Microburst and Macroburst – Understanding Downbursts

Downburst Microburst and Macroburst

A microburst and a macroburst are two forms of downburst. Meteorologist Dr. Ted Fujita coined the terms “downburst”, “microburst”, and “macroburst” in the 1970s. A downburst is a powerful, localized downdraft that spreads out rapidly when it hits the ground, producing damaging winds similar to those of a tornado. A typical event lasts anywhere between a few seconds to several minutes. Downbursts occur in association with thunderstorms and are potentially destructive.

Microbursts and Macrobursts

Most of the time, you hear about microbursts because they are the most common type of downburst.

  • Microbursts: These are small downbursts with an affected area less than or equal to 4 kilometers (2.5 miles) in diameter. Despite their size, microbursts produce intense winds potentially exceeding 100 mph (160 km/h). Microbursts can occur with any thunderstorm, but are common in the Great Plains and Southeastern portion of the United States during the summer months.
  • Macrobursts: These are larger downbursts affecting an area greater than 4 kilometers (2.5 miles) in diameter. Macrobursts typically last longer than microbursts and produce wind speeds over 130 mph (210 km/h). The same areas prone to microbursts also experience macrobursts, just less often.

Dry and Wet Downbursts

The two main categories of downbursts and dry and wet downbursts. Both occur in connection to thunderstorms and cumulus or cumulonimbus clouds. Most downbursts happen along the leading edge of the storm, although some also occur on the trailing end of a storm.

Dry Downbursts

Dry downbursts occur in environments with low humidity. They occur in the absence of significant precipitation, sometimes with virga. These downbursts form with high-based thunderstorms where rain evaporates before reaching the ground, cooling the air so it descends rapidly.

Wet Downbursts

Wet downbursts are accompanied by heavy precipitation. These occur when a thunderstorm’s downdraft drags rain-cooled air to the surface, leading to intense rainfall and strong winds upon reaching the ground.

Heat Bursts

A heat burst is a rare atmospheric phenomenon characterized by a sudden increase in temperature and a decrease in humidity. Heat bursts typically occur at night or in the early morning in connection with decaying thunderstorms.

Heat bursts form when a downdraft descends through a very dry layer of the atmosphere, warming the air adiabatically as it compresses upon reaching the surface. This results in a sudden, sharp increase in temperature.

Conditions of Microburst Development

Downbursts form under the following conditions:

  • Strong thunderstorms with significant updrafts and downdrafts
  • High instability in the atmosphere
  • Presence of dry air in the mid-levels of the atmosphere (especially for dry downbursts)
  • High moisture content near the surface (especially for wet downbursts)

Stages of Development

  1. Formation Stage: In this stage, a downdraft forms within a thunderstorm due to cooling from evaporation of precipitation or melting hail.
  2. Descent Stage: The downdraft accelerates downward, driven by the negative buoyancy created by cooling.
  3. Impact Stage: The downdraft hits the ground and spreads out horizontally, creating strong surface winds.
  4. Dissipation Stage: The winds gradually weaken as the downdraft spreads out and loses momentum.

Specific Conditions for Wet and Dry Downbursts

Wet Downbursts

  • High moisture content at lower levels of the atmosphere
  • Thunderstorms with significant rainfall
  • Presence of a strong downdraft that pulls rain-cooled air to the surface
  • More frequent in areas with abundant moisture such as the southeastern United States, the Indian subcontinent during monsoon season, and tropical regions.

Dry Downbursts

  • Low humidity levels, particularly in the mid-levels of the atmosphere
  • High-based thunderstorms where precipitation evaporates before reaching the ground
  • Significant cooling from evaporation leading to strong downdrafts
  • Common in arid and semi-arid regions such as the southwestern United States and parts of Australia.
Microburst and Airport

Risks Associated with Downbursts

Downbursts pose significant risks, including:

  • Aviation Hazards: Sudden and severe wind shear are particularly dangerous for aircraft during takeoff and landing. For example, in 1985, Delta Air Lines Flight 191 crashed while attempting to land at Dallas/Fort Worth International Airport due to a microburst, resulting in 137 fatalities.
  • Structural Damage: High winds damage buildings, uproot trees, and cause widespread power outages. Downbursts are often the building blocks of derechos, which are a line of related microbursts. A 1977 derecho event in Wisconsin and Michigan produced winds comparable to a Category 1 hurricane and caused widespread damage.
  • Safety Risks: People caught outdoors during a downburst can be injured or killed by flying debris, much like in a tornado or hurricane.

Microburst Detection and Warnings

Meteorologists detect downbursts using several methods:

  • Doppler Radar: Identifies areas of intense wind shear and divergence.
  • Wind Profilers: Measure wind speeds and directions at different altitudes.
  • Surface Observations: Automated weather stations and human observers report sudden changes in wind speed and direction.

However, microbursts are short-lived events, so there really isn’t any warning specific to them like a forecaster gives for a tornado. When the conditions are favorable for a microburst, meteorological agencies issue severe thunderstorm warnings that highlight the risk of damaging winds. These warnings alert the public and aviation authorities to take necessary precautions.

Microburst vs Tornado

Both microbursts (or any downburst) and tornadoes are extreme weather events with high winds that are associated with thunderstorms. Both cause comparable damage. However, the two events have different causes and damage paths.

FormationDowndraft from a thunderstormRotating column of air from a supercell
Wind SpeedsUp to 130 mph (210 km/h)Up to 300 mph (480 km/h)
DurationTypically a few minutesCan last from a few minutes to over an hour
Damage PathStraight-line damageCircular or spiral damage
DetectionDoppler radar, wind profilersDoppler radar, storm spotters
WarningsSevere thunderstorm warningsTornado warnings
LocationsWorldwide, more common in mid-latitudesTornado Alley in the US, other regions with supercell storms