When the conditions are just right, a rare event characterized by a sudden downpour of rain with extreme wind gusts occurs during the development of a severe storm. This is better known as a microburst.

A microburst usually occurs during a thunderstorm or heavy rain shower and often is relatively short-lived. It also dissipates as quickly as it arrived.

Wind speeds can reach up to 160 km/h (100 mph) as the air hits the ground and disperses. As a result, this phenomenon often gets confused with a tornado or hurricane.

This article examines what exactly a microburst is, how it develops, and also looks at the different types of microbursts.

Meteorologist Ted Fujita coined the term “microburst.” He defined it as a downburst that occurs over a small area, affecting a region with a diameter of no more than 4 kilometers (2.5 miles).

(He was also responsible for the term “macroburst” to identify downbursts larger than 4 kilometers (2.5 miles).)

Although a microburst displays many of the characteristics of a tornado (and sometimes a hurricane), it is a completely different type of meteorological event. Before examining how it develops in more detail, one needs to define what precisely a microburst is:

Microburst Definition

A microburst is a powerful localized downdraft created by a column of sinking air through the base of a storm or rain cloud. This meteorological phenomenon can be divided into dry and wet microbursts, both of which can cause severe damage to the surface below and surrounding objects in their path.

With a clear understanding of what a microburst is, it is important to know how it develops. This will help explain not only how and why it occurs, but also why it displays characteristics unique to this phenomenon.

A microburst starts to form in the upper part of a storm cloud, where pockets of cold, moist air accumulate and continue to build up. Two main factors trigger and contribute to a column of cold air beginning to drop and accelerate toward the ground:

1. 1. Precipitation Loading
   2. Evaporation Cooling

Sometimes, precipitation loading on its own can initiate a microburst, but it usually is a combination of both, working together to create a downburst with potentially devastating effects. A closer examination of each will show how they contribute to the occurrence.

Precipitation loading is the raindrops, hail, ice crystals, graupel, and other forms of water that accumulate in the upper regions of a storm cloud. It gets carried up and kept airborne by the strong updrafts that occur within a large storm system.

Water carries a lot of weight, and when updrafts are no longer able to keep it in the air, it starts to drop to the ground. Sometimes it is the sheer weight of the moisture, combined with a dissipating storm system and weakening updrafts, that triggers the event.

In the upper troposphere, external cold, dry air also comes in contact with the moist air in the cloud. It causes the moisture to start evaporating. Since evaporation is a cooling process, the air in the cloud begins to cool down.

The resulting pocket of cold air is much denser and heavier than the surrounding warmer air. As a result, this column of cold, heavy air will start to sink to the ground.

When the sinking air exits the cloud base, it comes in contact with more dry air. Here, evaporation continues to take place, which cools the air down even further. As a result, the cold air drops even faster and accelerates towards the ground.

The actual occurrence of a microburst can be broken down into three primary stages:

1. 1. Contact Stage
   2. Outburst Stage
   3. Cushion Stage

By looking at each of these three stages individually, it will be easier to understand the complete process through which a microburst develops:

During the initial stage of a microburst, cold air (often accompanied by raindrops) exits the cloud base and continues to accelerate to the ground.

The downdraft rushes towards and hits the ground at what is called the contact point (also called the splashdown point), from where strong winds diverge outwards.

(For clarity, you can use the image of a water-filled balloon, dropped from a height and bursting open when it hits the ground, with water being dispersed in all directions. The contact stage of a microburst closely resembles this image.)

The name of this stage itself is quite self-explanatory. During this stage, powerful winds are dispersed away from the splashdown point after contact with the ground.

These winds travel along the surface and can reach speeds of up to 160 km/h (100 mph) or more, powerful enough to cause severe damage to the environment and structures. Winds of this speed can flatten large trees and seriously damage large structures.

Depending on the type of microburst, the winds can be accompanied by heavy rain, which can add to the damage caused and pose additional dangers as well.

In the final stages of the event, a layer of cold air forms at the surface where the downdraft first made contact with the ground. This cold air accumulated from the start of the microburst and built up as it progressed.

While the air, already at the surface and outer limits of the affected area, continues to blow strongly, the “cushion” provided by the layer of cold air below the downdraft prohibits any more air from reaching the ground.

This final stage marks the beginning of the end of a microburst, which will blow itself out shortly after no more wind from the downdraft can reach the ground. The duration of a microburst can last anything from a few seconds to a couple of minutes.

As I briefly touched on in the summary of what a microburst is, there are mainly two forms of this event:

A dry microburst occurs when no rain is present in the column of air reaching the ground. It happens when there is very little moisture present in the cloud, and the cloud base is situated at a relatively high altitude.

The little rain that emerges from the cloud base with the downdraft quickly evaporates in the dry air below the cloud before it can reach the ground (also known as virga). As a result, it is only the dry air that reaches the ground and gets dispersed in multiple directions.

A wet microburst occurs when a combination of heavy rain and wind reaches and gets dispersed over the ground. This happens where there is a high percentage of moisture present in a cloud.

When the combined weight of the different sources of moisture in the top part of the cloud becomes too heavy, it starts to sink. As it drops through the cloud, it drags the surrounding air down with it, which accelerates the speed of the sinking column of air.

Subsequently, a combination of rain and wind is caught in the column of air that crashes into the ground, and both get propelled outwards with potentially devastating results. 

There are three characteristics of microbursts that make them especially dangerous:

• • Sudden onset with little to no warning.
  • Extreme wind shear is especially dangerous for aircraft.
  • Localized but severe damage.

They may not be as well-known as tornadoes and hurricanes, but microbursts are just as severe and destructive. It should be very clear after reading this post.

This article also illustrated that, even though many of the characteristics are the same as those of a tornado, a microburst is an entirely separate weather phenomenon that is formed and develops through very different mechanisms.

Until next time, keep your eye on the weather!