  One of the many unresolved questions that revolves around the Covid-19
pandemic is whether local outbreaks can depend on ambient conditions like
temperature and relative humidity. In this paper, we develop a model that tries
to explain and describe the temperature and relative humidity sensitivity of
respiratory droplets and their possible connection in determining viral
outbreaks. The model has two parts. First, we model the growth rate of the
infected population based on a reaction mechanism - the final equations of
which are similar to the well-known SIR model. The advantage of modeling the
pandemic using the reaction mechanism is that the rate constants have sound
physical interpretation. The infection rate constant is derived using collision
rate theory and shown to be a function of the respiratory droplet lifetime. In
the second part, we have emulated the respiratory droplets responsible for
disease transmission as salt solution droplets and computed their evaporation
time accounting for droplet cooling, heat and mass transfer and finally
crystallization of the salt. The model output favourably compares with the
experimentally obtained evaporation characteristics of levitated droplets of
pure water and salt solution, respectively, ensuring fidelity of the model.
Droplet evaporation/desiccation time is indeed dependent on ambient temperature
and relative humidity, considered at both outdoor and indoor conditions. Since
the droplet evaporation time determines the infection rate constant, ambient
temperature and relative humidity are shown to impact the outbreak growth
rates.
