Definition
Conventionally the change in mean sea level pressure over the past three hours, coded in synoptic reports. A fall of 1 to 2 hPa in three hours suggests a deepening or approaching system; falls of 3 to 5 hPa in three hours indicate a rapidly developing situation and likely gales. A drop of 24 hPa in 24 hours, adjusted for latitude, defines explosive cyclogenesis, often called a weather bomb.
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