Explained: What Are Bomb Cyclones And How Do They Form?
A dangerous weather phenomenon called a bomb cyclone that occurs in mid-latitudes - between Earth's tropics and the polar regions - can bring strong and damaging winds, torrential rains, heavy snowfall, flooding and frigid temperatures.
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- WHAT IS A BOMB CYCLONE? A bomb cyclone, also referred to as explosive cyclogenesis or bombogenesis, is a mid-latitude cyclone that has rapidly intensified. A cyclone is a low-pressure weather system - one where the atmospheric pressure is lower at its center than in surrounding areas - with winds rotating inward. It circulates in a counterclockwise direction in the northern hemisphere and a clockwise direction in the southern hemisphere. A bomb cyclone's winds can reach hurricane force - 74 miles (119 km) per hour - and stronger. These storms tend to form during winter and can spawn copious amounts of precipitation. They have life spans of about a week during which they grow to peak intensity over roughly four to five days and then dissipate over the last two, according to Jon Martin, a professor of meteorology at the University of Wisconsin at Madison.
- HOW DOES A BOMB CYCLONE FORM? Bomb cyclones form when the conditions at the surface and at the jet stream level are ideal for the storm to intensify. The jet stream is a narrow band of strong winds in the upper atmosphere. A variety of atmospheric processes combine to produce these storms. Almost all bomb cyclones have a precursor disturbance in the winds in the middle part of the troposphere - the lowest region of Earth's atmosphere - about 3-5 miles (5-8 km) above the planet's surface, Martin said. Another important feature common to many, but not all, explosive cyclogenesis events is a warm ocean surface. Many of the most intense bomb cyclones form over oceans. Precipitation can be prodigious. When water vapor changes into liquid and ice, as it does in these storms, enormous amounts of energy - called latent heat energy - are released. Some of that energy further intensifies the storm. By virtue of the atmospheric pressure getting so low, differences in pressure across the storm can become very large, powering strong winds that can have devastating effects.
- WHEN AND WHERE ARE THEY MOST LIKELY TO FORM? Explosive cyclogenesis occurs mostly over oceans and most commonly during the cold season in both hemispheres - roughly November to March for the northern hemisphere and roughly May through August in the southern hemisphere, though these storms can be earlier or later than that. Areas particularly prone are situated in so-called storm tracks along the east coast of continents because this is where the warmest ocean currents exist, such as the Kuroshio off Japan and the Gulf Stream off North America, Martin said. Bomb cyclones can be very destructive and pose particular danger to shipping interests, since many of them occur over the oceans, according to John Knox, an atmospheric scientist and professor of geography at the University of Georgia. Some bomb cyclones have occurred on the Great Lakes of North America and caused shipwrecks there, too, Knox said.
- WHAT HAPPENS TO THE ATMOSPHERIC PRESSURE? The average sea-level atmospheric pressure at middle latitudes is about 1012 millibars, or mb. In cyclones, this gets down to as low as 980 mb with regularity. With bomb cyclones, it can drop to 950 mb or lower, and the rate at which they intensify is at least 24 mb in 24 hours.
- WHY CAN WE NOT CALL IT A HURRICANE? While bomb cyclones can unleash hurricane-force winds and sometimes display characteristics of a hurricane, they are not hurricanes. They form from different physical processes and do not have the symmetry of hurricanes, which also are low-pressure systems. Bomb cyclones occasionally develop "eyes" resembling those at the center of a hurricane, Knox said. But a bomb cyclone has its origins in the mid-latitudes and is associated with weather fronts - a boundary between two air masses with different characteristics such as temperature - and a strong jet stream, Knox said. A hurricane originates in the tropics and is not associated with either weather fronts or a strong jet stream, Knox said.
- ARE BOMB CYCLONES BECOMING MORE COMMON? Global climate change, according to experts, is causing more frequent and more extreme weather events around the world. But are bomb cyclones becoming more common or more intense? Martin said it is not clear whether that is the case. The fact that Earth is warming has ramifications for cyclone dynamics that scientists are currently trying to figure out, Martin said. A warmer planet means more water vapor in the air and that would tend to make at least the latent heat portion of the empowerment of these storms stronger, Martin said. However, the warming is not uniform, Martin said. Since observations suggest more warming at high latitudes, Martin said, this could render the bomb cyclones weaker in general.
