Meteorological data and water temperatures from a 98-day period during the summer of 1980 are used to examine the suitability of routine weather observations for quantifying the heat energy budget of a shallow lagoon on the Atlantic coast of Florida. Hourly cloud-cover observations are used to calculate incoming solar radiation, and results are compared with pyranometer readings. The standard error of the estimate is 26% of the mean daily accumulation for simulations of one day, but this decreases to only 0.3% of the total accumulation for simulations of 21 days. Wind speeds recorded at a weather station 4 km from the study site must be modified by a site-specific wind ratio before they can be used to calculate latent and sensible heat fluxes. Even with site-specific corrections to routine weather data, the standard deviation of simulated temperatures is 2 to 3 times that of observed temperatures. Cloud cover and wind speed are the two variables most difficult to incorporate into the simulations.