Argonaut G-ALHE operated on the flight Lagos-Kano-Tripoli-London. The aircraft arrived at Kano at 16:40. At 17:21 GMT the aircraft took off again from runway 25. Reported weather was clouds 3/8 base at 2500 feet; wind 270deg/20 kts; visibility 1500yds in moderate rain. The gear was raised and the Argonaut passed the runway threshold at 100 feet at an airspeed of 125 knots. As power was reduced to 2850rpm and 54″ manifold pressure a slight updraught was noticed. Having entered an area of heavy rain normal climb was made to 240 feet when the flaps were raised; rate of climb was 300 feet/min at an airspeed of 125-130 knots, which dropped to 123 knots following retraction of the flaps. Then suddenly, the airspeed dropped to 103 knots which was close to the 97 knots stall speed. Full power was applied and the nose was pushed slightly down to gain speed, but of no avail. The aircraft entered a high rate of descent and descended to 15-20 feet agl. A tree ahead forced the pilot-in-command to bank to the right. The Argonaut impacted the 35 feet high tree with the left wing, rupturing fuel lines. The left outer wing detached as the aircraft crashed into trees, catching fire. PROBABLE CAUSE: “Loss of height and airspeed caused by the aircraft encountering, at approximately 250ft after take-off, an unpredictable thunderstorm cell which gave rise to a sudden reversal of wind direction, heavy rain, and possible downdraft conditions.”