Altocumulus Sunrise

The morning of August 10, 2019 dawned with the eastern sky full of backlit altocumulus. The contrast between light and dark was dramatic. Notice the cumulus elements in the midlevel cloud layer. Some are very small while others are larger and more dense. The dark cloud bases show where clouds are thickest. Altocumulus indicate instability in the middle levels of the atmosphere.

The second photo below shows the eastern sky a few minutes later. Clouds had receded eastward, meaning the layer was now lower in the sky. A hint of blue near the top of the frame shows what was coming for the day. In this case the cloud layer was created by an upper level disturbance (region of cooler air aloft) that crossed Iowa. Upward motion with the system released the instability causing the altocumulus to form. Lower clouds did not form because the upward motion and moisture was only sufficient above 6000 feet for cloud formation.

Think about air motion in terms of up and down and sideways. The sideways motion (horizontal) is generally stronger than the up and down motion. We call horizontal motion “wind.” When up and down motion becomes strong we often end up with thunderstorms - cumulonimbus clouds. This was not a thunderstorm day for us.