Chasing cloudbows: student builds a drone to measure cloud droplets

A Cambridge Engineering student has developed a drone to fly above clouds and measure droplets with a camera.

Luca Lin, who graduated this week from the Department of Engineering, built the drone and camera system as his fourth-year project with the Centre for Climate Repair. The Centre is researching marine cloud brightening, a proposed climate intervention that would help low-lying marine clouds reflect more sunlight. The idea is to spray seawater droplets of a particular size, as smaller droplets scatter more light back into space.

The approach is being tested in models and small-scale trials, but Luca’s project addresses a fundamental problem: how do you check, in the field, whether cloud droplets have actually got smaller?

Flying the drone in first-person view, Luca wears goggles fed by a live video transmission from two bug-like antennas mounted on the aircraft. Through them, he looks for a specific optical effect known as the “cloudbow” - a faint, rainbow-like ring produced by light scattering. The diameter of the ring corresponds to the size of the droplets producing it; a wider ring means larger droplets, while a tighter one means smaller droplets. 

He records the flight with the drone’s second camera. Powered by a Raspberry Pi and two batteries, it has polarisation filters built directly onto its imaging sensor to capture light at four angles: 0, 45, 90, and 135 degrees. Comparing the intensity of light across those four channels reveals the polarisation state of light scattered by the cloud below.

By flying over a cloud and recording the cloudbow, which he analyses later, Luca can estimate the droplet size distribution across the cloud top without physically sampling it. A spotter stays beside him to maintain direct visual line of sight with the drone at all times, in line with UK drone regulations for FPV flying.

To test his machine, Luca took a couple of trips to the hills of Wales and the Lake District, where it is easier to get above the clouds while keeping the drone in view.

The flights have not been without incident. In February, on the drone’s first outing away from Cambridge, unfamiliar terrain and windy conditions led to a rough landing. The drone flipped over and cracked the casing of its solid-state drive (SSD). Luca recovered the SSD and found, to his surprise, that it still worked.

After that, he redesigned the storage mounting, sandwiching the SSD between the carbon-fibre body and the upper assembly. In theory, the data should now survive even if the drone doesn’t - though he hasn’t yet had to put that theory to the test.

A flight in March, using the finished polarisation camera, went more smoothly. It produced solid results with an error rate so low that Luca initially suspected something had gone wrong with the analysis. Further checks suggested the numbers were simply accurate.

Luca thanks staff at the Centre for Climate Repair and Department of Engineering, as well as collaborators at the University of Manchester and authors of prior research on cloud polarisation, for helping guide the project to completion.

Whether marine cloud brightening becomes a viable climate tool remains an open scientific and policy question. But if it is trialled as scale, tools like Luca’s drone may provide a vital way of assessing its impact.