A cumulus cloud is usually a child of the sun, a product of fine weather. It resembles a cauliflower in appearance, and normally plenty of blue sky is visible between the individual clouds. The vertical currents of air in which cumulus clouds form occur when a volume of air near the ground has been warmed, often by contact with the surface, so that it acquires a certain buoyancy relative to the atmosphere surrounding it; as warm bubbles of air begin to drift upwards, the rising air becomes cooler with increasing height, and its temperature eventually falls to the dewpoint, the temperature at which condensation takes place. The result is a cumulus cloud.
At first glance, cumulus clouds might appear to be arranged randomly around the sky, but a closer look will often reveal a certain order in the pattern. Sometimes, for example, a family of cumulus may lie in a chain marking the course of large river, forming in this vicinity because of the abundance of moisture available. More often, when many such clouds form over an extensive area, they will be seen to have arranged themselves in parallel lines across the sky; these "streets" of cumulus are usually orientated in a direction parallel to that from which the wind is blowing.
Streets of cumulus occur because certain areas on the ground are better than others at triggering the convective processes that form the clouds. Such surfaces become a little warmer than their surroundings, perhaps because they are slopes facing the sun, or are darker in colour than other places in the vicinity, are sheltered from a cooling wind, or comprise built-up areas where warm air can accumulate between buildings.
In any event, a bubble of warm air begins to ascend over this area, and if the thermal structure of the atmosphere is suitable, a rising current is established. This updraft, however, does not remain in situ; it drifts bodily with the wind, and the eventual cumulus cloud that forms near the top of the pillar of rising air will appear some distance downwind of the "hot spot". After some time, another updraft is initiated over the hot spot, and another cumulus forms to drift downwind - and so a "street" evolves.
Once a street has formed, the ground in its shadow, protected from the sun for much of the time, becomes cooler than elsewhere, and this suppresses any tendency for other updrafts to be triggered along its shady line; this prevents the formation of maverick clouds which might otherwise interfere with the established regular and linear pattern.