An unusual cloud formation was seen today just before noon in Mid-Missouri and thought to be a funnel cloud by one resident who watched it form. It is actually a fallstreak hole which is a large circular or elliptical gap that can appear in altocumulus or cirrocumulus cloud layers. These are also called hole punch clouds.
A fallstreak hole forms in clouds containing supercooled water droplets which are droplets in below freezing air but still in liquid form. If ice crystals can form in this layer of supercooled droplets, they will grow rapidly and cause the droplets to shrink or evaporate completely, causing a hole in the layer of clouds.
Aircraft passing through a layer of clouds containing supercooled droplets can trigger the formation of heavier ice crystals which fall to Earth and leave a circular void in the blanket of clouds, with a streak of ice crystals below. This happens as air forced around the aircraft propellers and wings expands and cools below the original temperature of the cloud layer, causing ice crystals to form.
Here are a few photos of hole punch clouds.