CHEMTRAILS OR CONTRAILS?: Concerned readers were quick to contact the Nelson Mail about unusual vapour trails in the skies on Wednesday evening.

Persistent vapour trails criss-crossed the skies on a clear autumn day over Nelson yesterday, prompting concerned readers to contact the Nelson Mail about so-called 'chemtrails'.

The chemtrail conspiracy theory is based around the idea that some trails left by aircraft contain chemical or biological agents deliberately sprayed at high altitudes, supposedly as part of clandestine programs directed by government officials.

This theory is not accepted by the scientific community, which states that they are just normal contrails, and there is no credible scientific evidence to suggest otherwise.

When asked about the trails that remained over Nelson for approximately two hours on Wednesday evening, Phillippa Sellens of the Airways Corporation was not aware of any unusual activity in the Nelson area.

"In calm, moist atmospheric conditions the jet streams from regular jet traffic remain visible for a long time." Ms Sellens said.

"Nelson is in the middle of the country so jet streams from flights going from Auckland and Wellington to Christchurch and Queenstown, as well as trans Tasman flights into Wellington, are often visible over the Nelson skies."