Microbursts

(credit: Meteorologist Ash Bray)

Microbursts, coined by Dr. Testsuya “Ted” Fujita, are immensely powerful, localized columns of wind that occur when cooled air falls from the base of a thunderstorm at exceptional speeds – upwards of 60 MPH – and subsequently hit the ground, sprawling out in all directions. Upon the column of air reaching the ground and expanding outwards, it produces straight-line winds, which are capable of reaching speeds up to 100 mph – the equivalent of an EF 1 tornado on the Enhanced Fujita Scale, according to the National Oceanic and Atmospheric Administration (NOAA). Microbursts are more than capable of wreaking havoc by demolishing trees and powerlines, as well as causing extensive damage to buildings.

The most common weather event leading up to the formation of a microburst is dry air entrainment, which occurs when dry air mixes with precipitation inside of the storm cloud. The dry air then causes the droplets to evaporate, resulting in a rapid decline of air temperature at the top of the thunderhead. This patch of cooled air then begins to sink, gaining ample momentum as it falls, essentially turning into a speeding column of air. When this cool, dry air is pulled further down by the weight of precipitation, it becomes “water loaded”and falls to the surface rapidly.

Microbursts can be divided into two classes: wet and dry. Where you reside throughout the country will determine as to which class you’re most likely to experience. For example, if you reside in the Southeastern United States where conditions are primed for thunderstorms, you’re more prone to experiencing a wet microburst. Wet microbursts are typically fueled by both water loading and dry air entrainment. Dry microbursts normally begin with dry air entrainment from moisture within the upper levels, eventually turning into wind-driven weather events with minimal surface precipitation. These typically occur when the relative humidity within the upper atmosphere is moist yet drier beneath the surface. When this occurs, a storm can feed off of moisture high within the storm. As it produces precipitation, the precipitation falls into the dry air, evaporating and cooling the air around it. The Southwestern United States is more likely to experience this phenomenon.

There are microbursts that share both the wet and dry characteristics and are referred to as “hybrids”. Hybrids are fueled by multiple influences such as water loading, dry air entrainment, cooling beneath the base of the cloud and/or sublimation – ice crystals forming directly into vapor.

Though they’re far more common than tornadoes, microbursts are not as well-known. The National Weather Service estimates that for every single tornado, there are roughly ten microbursts reported. While there is not a specified study on how many microbursts occur on average, it is accepted that most wind damage occurring within thunderstorms is likely due to microbursts. The damages caused by microbursts can lead one to believe at first that they’ve been struck by a tornado. The sure-fire way of knowing whether a tornado struck is to study the damage pattern. Tornadoes tend to leave behind a more circular trail of destruction, while microburst winds create straight-line wind damage that typically have a central point of impact.

In terms of age, the study of microbursts is still relatively new within the atmospheric science world. Despite overwhelming advances in technology, it’s still difficult to detect and forecast microbursts. Meteorologists are able to predict an environment that may favor microbursts, but it’s not possible to predict an exact location or a specific storm that may produce a microburst. When forecasting for prime conditions, radar is by far the most helpful tool. Radars show air colliding above the Earth’s surface, which commonly means some of that air is being forcefully pushed downward. Radar technology also has the capability of showing air diverging or spreading outwards in the lower atmosphere right above the Earth’s surface, giving strong indication that a microburst is occurring. As with any form of technology, radar does have its limitations. For example, if a microburst forms on the outer rim of the radar’s scanning circumference, it may appear as a small blip that the meteorologist misses. It also doesn’t help that microbursts can form so rapidly that one could crash into the surface before a warning can be issued.

While there is no guaranteed way to predict microbursts, the best thing one can do to protect themselves is to stay weather aware. This can best be accomplished by paying attention to any severe thunderstorm alerts issued by your local National Weather Service.