Tower Cloud and Crepuscular Rays

September 13, 2010


Photographer: Károly Vicián
Summary Author: Károly Vicián; Jim Foster

The photo above shows a backlit cumulus cloud forming over a power plant and a set of rather pronounced crepuscular rays. It was taken just after sunrise near the village of Visonta, northern Hungary on September 7, 2010. Though there are a number of days when one can observe clouds building above this plant's cooling towers, it's unusual to see rays associated with such clouds.

Many refineries, factories and power plants move water heated in the production process to cooling towers so that it can be continually reused. Here, it's typically cooled by a forced airstream. A small portion of this water evaporates; though, most is delivered back to the plant after being cooled. The evaporated water (vapor) mixes with the airstream on its way out of the cooling stacks and often forms a visible cloud. In very warm and dry air, the vapor dissipates before it cools enough to condense. In cooler and more moist air, clouds will readily develop above the towers. As air rises its pressure decreases allowing it to expand and cool until saturation occurs. When this happens, and if there's sufficient condensation nuclei (dust, salt particles, etc.) available in the lower atmosphere, water droplets form and coalesce into clouds.

Photo details: Camera Maker: NIKON CORPORATION; Camera Model: NIKON D60; Focal Length: 18.0mm (35mm equivalent: 27mm); Aperture: f/7.1; Exposure Time: 0.0050 s (1/200); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; White Balance: Auto; Flash Fired: No; Color Space: sRGB.