Explosive cyclogenesis
Explosive cyclogenesis, also known as bombogenesis or bomb cyclones, is a meteorological phenomenon characterized by a rapid drop in atmospheric pressure within a 24-hour period, leading to the formation of powerful storms. These events typically occur during cooler seasons and are most common in the middle latitudes, particularly in regions like the North Atlantic and Northwest Pacific. The storms form when warm and cold air masses collide, resulting in significant atmospheric pressure changes that can lead to intense winds, heavy snowfall, or even flooding, depending on the temperature conditions.
While similar to hurricanes, explosive cyclogenesis events are not as widely recognized, often due to their tendency to form over oceans. Meteorologists define these storms using specific measurements of pressure drop—at least 19 millibars in 24 hours at 45 degrees latitude. These storms, which can occasionally reach intensities comparable to Category I hurricanes, are notable for their sudden onset, which can lead to severe weather impacts, including blizzards and flooding. Understanding explosive cyclogenesis is crucial for weather forecasting, especially as these storms can cause significant disruptions and damage when they strike coastal and inland areas.
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Explosive cyclogenesis
Explosive cyclogenesis is a cool-weather storm that very quickly gains strength. The intensification is caused by a rapid drop in atmospheric pressure in a twenty-four-hour period. These storms are also referred to as bombogenesis, meteorological bombs, weather bombs, or bomb cyclones. They are similar to hurricanes but happen mostly during cooler seasons. They are not uncommon, occurring around forty times a year in the Northern Hemisphere. However, they are not as well-known as some other weather phenomena because they are more likely to occur over oceans.
Background
Earth’s weather is strongly affected by global winds. Because of the planet’s tilted axis, the sun heats some parts of Earth more than others. This uneven warming creates differences in air temperature, which causes air to move at different speeds, forming wind that moves across the globe. The hottest air is generally near the equator, the imaginary line around Earth’s center. This warm air rises high into the atmosphere and spreads toward Earth’s poles. Once this air travels reaches about 2,000 miles (3,219 kilometers) above the equator, it falls back to the surface and forms convection currents.
Earth’s six convection currents push large, thick masses of air in front of them. Some of these masses are warm and humid while others are cool and dry. A front is created when masses of two different temperatures meet. Cold fronts are formed when cold air replaces warm air, and warm fronts are formed when a warm air mass replaces a cold air mass. The movement of these masses is impacted by strong high atmospheric winds called jet streams, Earth’s geography beneath them, and several other factors. Sometimes these factors combine in a way that changes atmospheric pressure of the air against Earth. When this happens, weather phenomena such as storms, hurricanes, and explosive cyclogenesis can occur.
Overview
Explosive cyclogenesis is caused when an area of high atmospheric pressure drops toward Earth’s surface and pulls a large amount of cold air with it. As it moves closer to Earth’s surface, the atmospheric pressure begins to drop. When the pressure drops quickly in a twenty-four-hour period, it results in what meteorologists call explosive cyclogenesis.
In America, New Zealand, and a few other countries, this condition is often referred to as a bomb cyclone. This term was coined in 1980 by Massachusetts Institute of Technology (MIT) professor Frederick Sanders and graduate student John R. Gyakum. They wanted to draw awareness to the intensity and potential destructive power of the sudden changes resulting from these storms.
Meteorologists use very specific measurements to determine when an explosive cyclogenesis occurs. These measurements are based on the atmospheric pressure and latitude at which the event is occurring. Atmospheric pressure is also known as barometric pressure, after the barometer, or device used to measure the pressure. The unit of measurement of barometric pressure is a millibar. Latitude is a measurement of how far a specific place on Earth’s surface is from the equator. It is measured in degrees ranging from zero through ninety. During a bomb cyclone, the pressure falls at least nineteen millibars in twenty-four hours at a latitude of forty-five degrees or twenty-three millibars in twenty-four hours at fifty-five degrees of latitude.
These events require a strong jet stream, which refers to the strong winds that blow high in Earth’s atmosphere. Warm ocean currents also play a factor and are part of the reason explosive cyclogenesis occurs most often at sea. The conditions that favor bomb cyclones are most common in the North Atlantic, South Atlantic, Northwest Pacific, and Southwest Pacific regions.
The development of explosive cyclogenesis is dependent on the collision of warm and cold air masses. This is why these storms are most likely to occur in the middle latitudes of about forty-five to fifty-five degrees. The presence of both warm and cold air fronts is most common in these areas during the cool weather months, such as during the late fall through early spring in North America.
The sudden drop in atmospheric pressure is caused when warm and cold air masses come together. As the air temperature attempts to regulate in the area between the two masses, it triggers a rapid rise of air from near the planet’s surface. This causes winds to begin to swirl and spiral. These upward spiraling winds suck air out of the top of the system. When the air rises more quickly than it is replaced at the bottom of the growing storm system, the atmospheric pressure drops. As more air is pulled out of the system at the top, the atmospheric pressure decreases more and more. The lower the pressure drops, the more intense and larger the storm becomes.
Despite the very dangerous sounding names, explosive cyclogenesis storms are not necessarily stronger than normal winter storms. Much of the potential for damage and harm from these storms comes from the suddenness with which they strike. However, some bomb cyclones are as intense as a Category I hurricane. They can cause very strong winds as well as heavy snow and blizzard conditions. In some cases, when the storm occurs in slightly warmer conditions, explosive cyclogenesis can result in heavy rains and flooding. For example, an explosive cyclogenesis event in early January 2018 resulted in snowfall up and down America’s East Coast. During that storm, the atmospheric pressure dropped fifty-nine millibars in twenty-four hours, more than twice the drop used to define these storms. More than a foot of snow fell in parts of New England. In Florida, freezing temperatures and snowfall paralyzed communities with no equipment for coping with ice and snow. A record high tide driven by strong winds closed subways and caused road flooding sufficient to float dumpsters and vehicles down the streets of Boston.
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