Happy #WeatherWednesday everyone! It’s severe weather season! Last week we talked about supercells! They offer some of the most damaging weather hazards to life and property. Today we will discuss microbursts!
Thunderstorms are formed when air rises and condenses in a conducive atmosphere for storms. What goes up must come down! As you probably know from learning about the water cycle in school, ample condensation in a cloud leads to precipitation as the mass (water, hail or snow) falls out of it due to gravity. As raindrops develop from droplet growth and collision/coalescence, they get too heavy to remain buoyant in the cloud.
Under certain conditions, a portion of cloud mass descends (on the order of a fraction of a mile to a few miles in area) at a higher acceleration than simply gravitational acceleration. This is called a microburst. Micro- is a prefix to describe small quantities, and in meteorology micro fits a description of an average storm cell, or what’s happening in just several square miles spatially. The second part of the word “burst” is the physical part of the event, which accurately describes how air/precipitation crashes downward faster than normal.
A microburst can happen anytime and anywhere, but the preferred environment for them consists of a relatively dry layer between the surface and the base of the cloud. One thing to note is that cold air is denser than warm air, and will sink in a fluid atmosphere while warmer air rises. If descending mass from a storm becomes colder than surrounding mass, the density gradient will contribute more downward force in addition to the inherent gravitational force.
If precipitation encounters considerable dry air, then a little, some, or all of it will evaporate. As a result the descending air cools due to latent heat absorption. From chemistry, you may have learned that water’s evaporation phase requires thermal energy to change from liquid to gas. This type of process causes the precipitation to absorb energy around it to change phase, cooling the air as it falls and letting the water vapor escape from the air column. This situation causes the air to sink faster, eventually crashing it to the ground.
A microburst can be damaging, and can cause as much damage as weak tornadoes…although a bit more widespread. The average microburst lasts a minute or two as the environment can equalize itself quickly. The strongest bursts can cause 100mph winds at the surface, called straight-line winds. This name is given to characterize the wind experienced as the microburst hits the ground and spreads outward radially from its center. Wet microbursts which haven’t evaporated out all of its water content will have a feature called a “rain foot” where the microburst air/water propels out just before hitting the ground or even curls back in on itself from the flow of the burst.
Radar technology is evolving with time to better detect microbursts, although they cannot be accurately forecast in advance. Even with perfect downdraft conditions in an environment, the development of a microburst isn’t a guarantee. Much like tornado forecasting! The most vulnerable infrastructure to microbursts is aviation. Turbulence and rapid changes in airspeed can hinder a pilot’s ability to keep a plane airborne and on course. There have been deadly accidents in the past from flight through microbursts before meteorologists knew much about them. Now we can observe and measure them as they’re early in progress and can react accordingly.