The colored contours indicate sea level pressure in millibars.
High pressure is red, low pressure in green or blue.
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 thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters. Since 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. Thus, low values of thickness mean relatively cold air.