GFS 48 hours Forecasts of Sea Level Pressure
and 1000-500mb Thickness

• The colored contours indicate sea level pressure in millibars. High pressure is red, low pressure in green or blue. Only the last 2 digits shown -- sea level pressure is usually around 1000 millibars, so add 1000 to values in the range of 00-50, and add 900 to values in the range of 50-98.
  • 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.
  • The 540 line is highlighted, since this line is often used as a rule of thumb to indicate the division between rain and snow for low terrain. When there is precipitation where the thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is usually rain (or sleet if the air next to the surface is below freezing).