Radio interferometers consisting of two small antennas separated by a few times their own diameter have a strong instantaneous response to atmospheric water vapour because the beams overlap in the lower atmosphere. Any cloudlet of water vapour situated in the overlapping beams may radiate into both antennas simultaneously, giving rise to a correlated signal that is unwanted when making astronomical measurements. Here it is noted that the water vapour is not stationary over the long period of a typical integration in a microwave background experiment but is carried by the wind through the antenna reception pattern, so that each cloudlet is better regarded as a uniform line source of radiation than as a point source. The trajectories of most cloudlets take them through positive and negative fringes of the interferometer response pattern, making it possible for the radiation from a cloudlet at any instant to cancel some of its own effect at an earlier or a later time. This self-cancellation is investigated numerically for a simple interferometer and found to be considerable: even for the worst case, in which the antennas are so close that the edges of the apertures are in contact, more than half the trajectories show less than half the response that they would if this effect did not operate, and the extent of the cancellation increases with the baseline separation. A similar self-cancellation takes place when a single antenna is operated in beam-switching mode, but it is less effective. Emission from atmospheric water vapour is expected to be the limiting factor when observing the microwave background radiation on angular scales of about one degree, so it appears that the correlation interferometer is the instrument of choice for such studies.