Measuring individual droplets was part of his master's research project when he was still a physicist at the University of Hong Kong and that work was to lead to his present standing as one of the world's foremost experts on tropical cyclones and the science of climate change.
"I always wanted to study something to do with the weather," says Chan. He traces this interest back to his time as a Boy Scout, wondering why a day's hike often started in sunshine but ended in a downpour.
"That initial project was very simple. I built equipment to shine light through a hole across which raindrops fell," Chan says. "By counting the pulses and with the necessary calibration, the light blocked from the receiver on the other side could tell you the size of the drop and, therefore, how much water there was."
Correlating this information with distribution patterns and radar readings of local weather systems made it easier to assess total rainfall and drew Chan further into the study of atmospheric science and the physics of cloud formation.
Having collaborated with the then Royal Observatory during his master's, he knew there were limited options to research meteorology in Hong Kong. He headed first to the University of Hawaii in the mid-1970s to measure raindrops in clouds, and then to Colorado State University to take on a project with immediate practical purpose and - for a Hongkonger - obvious appeal.
"It was to study tropical cyclones - typhoons - and the physics that cause their movement," Chan says. "A typhoon is not a solid object like a cork floating in a river; it is not that simple. My PhD explained the different flows involved [depending on the position and direction]."
Returning in 1986 for a role at the Observatory, Chan admits to feeling slightly "lost" at an organisation whose focus was on improving operational forecasts rather than doing research into what causes the weather. He leapt at the chance to join the just established City Polytechnic (now CityU)
where he was later able to research phenomena of special interest locally, such as the sudden drop in winter temperatures and heavy summer rain squalls associated with monsoons. "It intrigued me, and when air pollution started to become a problem in Hong Kong, I jumped into that as well," he says.
As a professor, Chan originally had to accept career options were limited for students specialising in his area. Most courses were part of a BSc in applied physics, providing broad knowledge of the principles and instrumentation of atmospheric physics and environmental science.
That, though, is set to change. Many more jobs that require climate knowledge and environmental expertise are opening up in a range of sectors, from consulting and government roles to a variety of positions in insurance and non-governmental organisations.
As a result, Chan took charge of the new school of energy and environment 18 months ago and has been developing a four-year BSc to start next year, focusing on the inter-related subjects of climate change, air pollution and energy efficiency.
"There is growing student interest in these areas and every sector now requires professionals with knowledge of the environment," he says.
Making a difference
Johnny Chan this year became the first Hong Kong scholar to be elected a fellow of the American Meteorological Society
Says everyone can help the environment by simply switching off unneeded lights