An isallobar is a line of equal pressure change in whole millibars while height change contours are lines of equal height change in geopotential meters. Each of these isopleths are important because each give information on whether low pressure is deepening and where it is moving. Low pressure develops toward the region with the greatest height and pressure falls. The isallobaric chart is available at: http://weather.unisys.com/surface/sfc_con.php?image=3p&inv=0&t=cur Pressures will fall by low pressure moving closer to the forecast region, low pressure deepening or both processes occurring simultaneously. Height change contours represent the change in geopotential meters from the last geopotential height measurement. If the height 12 hours ago was 3,160 gpm at 700 mb and the current height is 3,100 gpm, the heights have fallen 60 gpm. Heights will fall due to the cooling of air. This is accomplished by low pressure's association with rising and cooling air or CAA. Height change is given for every upper air station plotted on an analysis chart (i.e. 850 mb, 700 mb, 500 mb, 300 mb charts). Add a zero to the value given on the analysis chart. Example: -07 is -70 gpm; +15 is +150 gpm. The height change value is found just to the right of the station plot. Analysis charts for each level of the troposphere are available at: http://www.atmos.albany.edu/weather/difax.html |