The spread of COVID19 through droplets ejected by infected individuals during sneezing
and coughing has been considered a matter of key concern. Therefore, a quantitative
understanding of the propagation of droplets containing the virus assumes immense
importance. Here, we investigate the evolution of droplets in space and time under varying
external conditions of temperature, humidity, and wind flow by using laws of statistical
and fluid mechanics. The effects of drag, diffusion, and gravity on droplets of different
sizes and ejection velocities have been considered during their motion in air. In still
air, we found that bigger droplets traverse a larger distance, but smaller droplets remain
suspended in air for a longer time. Therefore, in still air, the horizontal distance that
a healthy individual should maintain from an infected one is based on the bigger droplets,
but the time interval to be maintained is based on the smaller droplets. We show that in
places with wind flow, the lighter droplets travel a larger distance and remain suspended
in air for a longer time. Therefore, we conclude that both temporal and geometric distance
that a healthy individual should maintain from an infected one is based on the smaller
droplets under flowing air, which makes the use of a mask mandatory to prevent the virus.
Maintenance of only stationary separation between healthy and infected individuals is not
substantiated. The quantitative results obtained here will be useful to devise strategies
for preventing the spread of other types of droplets containing microorganisms.