photo #6108838

When Tim Jordheim (Member 35,295) spotted this sky over Prescott, Arizona, US, he first assumed he was seeing part of the huge canopy of cloud, described as an anvil, which spreads out at the top of an individual Cumulonimbus storm cloud. This might have heralded a coming storm. But Tim checked the radar and saw that no precipitation was expected that day. He realised he was instead spotting the arrival of a weather front. This marks the boundary between two air masses of differing temperatures or densities where, as one region of air pushes the other away, cloud formations develop that may or may not lead to precipitation. As the front approached, thick Altocumulus stratiformis clouds crawled across the sky like the tide covering a beach.The cloud term ‘stratiformis’ derives from the Latin word meaning ‘to extend,’ or ‘to flatten out’, and we use it when clouds spread across much of the sky. Within 40 minutes, Tim tells us, these Altocumulus had indeed filled the sky completely, but they brought no rain. Look closely, however, to the left of his image, and you might be able to make out some dark streaks beneath the cloud layer. These streaks are features known as virga, precipitation falling from a cloud that evaporates away before ever reaching the ground. The cloud was doing its best, but raining never comes easy as spring approaches in Arizona.