Shows difference in pressure since last lap. Use in mode with barometric pressure chosen, not auto. And use it when you don't change altitude (like longer rest). Unfortunately I cannot add alarm sound for this. "6h before storm a drop in pressure of -1.5 mbar/h and 1h before the storm there is -2mbar/h." http://wiki.answers.com/Q/What_are_warning_signs_of_a_hurricane