Jump onboard an alpine flight to photograph some glaciers! The annual snowline survey has been running since 1977, but today new techniques are allowing researchers to go beyond 2D photos to make 3D models of the glaciers. Claire Concannon joins the team for a long day of flying and photographing.

It's a bright blue morning in Queenstown. Summer has been and gone, and the first hints of autumn are starting to appear. Leaves turning, a sharpness in the mornings, the first overnight frosts.

And as this shift begins, it's also time for another annual event - the end of summer snowline survey flight to monitor New Zealand's glaciers.

How to build a glacier

A glacier forms when snow builds up over time, turns to ice and then begins to flow downwards under the pressure of its own weight. For this to happen, you need snow accumulating.

The snowline is an imaginary line that traces along mountain slopes and marks the lower limit of permanent snow cover. Below this line, snow and ice melt away, above it, snow sticks around.

At the change of seasons, snow below this line from the previous winter will have melted, and if you time it right, and no new snow has fallen, you can fly a plane past a glacier and photograph the end-of-summer snowline. By repeating this each year, researchers can track changes happening to our glaciers over time.

'Trev used to run on excitement and liquorice'

The survey began in 1977. Back then, it was designed and led by a scientist called Trevor Chinn. After completing an inventory of all New Zealand's glaciers, and coming up with a total of more than 3100, Trevor realised that it just wasn't practical to monitor every individual glacier.

Instead, he developed a list of 51 'index' glaciers that would be surveyed each year. Using aerial photographs that showed the end of summer snowline height, they would be able to estimate ice volumes of the glaciers year-on-year - and keep an eye on changes.

Though the survey baton was passed on to Dr Drew Lorrey of NIWA in 2009, Chinn continued to go on the annual flights until his passing in 2018. As a nod to him, there's still a bag of liquorice opened and passed around on each flight.

"Trev used to run on excitement and liquorice," says Lorrey . "There's a really rich legacy that we've got to do justice to, in terms of carrying on, but also making sure that the science grows and that it actually is applied with a purpose." …

Go to this episode on rnz.co.nz for more details