Melting Snow Helps Police Spot Cannabis Dens

Dutch cannabis growers are reportedly being caught by police - thanks to melting snow.

Officers have released a photo of a marijuana factory in the city of Haarlem that was found due to an unusual lack of the white stuff.

While the surrounding rooftops were covered in a light dusting of snow, this particular roof was completely snow-free.

Growing marijuana requires temperatures far above those common in neighbouring residential buildings, so Dutch news reports say police looking for cannabis farms regularly look for roofs where snow appears to have melted.

Last week police raided a house in the city of Zutphen, where they discovered a bedroom containing 88 marijuana plants, according to the Washington Post.

Two days later, they arrested a person in Arnhem after finding a similarly-sized plantation, proof that their appeals to residents to report suspiciously snow-free rooftops are bearing fruit.

The Netherlands is known for liberal laws regarding the possession and growth of cannabis but laws forbid more than five cannabis plants or possession of more than five grams per person.

Police helicopters in the UK have used a similar heat-detection method for years.

West Midlands Police Deputy Air Operations Manager Martin Knowles said in 2013: "Our thermal cameras can detect unusually high temperatures coming from properties several thousand feet away – anyone growing cannabis on a commercial scale, using rows of heat lamps, runs the very real risk their criminality will be rumbled from the skies."