This was certainly a case of right place, right time. I was enjoying a drink with a friend on the grass outside the union on Friday the 4th when I witnessed something pretty spectacular. Spotting a circular motion of leaves over to my right, I didn’t think much of it since this is quite a common occurrence. But within a few seconds, it had moved towards me and developed into a thin funnel extending about 10 metres up into the air having picked up grass and debris. Then moving right behind me, it quickly dissipated after reaching the union building with the only marks of its occurrence being a few squeals from some students and the shocked faces on my friend and myself. Unfortunately, my friends of the non-weather-geeks variety I saw over the next couple of days failed to share my enthusiasm and instead could only resort to the cheeky remark: “So you then saw a unicorn next, right?”
Being so pleasantly surprised at witnessing something rather cool, I tried to seek some answers as to why this occurred. Strong solar heating is an important driving factor. The weather conditions that day were dominated by a ridge of high pressure extending over the UK (see synoptic chart) , resulting in a Tropical Continental airmass bringing warm conditions (maximum temperature of 26.6°C), clear skies and light winds. Therefore, strong solar heating of the ground would have led to strong convective activity in the form of turbulent eddies and rising hot air.
An important driving factor in the whirlwind formation is the turbulent motion due to the convection driven by the warm surface; any resulting spinning motion can get intensified due to vortex stretching effects (by conservation of angular momentum) and these can lead to substantial coherent vortices. An important factor in the visibility of the vortex near the surface is the effect of friction. With the main balance of the pressure gradient force pulling air towards the centre and the centrifugal force pushing air away from the centre, this produces a stable vortex. But friction at the surface will retard the flow and thus reduce the centrifugal force. Now the balance is lost: the air, along with dust and debris, will start to rush in towards the centre of the low pressure area, feeding the whirlwind.
Turbulence clearly goes on in the boundary layer all the time, particularly during calm sunny conditions that enhance convection and so invisible whirlwind motions are commonly present in the boundary layer. I was just fortunate to see one of these turbulent whirlwinds reaching the earth’s surface made visible by the present of grass and debris. Maybe this phenomena happens a lot more often than we think.
Thanks to Maarten Ambaum for his help with this. Any further comments/corrections are welcome.