  Coronavirus disease 2019 (COVID-19) has become a global pandemic infectious
respiratory disease with high mortality and infectiousness. This paper
investigates respiratory droplet transmission, which is critical to
understanding, modeling and controlling epidemics. In the present work, we
implemented flow visualization, particle image velocimetry (PIV) and particle
shadow tracking velocimetry (PSTV) to measure the velocity of the airflow and
droplets involved in coughing and then constructed a physical model considering
the evaporation effect to predict the motion of droplets under different
weather conditions. The experimental results indicate that the convection
velocity of cough airflow presents the relationship $t^{-0.7}$ with time;
hence, the distance from the cougher increases by $t^{0.3}$ in the range of our
measurement domain. Substituting these experimental results into the physical
model reveals that the small droplets (initial diameter $D \leq$ 100 $\mu$m)
evaporate to droplet nuclei and that the large droplets with $D \geq$ 500
$\mu$m and initial velocity $u_0 \geq$ 5 m/s travel more than 2 m. Winter
conditions of low temperature and high relative humidity can cause more
droplets to settle to the ground, which may be a possible driver of a second
pandemic wave in the autumn and winter seasons.
