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).

Low Cloud Season Begins in the Northwest

It happens every year sometime during mid-spring.

The low-cloud season began in western Washington today.

You know what I mean, if you have lived here any amount of time.  But if you don't, this morning's Space Needle PanoCam tells the story in dreary detail.  On many  such days, the low clouds thin or burn out by lunchtime.  But on depressingly too many days, it holds in through much or all of the afternoon.

During the low-cloud period, which often stretches from May through mid- July, the northeastern Pacific fills will low clouds.  Here is today's GOES-17 full-color visible satellite image this morning--a huge area of the Pacific is full of these clouds!

Fortunately, you can generally escape the low clouds by crossing the Cascades into eastern WA or Oregon.

Ironically, all this low cloudiness is the result of high pressure building into the northeastern Pacific, something illustrated by the sea level pressure forecast for later Thursday afternoon (see below).  In contrast, there is lower pressure over the continent.  The map also shows temperature around 3000 ft., with warmer air being orange and red.

This weather pattern is a low cloud producer for several reasons. 

First, with high pressure offshore and lower pressure inland there is an onshore pressure gradient, which tends to push cool, marine air inland.

One strike.

High pressure extends aloft and it associated with sinking air above the surface, which warms by compression.  This sinking decreases towards the surface (since air can't go through the ground or ocean).   Such warming aloft tends to produce an inversion (temperature warming with height) aloft that acts as a cap on the lower atmosphere-- allowing the ocean to cool and moisten a lower layer, without bring down any drier air from aloft.  Good for clouds.

Here is the vertical sounding today at Forks, on the WA coast.  Temperature is on the x-axis and height (in pressure) is on the y-axis.   700 indicates about 10,000 ft.  The right-hand line is temperature, the left hand one is dew point.   The lowest 5000 ft is near or at saturation (the temperature and dew point are right on top of each other)--that is cloud layer.  An inversion is right above it.

Two strikes.

The high pressure also brings northerly (from the north) winds over the coastal zone, which causes upwelling of cold water from below the surface.  As warm air from farther out over the ocean moves towards that coast, it cooled by the coastal waters, resulting in saturation and clouds.

Three strikes.   Low clouds rule.

The good news is that in May there is still enough weather activity that high pressure is often displaced, moved about, or replaced by low pressure, which can get rid of the low-clouds for a while and even give us a temporary warm spell.  That should happen next week.

But then in June, when the weather activity quiets down and the high pressure become persistent and stronger, the low cloud settle in for much of the month.  You know what that means:  June gloom.

One good thing about the low clouds--it keeps many Californians away, thus keeping our real estate prices in check and the traffic from getting even worse than it is today.