The Dundas Island polynya is a natural laboratory for the study of atmospheric turbulence and heat exchange between the Arctic Ocean and the atmosphere in extreme (20--34 ¿C) sea-air temperature differences. A sonic anemometer-thermometer measured wind and temperature fluctuations at a height of 4.4 m at the downwind edge, where turbulent heat flux was characteristic of the surface flux from the polynya. Profiles of wind, temperature, and temperature fluctuations were also measured. Temperature ramps associated with the passage of convective plumes give skewed temperature distributions. Sensible heat flux is related to wind speed and sea-air temperature difference by a stability-dependent bulk coefficient. Formulas describing turbulence structure in convective boundary layers in terms of 'similarity' theory are verified and compared with published results taken in less extreme temperature gradients. By using these formulas, heat flux can be estimated from either temperature variance or temperature spectra.