cyclonic (counterclockwise) wind pattern. Wind barbs surrounding this region show:

• Easterly winds across southern Mississippi and Alabama,
• Southerly to southeasterly winds over southeastern Louisiana,
• Westerly winds in southern Texas,
• Northerly winds in northern Louisiana and southern Arkansas.

Cloud cover in this region is generally overcast, and the high dew points indicate the presence of deep moisture, supportive of widespread rainfall or ongoing showers—though specific present weather symbols are limited in this plot.

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