Today: Tue 23 May 2017

Sea Level Pressure and 1000-500mb Thickness

The thickness is the vertical distance in meters from the 1000 to the 500 millibar level. The 1000 to 500 mb thickness is a function of two properties, (1) the average temperature of the air between 1000 and 500 millibars and (2) the average moisture content of the air between 1000 and 500 millibars. These two properties are combined together to produce the virtual temperature.

  • 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 shaded 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 in black, 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).

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