Forecasters says a 'weird” white streak left in the sky above Tauranga is a contrail but, maybe not from an exhaust contrail.

A photo sent to SunLive by a reader shows a white streak running in the opposite direction to the normal cloud coverage in the sky.

The photo was taken in Tauranga yesterday morning.

SunLive sent the photo, which was taken in Tauranga yesterday morning, to the MetService to see if they could explain the anomaly.

'I consulted with a fellow forecaster Ross Marsden and that looks like a contrail for sure, though maybe not an exhaust contrail,” says senior communications meteorologist Lisa Murray.

'We had a look on FlightRadar24 replay over Tauranga, but there were no aircraft showing at typical contrail altitude at the time of the photo.

'So we had a look at lower levels and the only flight passing Tauranga at that time was this one.”

It was at 17,025 feet. The freezing level there today is 4500 feet.

Lisa says 17000 feet is not unreasonable for the altitude of that altocumulus in the photo. Altocumulus clouds are comprised of supercooled water droplets.

'The trail seems to have been disturbed out of a straight line in the places near the bottom of the photo. This means the cloud is more or less at the same altitude as the trail.

'This means that the air at 17000 feet is very humid (lots of moisture). The relative humidity with respect to water is near 100 per cent, and well over 100 per cent with respect to ice,” says Lisa.

'In cloud physics we say that the air is ice-supersaturated. But because there are no naturally occurring freezing nuclei in the atmosphere (there are plenty of cloud condensation nuclei) ice clouds do not spontaneously form in these conditions.

'Today the temperature at 17000 feet is about -20°C. '

That environment is ideal for the formation of aerodynamic contrails, says Lisa.

These are different from exhaust contrails, so how do those form?

There is low pressure over the wings, and because of that the temperature lowers too, as the air passes over them.

Lisa says if the air is humid enough the lowering of pressure (and temperature) will cause condensation.

When the air is well below freezing, these condensation droplets can freeze to form ice particles.

'Once that happens in ice-supersaturated conditions they can't evaporate because the relative humidity is too high – there is already too much moisture in the air, no enough for condensation, but too much for ice to evaporate. So that is why a trail is formed.

'Very interesting case.”