- The strong tornado in Springfield, Mass., demolished some buildings but left others nearby standing intact. Why is that? (Photo: Associated Press)
For me, this footage of the Springfield, Mass., tornado is one of the most exceptional videos of a tornado forming I have ever seen. It’s not so much what is happening in the air, but what is happening over the Connecticut River as the tornado forms. You can see the detailed air flow as the surface air is drawn into the developing vortex and the low level spin accelerates as the air streaks into the rotating supercell-thunderstorm updraft. It was a “Tornado Alley” type of supercell, for sure. Look at the Doppler radar image at the time of the tornado:
Within the circle you can see blocks of colors change from red to green right next to each other. This is where the air was moving away from the radar at about 60 m.p.h. while about 100 yards away the air was moving toward the radar at more than 60 m.p.h., creating the classic hook of a tornado vortex signature.
As the air was rapidly spinning into the center of the developing tornado, much like a spinning ice skater who draws their arms in, the spin increased plus the pressure at the surface was dropping and a layer of fog formed over the river. The dew point was as high as here in Washington at 72 and the relative humidity about 80 percent, so as the tornado formed and the surface pressure dropped, a cloud formed. The visible cone of the tornado is essentially the extension of the cloud surface along an area where the air pressure is the same. For a similar example, look at the trace of barometric pressure where a tornado passed over a weather station in Oklahoma this May:
The pressure dropped an ear popping 10 percent in minutes. In Springfield, the pressure drop was the reason that ethereal, entrancing misty fog formed over the river. Imagine what is was like for the drivers on the bridge as they saw it coming. The twister was probably just an EF-0 at that point, but as it grew in intensity it became a major tornado that NWS surveys have determined produced EF-3 damage with winds of about 150 m.p.h.
So why were some homes destroyed and others across the street were barely damaged?
Well look back at that incredible video of the tornado forming and look again at the airflow on the river as the vortex is forming;
The river is smooth and the air, even moving into the intensifying whirlwind at 50 to 80 m.p.h, is fairly smooth or laminar. But then when that whirlwind moves over the rough surface of trees and buildings the airflow becomes very turbulent and just as you are flying along in a plane and you hit some “turbulence,” boy do things and the wind speed change and swirl in only a few feet. Watch this great video of a test of buildings in a wind tunnel with “smooth” winds and see what happens around buildings.
We all really notice this in winter when we walk around the corner of a building and wham, get hit with a blast of cold wind gusts. The air is a fluid, just like the ocean or a river and when it gets forced around solid objects like rocks or houses the speed increases and the difference of only a few feet can be the difference between a scary wind of 70 m.p.h. and a wind of 150 m.p.h. that lifts the roof off a house and collapses the walls. And what is the safest spot in a tornado? Well, underground in a tornado shelter or as some folks have done, build a monolithic dome home.
No sharp edges, no real barrier to the wind. Yes, you will get lots of folks stopping by to look at your strange home but in a tornado you could also be living on the laminar edge.