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What was a "Rhubarb" operation?
Short answer
A "Rhubarb" was an RAF offensive sortie in which small patrols crossed under cloud cover to attack any aircraft they encountered. The first was flown by 66 Sqn on 20 December 1940, with two Spitfires over Le Touquet.

Two Ways of Going on the Offensive
Fighter Command's operational instructions were ready in December 1940. They provided for two types of offensive operation: the "Rhubarb", at first called "Mosquito", and the "Circus". In the former, small patrols crossed over under cover of cloudy conditions and attacked any aircraft they found; the latter was reserved for fine-weather days, with several squadrons — possibly with some bombers — sweeping northern France.
A Command Determined to Cross the Channel
The approach was a consequence of Hugh Dowding being replaced by the more aggressive Sholto Douglas. It was enthusiastically backed by Trafford Leigh-Mallory, who had replaced Keith Park at the head of No. 11 Group RAF.
The First Patrols and Their Results
No. 66 Sqn flew the first Rhubarb patrol on 20 December 1940, with two Spitfires over Le Touquet. By 13 June 1941, 149 Rhubarb patrols had been launched, amounting to some 336 sorties, as Sholto Douglas recalled. In them, seven enemy aircraft were claimed shot down at the cost of eight British pilots lost.
The first Circus operation, by contrast, was flown on 11 January 1941 against Guînes, with No. 114 Sqn's Blenheims and nine fighter squadrons. Circuses were usually formations of 20 to 30 bombers escorted by up to 16 fighter squadrons.
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