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The October 1987 storm - a 'hurricane', or not?

Well, it all depends who you talk to! Meteorologists make a clear distinction between hurricanes, a regional name given to tropical cyclones occurring in the tropical Atlantic and east Pacific basins; and intense mid-latitude storms. Reference to the following will show why:-

Hurricanes, or tropical cyclones, form in an environment of little or no vertical wind shear. Vertical shear (which doesn't have to be throughout the troposphere, but can also be over a very shallow layer) destroys the convection around the centre of the tropical cyclone (the 'eye'). For a tropical cyclone to continue to develop, there must be inflow of warm air at lower levels, and upper level outflow: the convection provides the 'pathway' for the necessary rising air. They form over very warm waters - sea surface temperatures (SST) values above about 27 degC, equator-ward of the sub-tropical anticyclone belt, with the capture/inflow of water vapour and sensible heat from the tropical oceans being essential to the physics of the storm. The storm is warm core, especially in the lower troposphere, with little overall (synoptic scale) 500-1000 hPa thickness gradient, and therefore hurricanes have no 'Norwegian model' fronts associated with them. Low pressure is primarily due to the contrast between the warm core of the storm, and the unperturbed tropical environment, with some contribution from compressional warming of descending air within the 'eye' of the storm, and the speed of movement of the storm is generally less than 15 knots.

(NB: the name 'tropical' should not be taken to mean that these features only form between the Tropics of Cancer and Capricorn; they can develop well poleward of the 'tropics' and indeed persist with tropical characteristics [up to 'Hurricane' category], well into the normally accepted 'mid-latitudes': it is the storm structure - as defined above - that is important, not the exact location of the disturbance.)

Mid-latitude severe storms form in a strongly sheared environment, such as is found at high levels around the parent Polar Front jet core - jetstreams play no (direct) part in the formation of a hurricane. The formation of a mid-latitude storm is triggered by a short wave eastward moving disturbance embedded in the upper flow, with consequent distortion of the pre-existing baroclinic (i.e. frontal) zone. There is a strong 500-1000 hPa gradient involved. There is appreciable disturbance of the tropopause in the vicinity of the storm, particularly to the rear in the 'dry slot', and this is thought to be important in that it indicates the intrusion of dry stratospheric air - a key ingredient in the 'explosive cyclogenisis' aspect of these storms. (At the present time, it is not known for certain whether stratospheric air is involved with tropical cyclones: studies are underway to investigate this). The low (or lowering) pressure at the surface is due to an excess of divergence of mass aloft over convergence below, coupled to strong warm advection. The speed of movement of such storms are often in excess of 30 knots.

So far, so good - meteorologists are not going to get the two phenomena mixed up, but when looking at the October 1987 storm that hit the southeast of England, these clear scientific differences must be balanced against the reality of the event. For example, some very warm/moist air was entrained in the storm, possibly the remnants of a former tropical cyclone. Although the 10 minute mean winds in most cases failed to reach the threshold of 64 knots for a hurricane, two reports within the circulation over/adjacent to the English Channel did exceed this threshold, and although not analysed, 1 minute means, which the Miami NHC uses to classify hurricanes, almost certainly would have reached or exceeded this level, particularly when set against observed gusts of 70-90 knots or more, which are easily attained in mature tropical cyclones. There was widespread damage and disruption, with millions of trees damaged or felled, several people dead, ferries stranded on windward shores and given these facts, it easily matched the OED definition of a hurricane.

Prior to dawn on the 16th October, 1987, the image most members of the general public had of the damage wrought by hurricanes came from television pictures from the US or the Far East. The folk of the south east of England then are surely to be forgiven if venturing out and finding the car under a substantial tree, or whole communities cut off from electrical power, they refer to this event as "... the 'hurricane' of 1987".

(help with information relating to tropical cyclones was supplied by: Sim Aberson, a meteorologist with NOAA's Hurricane Research Division in Miami, Florida.) For more detail, visit the Tropical Cyclone FAQ at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html.

[ For more on the use/abuse of the phrase "Hurricane Force 12" see:- HERE ]