One day mankind may be capable of controlling the weather on a global scale. The key factor enabling control of the weather is that the atmosphere appears to be chaotic and chaos implies sensitivity to small perturbations. Extreme sensitivity to initial conditions suggests that small perturbations to the atmosphere may effectively control the evolution of the atmosphere if the atmosphere is observed and modeled sufficiently well. It is shown that four-dimensional variational analysis∈dex four-dimensional variational analysis (4d-VAR) is a data assimilation technique that has promise for calculating optimal perturbations for weather modification. Experiments described here demonstrate the ability of 4d-VAR to calculate perturbations to influence the evolution of a simulated tropical cyclone. In “damage cost function” experiments described here, 4d-VAR simultaneously minimizes the size of the initial perturbation and an estimate of property loss that depends on wind speed. In these experiments the hurricane surface winds decrease over the built-up area at landfall. It is as if the simulated hurricane "blinks its eye" at a precisely controlled time. The optimal perturbations usually include quasi-axisymmetric features centered on the hurricane. It appears that the perturbation evolves as a concentric wave disturbance that propagates to a focus at the hurricane center, and converts the kinetic energy of the hurricane into thermal potential energy at the appropriate time. The hurricane surface winds regenerate soon thereafter, so a continuous series of perturbations may be needed in practice. Experiments are described with different control vectors, including all prognostic variables, temperature only, and temperature only outside of the center core of the hurricane. The temperature only experiments suggest that precisely prescribed heating might serve to control hurricanes and other weather phenomena in the future. Microwave heating∈dex Microwave heating rate calculations are presented in support of the concept of dual use space solar power satellites for electric power and for weather control. These calculations show that by tuning within the 183 GHz water vapor absorption interval it would be possible to control the height of the maximum heating. The prototype experiments presented here suggest that global weather control will eventually become a reality especially since many of the supporting disciplines will naturally evolve at a rapid pace. The costs associated with recent damaging hurricanes should cause this pace to accelerate. It is plausible that two generations from now controlling the global weather may be within the capabilities and resources of several nations or groups of nations. In the future, NASA’s mission may explicitly include mention of research to control the weather for the benefit of mankind