Bar of a Tropical Cyclone

Also referred to as the bar of a hurricane, the bar of a tropical cyclone is the gray-black wall of cumulus and cumulonimbus clouds that appears on the horizon in connection with the approach and passage of a mature-stage tropical system. Described in 1697 by the British adventurer William Dampier and coined during the 19th century to describe the appearance from a distance of a tropical cyclone’s eyewall over the ocean, the bar is preceded by the onset of high-level cirrus and cirrostratus clouds, wispy collections of ice crystals that seem to the viewer to converge on the horizon. In turn, the advancing edge of the outflow layer slowly gives way to a thicker canopy of cumulus clouds, grayish-white tufts that fringe the lower reaches of the bar and closely resemble the shape and texture of an enormous mushroom cloud. Contingent upon the intensity of a respective system, the bar can range in color from light gray to indigo to black (a result of the vast quantities of moisture contained within the lower cloud towers of the eyewall) and may assume shades of yellow, red, green, and even purple depending upon the time of day it is observed. Numerous eyewitnesses have noted the vivid sunrises and sunsets that often precede a tropical cyclone’s landfall, the prismatic result of the sun’s orange rays being filtered through the system’s thin outflow shield; and later, as the bar draws closer, through the successive layers of moisture that darken its core.