This panel shows the most recent 850mb observations for geopotential height and temperature. 850mb charts depict conditions in the lower troposphere (roughly 1500 meters).
Geopotential height approximates the actual height of a pressure surface above mean sea-level and is represented by the solid light blue contours. The geopotential height field is given in meters with an interval of 30 meters between height lines. The locations of surface cyclones and anticyclones hold similar positions in the 850 mb geopotential height field.
Lines of constant temperature, or isotherms, are represented by the dashed yellow contours and are drawn at intervals of 5 degrees Celsius. Temperature at 850 mb does not experience the diurnal variation (changes with the rising and setting of the sun) as seen at the surface, so this is a good indicator where warm air and cold air is really located. Precipitation type can also be estimated from the 850 mb temperature field. Snow is likely to occur where the 850 temps are below freezing (zero and below), while liquid precipitation is found at warmer temperatures.
Advection of heat and moisture along the 850mb surface can be used to predict what will occur at the surface. Thermal advection can indicate forthcoming surface temperature changes, amplification of upper level troughs and ridges, and changes in pressure at the surface. Moisture advection is useful for predicting areas of precipitation.