Fires simultaneously burning in hundreds of square kilometers could result from a nuclear weapon explosion. The strong buoyancy field of such large area fires induces high-velocity fire winds that turn upward in the burning region. This results in the vertical transport of a large quantity of water vapor. We numerically model the rise of moisture-laden, free-convection columns and examine the size increase of clouds as a function of relative humidity, fire size, and burning or heat release rate. Our simulations indicate that the rise is controlled principally by the fire heat release and the atmospheric stratification. In most cases, enough moisture is lofted to form large cumulus clouds early in the plume development.