On the 12 Hour Sea Level Pressure Change image, areas of 12 hour positive and negative pressure change are contoured. This image is useful to predict the direction that the low or high pressure systems will likely travel.

Positive pressure change areas are contoured in warm colors and indicate that the pressure is rising. Negative pressure change areas are contoured in dashed cool colors and indicate that the pressure is falling. Both pressure change areas are contoured in intervals of one millibar. Therefore, a positive pressure change contour of 2 would indicate that the pressure has risen by 2 millibars in the past 12 hours within that contour.

This image is useful since low pressure systems (i.e., cyclones) tend to move to regions of the greatest negative pressure changes (where the pressure is falling most rapidly) and high pressure systems tend to move to regions of the greatest positive pressure changes (where the pressure is rising most rapidly). Therefore, from this image one can predict the direction that the low or high pressure systems will likely travel. Also, one can estimate whether or not the system is strengthening or weakening by examining the pressure change. For example, if there is a large drop in pressure ahead of a cyclone, that would indicate that the cyclone is increasing in strength (or as meteorologists say, "the cyclone is deepening").

3hr pres. change

Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign.

pres & mixing ratio