Altocumulus Cloud Streets
November 10, 2015
Cloud streets, such as the altocumulus cloud street featured above, tend to form parallel to the wind direction. As air warmed at the surface rises it pushes against a cooler cap (inversion layer) and begins to spread out. Rising and sinking air currents within a low-level wind field may create long, horizontal convective cells. They're invisible unless the air is just at saturation. In this case, clouds form as the air in the rising currents (updrafts) cools and then evaporates in the drier air of the sinking currents (downdrafts), resulting in geometric rows or lanes between the clouds -- cloud streets. Altocumulus cloud streets form most commonly when cool air streaming over land flows over warmer water, warming the lowest portion of the cool air, creating an unstable environment. Photo taken over the Squantum Peninsula of Massachusetts, in late fall 2014.