850 mb Maps The 850 mb map provides a view of the atmosphere above the boundary layer where diurnal temperatures are strongly influenced by warming and cooling of Earth s surface The 850 mb chart includes raw data at each station height contours and isotherms Figure 9 15 Meteorologists use the 850 mb layer to find areas influenced by the movement of warm and cold air masses Warm air advection exists when winds blow across isotherms from warmer to colder areas cold air advection occurs where winds blow across isotherms from colder to warmer areas During the winter months information on an 850 mb map can help forecasters determine the likely form of precipitation Forecasters often look at the 0 C isotherm at 850 mb as the division between snow and rain 1380 1380 1410 1440 1410 1470 1500 10 L 1516 10 H 1528 1410 1380 1350 H 1534 1530 1320 H 1538 10 What range of heights do you see in this example of the 850 mb surface 11 Circle and label areas of warm and cold air advection on the 850 mb map 12 Highlight the 0 C isotherm 1320 1350 1380 1410 1440 1260 1320 1290 1230 1350 1530 Figure 9 15 An 850 mb map 1560 1470 H 1612 10 10 1590 1500 1590 1530 0 1560