If you saw any local news this week you undoubtedly saw the damage storms earlier this week caused in the Sealy area. The scale and severity of the damage initially led people to think a tornado hit, but upon further investigation the National Weather Service determined the damage was actually caused by a strong wind event called a microburst. Although a microburst can cause just as much damage as the typical type of tornado we see in Southeast Texas, these events usually receive little attention. However when you understand what a microburst really is you can see how they can be just as dangerous as a tornado.
A microburst, also known as a downburst, results from the core of a thunderstorm plummeting to the ground. When this rush of air hits the ground it spreads out in all directions, much like what happens when you pour a cup of water on the ground. When that air hits the ground it can reach speeds over 60 mph, and in Sealy this week winds were estimated to be near 100 mph if not a bit higher. Although the resulting damage can be similar to that of a tornado the distinction can be made through the pattern in which debris is dispersed. A tornado will leave swirling or curved pattern to the debris field, but with a microburst debris is all blown in the same direction radiating out from where the microburst in hit the ground.
So how does a microburst happen? One of the main ingredients in creating a microburst is actually dry air. Yes dry air in a thunderstorm – seems weird right? While it may be moist and humid in the lower few thousand feet of the atmosphere, the air can be much drier in the higher levels. As thunderstorms grow taller they can suck in this drier air which starts to evaporate water droplets inside the storm. The evaporation of water results in cooling temperatures (why you feel cold when you get out of a pool). As precipitation inside the storm evaporates the air temperature decreases causing the air become more dense and sink. As the environment gets progressively warmer closer to the ground this bubble of cold air accelerates as it descends. In the case the storms this past week weather models showed that the atmosphere over the Sealy area had quite a bit of dry air present and a steep temperature change with altitude as the storms rolled in.
The other factor contributing a microburst is called precipitation loading. In strong storms the updraft – rising warm moist air fueling the storm – will be strong enough to suspend precipitation, including hail, inside the storm. The large hail seen in the Sealy area is a good indication that there was a very strong updraft present. The precipitation being held in the cloud by the updraft creates a huge amount of weight. When a microburst occurs the bubble of cool dense air rushing down from higher up combines with the weight of precipitation in the cloud becoming too much for the updraft to hold and gravity takes over. The result is the storm essentially explosively vomiting its guts on to the surface below. If you are below a microburst you will just see very high winds, something like this. However as seen from a distance you can see how the bottom drops out of the storm and crashes to the ground (also see the GIF at the top of this post).
A perfect combination of ingredients came together to create a microburst as strong as the one that happened in Sealy this week. These types of occurrences should be reason enough not to discount a severe thunderstorm just because it doesn’t have a tornado.