Abstract:  Based on the main existing form of viral in the air being attached to tiny droplets from normal breathing or coughing and sneezing, we mainly consider those droplets’ temporal and space dynamics with radius of 0.05~100μm in calm air. It is found that the drops with radiuses are close to 100μm stay in the air less than 2 seconds, which can carry the virus away up to 0.61m when one cough or sneeze. When the radius is in the range of 1~10μm the floating droplets carrying with virus can survive 3.7 hours in the air. While the droplet’s dimension is close to the viral’s it will be present in calm air for nearly one month, and will also have significant diffusions in the air.

Key words: COVID-19, liquid drop, kinetic equation, diffusion equation