During a period of strong SW winds, a local anomalous hot air mass was observed at sea level on the lee side of Pantelleria Island in the Strait of Sicily. The hot air, some 10 C above ambient, was produced by downward transport or mixing from an intense inversion layer occurring above 500 m. This rapidly moving layer, associated with a flow from the Libyan Desert, was apparently perturbed by the 836-m high volcanic cone of the island. Laboratory models suggest that such an interaction could produce phenomena, i.e., vortex shedding and lee waves, all of which would mix or advect the upper warm air downward. This, together with the adiabatic heating effect, would produce the hot air phenomena.
1. The Phenomena Observed
During May, 1970 the Research Vessel MARIA PAOLINA G. of the SACLANT ASW Research Centre was making oceanographic observations in the Strait of Sicily. On the afternoon of 9 May, while working 25 km W of Pantelleria Island (Figure 1), strong winds from the SSE developed, increasing steadily from 10 m s-' at 1400 (GMT or Z time) up to 25-27 m s -' by 1600Z. During the same period the French minesweeper ORIGNY, also working in the Strait, reported winds exceeding 30 m s-. Because of the increasing sea state, observations were terminated and MARIA PAOLINA G. was taken to Pantelleria for safe anchorage.
The Island of Pantelleria is an extinct elliptical volcanic cone with a maximum altitude of 836 m (Figure 1). It has a steep south slope inclining from 30°-35 °, the northern slope being much more gradual.
At about 1800Z the vessel approached the lee side (NNW) of the island on a westward course (Figure 1). The winds were 20-25 m s -1 from the SE and the air temperature had risen gradually from 16.5 °on the open sea to 17-18 0 C. Upon entering the lee of the island, the wind dropped considerably to 5-10 m s-'. However, simultaneously the air temperature dramatically increased within five minutes to values estimated between 25 °to 30°C. This phenomenon, first observed by personnel on the ship's bridge, was described as a 'hot draft' coming directly from the shore of the island some 700 m away. (One of the ship's officers seriously suggested the occurrence of volcanic activity.) Not having found an appropriate anchoring position, the ship was sailed slowly through and out of the sheltered zone of the island; upon moving out into open water, the air temperature returned to the ambient value of about 16 °C.