So far this year more than 250 cannabis farms have been spotted by the police helicopter, thanks to the advanced thermal imaging cameras it carries.

Many cannabis factories are excessively heated to help produce the drug and this heat literally makes cannabis farms stand out a mile when the helicopter is in the area.

Police say the infra-red camera means there is no hiding place for criminals.

Head of West Yorkshire Police's air operations unit, chief executive John Whittaker said: "As the picture shows, through the thermal imaging cameras on board the helicopter, hot areas show as an area of black in the camera, so a house being used as a cannabis factory will be far blacker than surrounding properties.

"Growing cannabis to sell is a serious offence for which a sentence of three years imprisonment can be expected. Some people do it as a sort of cottage industry and probably do not know the possible consequences. I want those people to get the message that if you grow it, we will find you and you will go to prison. It's as simple as that," said Chief Supt Chris Hardern, head of operations support division.

When the helicopter spots a suspected cannabis factory, the information is passed on to divisional drugs team officers who are able to search the properties once they are granted a warrant under the Misuse of Drugs Act.

Bryan Dent, the force's drugs co-ordinator said: "The force helicopter has proved to be a great tool in helping us tackle cannabis farms in West Yorkshire.

"It's thermal cameras identify possible factories and drugs team officers can follow this information up with warrants and raids. The results we have had show that regardless of the size of the cannabis factory, the thermal cameras can effectively pick out this type of drug activity.

"Cannabis production should be relatively obvious to spot as the factories will often be emitting an unusual and distinctive odour.

"Neighbours may also hear a lot of banging noises as the factory is set up and see people coming and going with bags of fertiliser and compost and other equipment such as powerful lights.

"It is likely the curtains will be kept closed at all times but bright lights may appear to be left on around the clock. Residents living in the street are again likely to see various comings and goings when the crop is harvested, such as bags being loaded into a vehicle."

One example of how effective the helicopter has been in discovering cannabis factories was in March 2007 when, while out on another operation, the force helicopter passed over a house in Upton that was seen to be giving off a particularly high heat signal.

A subsequent raid on the house last year led to the seizure of 57 cannabis plants. As a result a 36-year-old man is awaiting sentencing after being found guilty of possession with intent to supply class C drugs.

More recently on July 16, as a result of information received from the helicopter, officers attended an address in Ferry Fryston where they discovered what was believed to be more than 30 cannabis plants and growing equipment. A man has since been charged in connection with the incident.

To pass on drugs information to police in confidence call Dob In A Dealer on 0800 555111.