This paper describes a method for estimating wind field (wind velocity, rate of change of wind velocity and wind gradient) for small and mini unmanned aerial vehicles. The approach utilizes sensors which are already part of a standard autopilot sensor suite (GPS, inertial measurement unit, airspeed, magnetometer). The primary motivation is enabling energy harvesting; a secondary motivation is development of a low-cost atmospheric measurement and sampling system. The paper presents an error analysis and discusses the primary contributions to error in the estimated wind field. Results of Monte Carlo simulations compare predicted errors in wind estimates with actual errors and show the effect of using estimated winds for energy harvesting from gusts.