Weather bomb

Explosive cyclogenesis (also referred to as a weather bomb,meteorological bomb,explosive development, or bombogenesis) refers in a strict sense to a rapidly deepening extratropical cyclonic low-pressure area. To enter this category, the central pressure of a depression at 60° latitude is required to decrease by 24 mb (hPa) or more in 24 hours.

This is a predominantly maritime, winter event, but also occurs in continental settings. This process is the extratropical equivalent of the tropical rapid deepening.

In the 1940s and 50s meteorologists at the Bergen School of Meteorology began informally calling some storms that grew over the sea "bombs" because they developed with a great ferocity rarely seen over land.

By the 1970s the terms "explosive cyclogenesis" and even "meteorological bombs" were being used by MIT professor Fred Sanders (building on work from the 1950s by Tor Bergeron), who brought the term into common usage in a 1980 article in the Monthly Weather Review. In 1980, Sanders and his colleague John Gyakum defined a "bomb" as an extratropical cyclone that deepens by at least (24 sin φ/ sin 60˚)mb in 24 hours, where φ represents latitude in degrees. This is based on the definition, standardised by Bergeron, for explosive development of a cyclone at 60˚N as deepening by 24mb in 24 hours. Sanders and Gyakum noted that an equivalent intensification is dependent on latitude: at the poles this would be a drop in pressure of 28 mb/24 hours, while at 25 degrees latitude it would be only 12 mb/24 hours. All these rates qualify for what Sanders and Gyakum called "1 bergeron".

Baroclinic instability has been cited as one of the principal mechanisms for the development of most explosively deepening cyclones. However, the relative roles of baroclinic and diabatic processes in explosive deepening of extratropical cyclones have been subject to debate (citing case studies) for a long time. Other factors include the relative position of a 500-hPa trough and thickness patterns, deep tropospheric frontogenetic processes which happen both upstream and downstream of the surface low, the influence of air–sea interaction, and latent heat release.

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