In 1982, a tragic incident occurred at New Orleans International Airport. A severe downdraft and strong winds hit Pan Am Flight 759, causing the plane to crash shortly after takeoff. A similar catastrophe took place in 1985 in Dallas, Texas. A Delta Air Lines aircraft encountered a strong downdraft, leading to a loss of control, and crashed short of the runway. Both incidents occurred during a microburst-induced wind shear. In this article, we are going to discuss microbursts – weather phenomena that pose significant risks both to aviation and ground-based structures.
Image source: Wikipedia
According to a microburst definition, these are sudden and powerful downdrafts of air that occur in a localized area, typically during thunderstorms. They can cause rapid changes in wind speed and direction, leading to severe turbulence. There are two types of microbursts: wet, which produces heavy rainfall, and dry, which does not produce noticeable precipitation.
Microburst Characteristics
Key features of a microburst weather phenomenon include:
- Size: A microburst is relatively small in scale, typically less than 2.5 miles (4 kilometers) in diameter.
- Duration: A microburst does not last long – its duration is between 5 and 15 minutes.
- Wind pattern: Microbursts produce a distinctive pattern of strong and sudden downdrafts that reach the ground and then spread out horizontally in all directions.
- Wind speed: The winds within a microburst can often exceed 100 mph (160 km/h).
Sometimes microbursts can be confused with downbursts. Both of these weather phenomena are associated with intense downward-moving air currents, but they differ in terms of scale and their impact on the surrounding environment. Here’s a quick comparison of a microburst vs. downburst:
| Microbursts | Downbursts |
|---|---|
| Small-scale weather events | Larger-scale weather events |
| > 2.5 mi (4 km) in diameter | Up to 6 mi (10 km) in diameter |
| Affect small areas | Affect large areas |
How Do Microbursts Form?
Microbursts form due to a complex interplay of atmospheric conditions within a thunderstorm. Here’s how it works:
- Cumulonimbus cloud development. Like tornadoes, microbursts are associated with mature or dissipating cumulonimbus clouds, which are large, towering clouds that develop vertically. These clouds are formed when warm, moist air rises and condenses, creating a vertically layered structure.
- Precipitation formation. Within the cloud, water droplets and ice crystals collide and merge, forming larger raindrops or hailstones. As these particles become heavy, they begin to fall through the clouds.
- Evaporation and cooling. As the precipitation falls through the cloud, it encounters a layer of dry air. This dry air causes the raindrops or hailstones to evaporate rapidly, which leads to the cooling of the surrounding air.
- Cooling and enhanced downdraft. Evaporation causes the air to cool significantly. Cooler air is denser and heavier, and it goes down because of this density. This intensified downdraft creates a localized, powerful downward rush of air.
- Impact. When the descending air reaches the ground, it spreads outward in all directions. The impact of this downdraft hitting the surface can create a strong burst of wind, which is known as a microburst.
- Outflow. As the downdraft hits the ground, it spreads out horizontally, forming an outward flow of air known as an outflow boundary. This outflow can produce damaging straight-line winds over a broad area.
Why Are Microbursts So Dangerous?
Microbursts pose significant dangers and impacts on aviation, especially during takeoff and landing. Here are the potential risks and consequences they present:
Wind Shear
A microburst generates intense downward winds that can abruptly change direction, causing wind shear. Wind shear can lead to sudden changes in airspeed and vertical lift. Pilots rely on predictable airflow during takeoff and landing, so encountering a microburst-induced wind shear can be extremely hazardous.
Loss of Lift
The strong downdrafts accompanying microbursts can rapidly decrease the amount of lift generated by an aircraft’s wings. This sudden loss of lift can result in a significant drop in altitude, making it challenging for pilots to control the aircraft.
Reduced Performance
A microburst produces downdrafts and strong winds, which can impede an aircraft’s performance during takeoff and landing. Reduced lift, increased drag, and decreased engine thrust can lead to longer takeoff or landing distances, potentially causing runway overrun accidents.
Turbulence
Microbursts generate severe turbulence, characterized by rapid and violent air movements. This turbulence can result in injuries and objects flying within the cabin.
Airports and pilots use various strategies of microburst mitigation and detection to minimize the risks associated with these phenomena. For example, pilots receive thorough training on weather patterns, including the identification and characteristics of microbursts.
At the same time, airport traffic control staff constantly monitor weather conditions, including the detection of thunderstorms, which are often associated with microbursts. They may issue a microburst warning and adjust landing and takeoff clearances to allow pilots to avoid areas of potential microburst occurrence.
Microbursts also carry risks for trees and ground-based structures:
- Trees: The intense downdrafts can uproot or break trees, potentially leading to falling branches or entire trees that pose risks to nearby structures and people.
Image source: WCVB, David Querze
Buildings: Microbursts can exert strong lateral forces on buildings, particularly if they are accompanied by strong winds. The combination of powerful gusts and wind direction changes can place significant stress on the structure, potentially leading to structural damage, collapse, or compromised integrity.
Power lines. The strong winds and downdrafts associated with a microburst can pose risks to power lines. Falling trees, debris, or damaged structures can hit power lines, causing outages and hazards from live wires.
Microburst Forecasting
Meteorologists use a variety of technologies and tools to identify and forecast microbursts. One of the key technologies used by meteorologists is weather radar, which plays a crucial role in monitoring and detecting microbursts. Advanced weather radar systems, such as Doppler radar, are especially valuable for this purpose. Doppler radar measures the motion of precipitation particles within a storm, providing valuable information about wind patterns and velocity.
To analyze radar data, forecasters look for air that comes together in the middle layers of the thunderstorm. This is called a mid-altitude radial convergence (MARC) signature. It can be quite difficult to spot these signatures because microbursts last for a very short time and may happen between radar scans. It means that sometimes there may be little or no warning before a severe thunderstorm with microbursts.
A microburst makes a pattern on the radar when it hits the ground. It looks like the wind is spreading out from the center. Like with a tornado, it is possible to spot a microburst on the radar. The red color shows the wind blowing away from the radar whereas the green color shows the wind blowing toward the radar.
Image source: National Weather Service
By integrating these radar observations with other meteorological data, such as satellite imagery, surface observations, and atmospheric models, meteorologists can develop forecasts and warnings for microbursts.
Summary
A microburst is a sudden and powerful downdraft of air that occurs during thunderstorms, posing significant risks to aviation and ground-based structures. It can cause rapid changes in wind speed and direction, resulting in severe turbulence.
Microbursts are relatively small in scale, lasting between 5 and 15 minutes, with wind speeds often exceeding 100 mph (160 km/h). They form within cumulonimbus clouds through a complex process involving evaporation, cooling, and an intensified downdraft.
Microbursts are dangerous due to the strong downward winds that can cause wind shear, loss of lift, reduced aircraft performance, and turbulence. Meteorologists use advanced weather radar systems like Doppler radar to detect microbursts by analyzing the MARC signatures.
