500 Millibar Chart. Approxximately 18,000 feet above sea level. AGL

At this height of the 500 mb, 1/2 of the Earth's atmosphere is below.

1. Solid lines (Contours) are constant heights of the 500 mb height above sea level in meters. AGL

2. Black arrows are the jet winds, 50 mph or greater, called, "Isotachs".

3. Circles are staion locations.

4. Clear circles are areas of low moisture.

5. Dark circles are areas of high moisture content.

6. Looking at Pittsburg, Pa. The circle is dark: High moisture content. -29 is the
temperature in degrees Celsius. 5 is the temperature - dew point spread. 544 to the
right of the dark circle is the height of the 500 mb in meters above sea level.
AGL. The zero (0) is dropped to save room on the chart. Example 544 = 5440 meters.

7. The shaded areas in Green are
areas of high moisture content.

8. The "H" is and area of high pressure, colored "Blue".