# Explanation for growing circular echo on weather radar?

Does anybody have an explanation for the growing ring/growing circular echo that I occasionally see on weather radar? Attached is a gif of a NEXRAD loop showing areas around Denver, CO, Aug. 2, 2015 3:46 PM to 4:32 PM local time.

• Given today's date, and the timestamp of MM/DD/YY - that looks more like August 2nd, 2015. – user227 Aug 3 '15 at 13:26

You are seeing the gust fronts or outflow boundaries of the thunderstorms around Denver. The rainfall from the storm provides an evaporative flux that cools the air it falls into (preserving $\theta_e$ of air descending from aloft). This cool air is more dense than the surrounding air at and near the surface and spreads out as a density current. The cooler the air (relative to ambient air) the faster it will spread out. You experience the passage of the boundary as the gusty wind conditions that often precede a storm.