Life Cycle of the Union City, Oklahoma Tornado and Comparison with Waterspouts

Abstract

A major tornado struck the small farming community of Union City, Okla., on 24 May 1973. It was on the ground 26 min, attaining a maximum width (at cloud base) of nearly 600 m. Even though the funnel narrowed toward the ground, the width of the damage path consistently equalled funnel width at cloud base. The tornado life cycle consisted of four distinct parts: organizing stage (visible funnel intermittently touching ground with continuous damage path), mature stage (tornado at largest size), shrinking stage (entire funnel decreasing to thin column), and decaying stage (fragmented, contorted funnel). Even in its final stages, the tornado retained its destructiveness. The tornado life cycle resembles, in many respects, that typical of Florida Keys waterspouts. Both commence with surface evidence of vortex existence before a visible funnel cloud has descended a significant distance toward the surface. Approaching the mature stage, the tornado and waterspout exhibit spiral inflow characteristics with a distinct boundary between warm, moist air and cool, dry air. The cooler air mass from a nearby precipitation area apparently cuts off flow of warm, moist air into the tornado's circulation, leading to vortex decay. The visible funnel becomes thin, increasingly tilted and distorted as it dissipates. Major differences between the tornado and waterspout appear to be vortex and parent cloud scales and, to a lesser extent, vortex lifetimes and intensities. Both vortices may evolve rapidly through their respective life cycles without evolving through every stage.