Anomalous Propagation, also called anaprop (or more simply AP), is a phenomenon often seen on the weather radar and is something that many of our PWL viewers have asked about.
In very simple terms, AP refers to a false echo on the radar, which to the untrained eye, looks like rain. There are several reasons why the radar might show AP, but here in Perth the most common is due to something called temperature inversion. See our article on how the weather radar works here.
Under normal atmospheric conditions, the warmest air is found near the surface of the earth. The sun warms the surface of the earth (or ocean) and that warm air rises. The air gradually becomes cooler as altitude increases. In the evening, when the sun is low (or after it has set) the surface temperature can drop quite quickly. If the conditions are right, this creates a situation where there is a layer of cooler air sandwiched between the surface of the earth and the warmer air that rose earlier. This condition is known as temperature inversion.
Normally, when a radar signal encounters rain in the atmosphere, the signal bounces off the water particles and returns to the radar.
The following ‘normal’ radar image shows a line of thunderstorms approaching Port Hedland.
If there is an inversion layer however, the radar signal hits this layer of warmer air and is deflected downward towards the surface of the earth. The signal then bounces back and the radar reads this return signal as it would any other particle in the atmosphere.
The radar image below is from Broome, and shows the AP just off the coast.
Typically, when looking at a radar loop, you can tell if it is AP because it generally remains in the same location. Rain cells typically move in one direction.
Disclaimer: As you might appreciate, understanding AP, inversion layers and radar is a lot more complex than my simple explanation. If you are interested in the science behind AP you can look here for starters. Good luck!
Image credits: Radar images from BOM, illustrations by Matt Fricker.