LAND DEVIL EVENT: 11TH JULY 2002

Rotating water-droplet column ('Land Devil') - Bracknell (Tawfield), east Berkshire.

Date: 11th July, 2002
Time: 1230UTC
State of ground: wet (but no puddles) after recent moderate rain shower (which ceased circa 1220UTC).
General wind field: (gradient/isobaric - i.e. quasi-frictionless flow at around 900 metres) - WSW'ly.
Surface wind: - SW/SSW'ly Beaufort force 3 occasionally 4, but varying typically as in a residential area, though general drift still south-west to north-east.
Clouds (overhead): Sc Cbgen (debris from past Cb/TCU)
Clouds (vicinity) (> 2 km distant): Cb cap - retreating shower.
General weather: bright/strong sunshine, with total cloud amount 3 oktas.
Event period: < 60 secs.

I had just sat down in a chair - looking out of the large window across the road outside to the opposite house and its front lawn: my eye caught by a 'misty' column moving from right-to-left (east to west, against the general wind flow), which lasted until column passed over a tarmac drive - it being over lengthy grass lawn when first observed & whilst it was visible (see below).

Dimensions (roughly): 0.8 m diameter; 2.5m depth (discrete top seen against building opposite; dimensions assessed with respect to known objects on house opposite).
Rotation observed, I think cyclonic (or anti-clockwise to an observer looking downward), with a velocity of rotation typical of that of a 'dust-devil', i.e. vigorous, but not extreme.

Presumably this whirlwind event was betrayed either:
(a) as a consequence of lowered pressure within the column, reducing temperature to local (enhanced?) dew-point - or,
(b) by its collecting water droplets held in suspension (after very recent sharp shower), from the tops of rather lengthy lawn grass over which it passed - the occupant of the house was on holiday - hence grass 'over-long' relative to normal.

The column could not be seen after it passed over the next tarmac drive it came to - which was strongly heated, so I assume that either the water-vapour source, or the water-droplet source (of the lengthy lawn grass) was cut-off, with a reduction in humidity (or absence of water droplets), therefore rendering the column 'invisible' - there being no debris (litter) or dirt/soil etc., to then betray its presence.

The really fascinating thing about this "land devil" was that it travelled against the prevailing flow, at just under a steady walking pace. I have not seen anything like it before - though I have seen many dust-devils of course, both in UK and in the Middle East.

Why the land devil should run to counter to the general wind field is unclear. It wasn't obvious that the local wind field had significantly reversed, but with the vigour of convective development, it would be argued that the storm complexes so distorted the gradient field that micro-scale heating allowed very localised and major change in wind field.

SATELLITE IMAGE, SURFACE PLOTS & TEPHIGRAM PLOT

Visible image early afternoon (red spot shows Bracknell) (1): This is the visible channel satellite image covering Ireland, England & Wales early afternoon. The lines of convection can clearly be seen over southern England, as can the clear area covering large regions of south-eastern England.

Synoptic chart at 12Z with streamlines of surface flow. (2): This is the synoptic plot of the area timed at 12UTC. The red dot shows the location of Bracknell, the synoptic station immediately to the south is the automatic station at Beaufort Park, on the southern outskirts of the town. This shows that the air temperature was 17degC, the dew-point temperature 7degC, the visibility 27km (excellent in other words), 1 okta of cloud with a base at 3300 ft (agl) and a SSW'ly wind of 10 knots - all data accord with my own observations made just after the event described here. Note the shower in progress at Heathrow airport, probably the one (or part of the complex) that caused the damp ground/grass here, and the observation (human observer) of Cumulonimbus with anvil. Note also the confirmation of the general (surface) wind flow as SSW or SW'ly, and isobaric drift as WSW'ly.

Tephigram plot for 12Z from Herstmonceux - see text for explanation. (3): This is the tephigram plot of Herstmonceux timed at 1200UTC (probably released circa 1130UTC), which shows a classically unstable ascent - the features marked are: ** adjusted dry-bulb trace assuming the surface temperature had risen in the strong sunshine to at least 18degC, though in fact the ground temperature was probably well in excess of this - though mixing with the environment would soon offset the high ground value. ** The isotherms at 10 and 20 degC are highlighted for ease of reference. ** The isobars for 900 and 1000 hPa are labelled for ease of reference. ** DPC=Dry-parcel convective top - assuming dew point mixed over lowest 100 hPa of ascent, T (initial)=18C, and allowing for an extra 30 hPa for non-precipitating convection. This gives a depth of dry convection: circa 135 hPa, or about 1.3 km. As can be seen, the vigour of dry convection to the estimated surface temperature was high, with a maximum excess of parcel temperature over environment temperature of at least 5 degC. ** The dashed line parallel with the Dry Adiabatic Lapse Rate from the surface (T=18C) to the DPC shows a 'parcel-path' assuming no mixing with the environment - the dry parcel path (DPP).

Martin Rowley, Bracknell. Berkshire 21st July, 2002