At 8:00 am on Friday morning, a smattering of high clouds could be seen passing over Bass Strait and southern Victoria. By 8:30 am, just half an hour later, a thin line of cloud had suddenly appeared, stretching 200 km across the sky to the southeast of Lorne.

The peculiar cloud was captured in visible satellite images, appearing as a pencil-thin line of white cloud that was clearly distinguishable from the more ragged cloud formations around it.

The cloud was one of several contrails that formed over the Bass Strait region on Friday morning.

What is a contrail?

Contrails refer to clouds that form in the wake of aeroplanes. As the name suggests, these clouds are produced by condensation in the wake of an aeroplane, making a visible condensation trail, or contrail.

Condensation in the atmosphere refers to the process where water vapour (a gas) condenses into water droplets (a liquid). This usually happens when the air is cooled until it becomes saturated and can’t hold any more water vapour.

Aeroplane engines emit relatively warm and humid air into a very cold layer of the atmosphere when they cruise along at high altitude. This jet engine exhaust is made up of about 30% water vapour. As this warm and humid air exits the jet engine, it quickly cools and mixes into the atmosphere in the jet’s turbulent wake. This rapidly cooling air becomes saturated, causing condensation to occur. The water droplets then quickly freeze into ice crystals.

Image: Contrails over northern Florida on the morning of January 9, 2004. Source: NASA Worldview / Terra.

Particulate matter emitted by the jet engines also acts as cloud condensation nuclei, providing a surface for the water vapour to condense onto. This particulate matter helps initiate the condensation process, making it easier for contrails to form.

What caused the contrails over Bass Strait on Friday?

Conditions were ideal for contrails to form above southern Vic and Bass Strait on Friday morning.

A layer of upper-level moisture sitting about 9 to 12 km above the ground meant that the air was already close to being saturated. The temperature of this layer was also around -40 to -60°C. When planes flew through this layer of air on Friday morning, their jet engine exhaust easily caused condensation and produced contrails.

Are contrails dangerous?

Contrails are like naturally occurring clouds and pose no direct health risk to humans. However, contrails are influencing climate change.

The cirrus clouds caused by contrails affect how much of the Sun’s radiation enters and exits Earth’s atmosphere. By adding more high-level clouds to Earth’s atmosphere, contrails help trap outgoing longwave radiation, which enhances warming in the atmosphere.

This is still an active area of research as scientists try to figure out how much the cirrus clouds caused by contrails will influence climate change in the future.