A US study led by Indian-origin researchers found that respiratory droplets from cough or sneeze travel farther and last longer in humid, cold climates than in hot and dry ones.The research team developed this new model to better understand the role that droplet clouds play in the spread of respiratory viruses, the study, published in the journal Physics of Fluids.Their model is the first to be based on a fundamental approach taken to study chemical reactions called collision rate theory, which looks at the interaction and collision rates of a droplet cloud exhaled by an infected person with healthy people.Their work connects population-scale human interaction with their micro-scale droplet physics results on how far and fast droplets spread, and how long they last."The basic fundamental form of a chemical reaction is two molecules are colliding. How frequently they're colliding will give you how fast the reaction progresses," said study author Abhishek Saha from the University of California in the US."It's exactly the same here; how frequently healthy people are coming in contact with an infected droplet cloud can be a measure of how fast the disease can spread," Saha added.They found that, depending on weather conditions, some respiratory droplets travel between eight feet and 13 feet away from their source before evaporating, without even accounting for wind.This means that without masks, six feet of social distance may not be enough to keep one person's exhaled particles from reaching someone else."Droplet physics are significantly dependent on weather. If you're in a colder, humid climate, droplets from a sneeze or cough are going to last longer and spread farther than if you're in a hot dry climate, where they'll get evaporated faster," Saha said."We incorporated these parameters into our model of infection spread; they aren't included in existing models as far as we can tell," he noted.The researchers hope that their more detailed model for the rate of infection spread and droplet spread will help inform public health policies at a more local level, and can be used in the future to better understand the role of environmental factors in virus spread.