Meteorologists call a storm a bomb when its pressure plunges fast enough
If a mid-latitude storm's central pressure drops by more than a millibar an hour, forecasters call it explosive cyclogenesis, and the storm earns the nickname bomb. These extratropical cyclones, fed by clashing air masses rather than warm seas, shape most of the weather across Earth's middle latitudes.
Extratropical cyclones, also called mid-latitude or wave cyclones, are low-pressure systems that, together with high-pressure anticyclones, drive the weather between roughly 30 and 60 degrees of latitude. They can deliver anything from cloud and light showers to hail, thunderstorms, blizzards and tornadoes. Unlike hurricanes, they draw energy from temperature contrasts along fronts, which is why they are classed as baroclinic.
They typically form along bands where temperature and dew point change sharply and winds shift strongly with height, near a favourable part of a jet streak, a core of fast wind in the upper-level jet stream. Strong forcing aloft can make pressure fall very quickly, and bombs can sink below 980 millibars. The Coriolis effect makes air spiral anticlockwise around northern storms and the opposite way south of the equator, with gusts strongest just poleward of the fronts where pressure changes most steeply.
Their structure is the reverse of a tropical storm's. They tilt back into cold air and strengthen with height, sometimes extending deeper than 9 kilometres, and their centres grow colder than surrounding air aloft, earning the name cold-core lows. Late in life, as heat evens out around the centre, they can become barotropic.
Hurricanes often end their lives by turning into extratropical storms, usually between 30 and 40 degrees latitude, as upper-level troughs in the westerlies take hold. The system tilts into colder air, its fuel switches from heat released by thunderstorm condensation to temperature contrasts, and its warm core disappears. In the North Atlantic, the reverse subtropical transition peaks in September and October. Forecasters use two main life-cycle models: the older Norwegian model, built from surface observations during the First World War, and the Shapiro-Keyser model.
Source: Extratropical cyclone
