<p>You are seeing the gust fronts or <a href="https://en.wikipedia.org/wiki/Outflow_boundary" rel="nofollow">outflow boundaries</a> 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.  </p>

<p>The movement of this density current causes the warmer air it displaces to rise (which can initiate further convection) and this concentrates and lofts insects, which is what you are seeing on radar.  In radar nomenclature you will hear these features referred to as "fine lines".  </p>

CC BY-SA 3.0
2015-08-03T04:15:45.973Z
2015-08-06T13:21:43.397Z
2015-08-06T13:21:43.397Z
casey
<p>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.</p>

<p><a href="https://i.sstatic.net/nxTBj.gif" rel="noreferrer"><img src="https://i.sstatic.net/nxTBj.gif" alt="NEXRAD loop around Denver, CO, Aug. 2, 2015 3:46 PM to 4:32 PM local time"></a></p>

<meteorology><storms>
Explanation for growing circular echo on weather radar?
Patrick
# Explanation for growing circular echo on weather radar?

(Asked by: username_0 on 2015-08-03)

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.

[![NEXRAD loop around Denver, CO, Aug. 2, 2015 3:46 PM to 4:32 PM local time](https://i.sstatic.net/nxTBj.gif)](https://i.sstatic.net/nxTBj.gif)

**Tags:** meteorology, storms


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## 1 answers

### Answer by username_1 on 2015-08-03. Score: 11 | Accepted answer

You are seeing the gust fronts or [outflow boundaries](https://en.wikipedia.org/wiki/Outflow_boundary) 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.

The movement of this density current causes the warmer air it displaces to rise (which can initiate further convection) and this concentrates and lofts insects, which is what you are seeing on radar. In radar nomenclature you will hear these features referred to as "fine lines".


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