Angels are echoes caused by a physical phenomenon not discernible by the eye at the radar site. They are usually caused by bats, birds, or insects. Angels typically appear as a donut-shaped echo with low reflectivity values (see Figure 15-16). When animated, the echo expands and becomes more diffuse with time.

Angels typically only appear when the WSR-88D radar is in Clear Air Mode because of their weak reflectivity (see Section 24.6.1.3 for additional information on modes). Echoes caused by birds are typically detected in the morning when they take flight for the day. Echoes caused by bats are typically detected in the evening when they are departing from caves.

This angel was caused by bats departing Selman Bat Cave at Alabaster Caverns State Park, OK, around sunset.

AP (see Figure 15-17) is an extended pattern of ground echoes caused by superrefraction of the radar beam. Superrefraction causes the radar beam to bend downward and strike the ground. It differs from ground clutter because it can occur anywhere within the radar’s range, not just at ranges close to the radar.

AP typically appears as speckled or blotchy high-reflectivity echoes. When animating images, AP tends to “bloom up” and dissipate, and has no continuity of motion. AP can sometimes be misinterpreted as thunderstorms; differentiating between the two is determined by animating the images. Thunderstorms move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly.

Wind farms can affect the return signal of the radar beam. Depending on the proximity of the wind farm to the radar site (generally within 10 NM), wind farm turbines can result in beam blockage, false echoes, or high reflectivity values (see Figure 15-18).

Chaff is a counter measure used by the military to distract enemy radars from their true targets (see Figure

The intensity of precipitation is determined from the amount of energy backscattered by precipitation, also known as reflectivity. Reflectivity is determined by: