Sometimes the sky has long, straight and evenly spaced rows of cumulus clouds streaked across it. Your correspondent doesn't say whether the lines of cloud he has observed are across or along the direction of the wind. This is important because each orientation indicates a different phenomenon.
Cumulus clouds form when thermals carry moisture upwards.
As a thermal rises, its pressure and temperature drop. Eventually it reaches the dew point, when the moisture condenses. This forms a cumulus cloud, and because the thermal will have been drifting with the wind during its ascent, the cumulus will be downwind of the point on the ground where the thermal started. Certain areas on the ground are better than others at triggering this convective process. In particular, locations where the temperature is a little higher than the surroundings will tend to start convection.