When there’s no precipitation in the area, it’s common for the radar to be operating in what is called “Clear Air Mode.” In this mode, the radar is scanning more slowly so that it can be more sensitive and pick up much weaker returns. This allows it to see more details and detect finer particles in the atmosphere, including things like dust and insects.
This more sensitive mode of operation allows meteorologists to see what’s going on in the atmosphere even though no rain is falling. Clear Air Mode gives meteorologists the ability to see things like cold fronts and subtle air mass boundaries. When conditions are right, these boundaries can become the focal point for storm initiation, so being able to see them is extremely important. This mode is also useful for detecting very light drizzle and light snow. Sometimes these phenomena do not generate a strong enough return signal to be detected in precipitation mode, but are clearly visible in the more sensitive clear air mode.