Powerful Dust Devil Takes a "Bounce House" Airborne, Injuring Students

This week there were a number of media stories about a "bounce house" in a field near Zillah High School going airborne, seriously injuring one student.

The origin?  

A strong "dust devil" resulting from intense heating of the surface.

To understand this event, let's start with the video that documents the situation around 2:30PM on Wednesday, May 1.  You can see the bounce house go airborne and the vortex associated with the dust devil is very evident.   This was an extraordinarily strong dust devil.   The video also has a few other examples of similar events around the world.




Maximum wind speeds in dust devils are typically 15-30 mph, but can get as high as 50-60 mph for the extreme cases.

The location of the incident was behind Zillah High School (see map), in the warm Yakima river valley.

A blow-up shows the location of the lofted bounce house, just south of the high school (there is a pointer at what I believe is the correct location),

Dust Devils are associated with warm, sunny days in which a rising current of air (known as a thermal), pulls surrounding air into the updraft, gaining rotation due to the horizontal variations of the wind speed of the surrounding air.   Strong winds are actually bad for the organization of these features, so they typically occur on light wind days (less than 10 mph).   Dust devils depend on the instability of the lower atmosphere--situations when temperatures cool rapidly with height.  Thus, a strongly heated surface helps them, as does a cool atmosphere above.

Such a situation is most probable in spring under clear conditions, since the atmosphere is still relatively cool, but the sun in near maximum strength (June 21st).  Flat, arid terrain is good for dust devils, such as a desert landscape or recently plowed, bare fields.

Many of these conditions were evident last Wednesday near Zillah High School.

A high-resolution visible satellite image at the 2:30 PM shows mainly clear skies, with some scattered cumulus clouds--suggesting low level instability and thermals.  But lots of sun.

The surface weather observations at the nearby Washington State Department of Transportation site along highway 12 had light southeasterly winds at 2 PM, with a temperature of 67F,

But let's look closer at the observations there that day (below).  

Until 2:20 PM, winds were weak (sustained winds around 5 mph), but there was a transient gust to 16 mph at 2:35 PM.    The road temperature (last column) rose to 103 F at the time of the incident--showing strong heating at the surface.   There was over a 35F difference between the temperature of the road and the air temperature at 2-m above the surface in the area of the road (68F).  The nearby fields were undoubtedly cooler than the road, but there was certainly a very large vertical temperature change above the nearby heated soil surfaces.

As shown by the satellite imagery above, there were fields to the south, where the dust devil could have originated.

And then we need rotation in the dust devil, which can be encouraged by obstructions to the wind.  You have seen this in action on windy fall days when rotating vortices of leaves are clearly apparent downstream of buildings.   The boundaries between irrigated fields with vegetation and non-irrigated fields can also produce gradients in wind that produce rotation. 

Both of these potential sources of different wind speeds and thus rotation were evident around Zillah High School, including the school itself, the church to the south, and the varied fields in the same direction.

There is a lot of scientific literature about dust devils, with an excellent reference here.   One reason for all the interest is that we can spot dust devils on Mars and knowing about their characteristics tells us about the meteorology and geology of the Red Planet (see below).