"Swiss Cheese Clouds Produced by Aircraft Over Texas" (click on the image to view the full size image)

The TERRA MODIS true color image taken on Jan 29, 2007 at 17:10 UTC shows an altostratus cloud deck over north central Texas that consists of relatively small, supercooled water droplets. The holes and lines in the cloud deck correspond to aircraft that either flew up or down through the cloud (holes) or flew just below or within the cloud. The holes, sometimes referred to as hole-punch clouds, are the result of glaciation (liquid water turning to ice) that occurs when the exhaust from the aircraft provides additional water vapor and nucleating aerosols that are sufficient to initiate the formation of ice crystals. Because the cloud is supercooled (the temperature is below freezing, in this case, almost -30°C), the presence of ice crystals starts the process of glaciation either by collision of the ice crystals with droplets or by drawing water away from the droplets and evaporating them. The ice crystals grow large enough to snow out of the cloud and leave a hole as they fall into lower, drier layers and evaporate. The same thing happens as the plane flies within the cloud, but it produces a line. When it is below the cloud the warm exhaust can loft into the cloud and mix with the cloud causing the same effect. Turbulence produced by the plane could also induce evaporation of the deck along the line of the plane's track. If the cloud layer is either lifting or thickening, the holes can eventually fill in again. If the layer is dropping or thinning, the holes will remain until the entire deck disappears. The GOES false color loop shows the cloud deck progressing across Texas into Louisiana. As it moves, the holes grow and disappear until this "Swiss cheese" cloud begins to dissipate as it moves over the Mississippi valley. This phenomenon requires certain conditions such as a very cold, relatively thin water droplet cloud formed in a stable layer. The phenomenon, known as a distrail, is similar to the formation of a contrail, except that the ice crystals remove water from the cloud droplets instead of from the water vapor in the atmosphere. Thus it creates a hole in the cloud. For more information, see Duda et al. (2001), a paper describing a detailed study of the distrail phenomenon.      + Click here for more "Distrail" Images ...