The coloured contours indicate sea level pressure in millibars.
High pressure is red, low pressure is green or blue.
Only the last two digits are shown. Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather. The black contours indicate the vertical distance, or the thickness, between the 1000 millibar surface and the 500 millibar surface, mea- sured in tens of meters. Since the air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels.