This paper examines an event in May 2016 when smoke from forest fires in Canada reached the UK at altitudes between 3 and 11 km above the surface. Although events of this kind are fairly common in the summer months, this one was unusual because it persisted for 9 days due to a stationary flow pattern that kept the smoky air from travelling away to the east. A network of lidars and ceilometers around the UK provided round-the-clock observations of the smoke event.