The Hoggar Mountains in Algeria saw a number of severe thunderstorms on 26–27 June. These appear to have blown a large quantity of Sahara dust into the atmosphere, where it was caught in a southerly wind in an atmospheric layer between 10,000 feet (3,000 m) and 17,000 feet (5,200 m) in altitude, forming a classic Spanish plume. Unusually, no clouds formed as this dust blew over continental Europe, and the layer reached England on 30 June largely unrained out.
The warm desert air brought a heatwave over Southern England, with temperatures in London on 1 July measured at 32 °C (90 °F), until it met the much cooler, moister Atlantic airstream. The boundary between the two formed a squall line stretching from Devon, along the England–Wales border and up across Northern England to the River Tees. Thick clouds, darkened by the Sahara dust, rose to 44,000 feet (13,000 m), plunging areas along the squall line into total darkness. In some areas, the lightning continued for 24 hours, and ball lightning was seen at RAF Chivenor in Devon.
The dust particles served as seeds for nucleation, causing water to rapidly precipitate out and form especially large raindrops and hailstones. The thunderstorms resulted in one of the most intense hail falls ever recorded in the UK, with hail events in 9 separate places reported as "severe" (H3) or greater on the TORRO hail scale, and the strongest rated "destructive" (H6). Hailstones 75 millimetres (3.0 in) across – the size of a tennis ball – were measured at Cardiff Airport, and the local newspaper for Hartland, Devon, reported the finding of "a piece of ice 4 inches (100 mm) long".
Along with the hail came heavy rain, with the 9 minute 35.7 millimetres (1.41 in) fall at Leeming Bar setting a record for a sub-10-minute total. The Isle of Man measured 184 millimetres (7.2 in) of rain over the 48 hour time period associated with the storms. Areas south and east of the squall line saw less severe storms, but the rain that fell in the night of 1–2 July 1968 was rich in Saharan dust, turning it blood red and leaving dusty deposits on the surfaces it fell on – only the south coast and uplands of Wales avoided the red rain. The last comparable storm associated with Saharan dust was seen in October 1755.