The radar legend is centered immediately above the map. It provides information about the currently selected radar and a means for accessing the radar, location, and spotter lists.
1. Selected Radar
The ID and name of the currently selected radar appear at the top the radar legend. Tapping the blue radar ID reveals a full list of available radars, as well as the custom location and spotter lists when those layers are displayed.
2. Volume Coverage Pattern
Weather radars continuously scan the atmosphere by rotating and tilting the radar dish. Typically, the radar will make a full 360° rotation at one tilt or beam angle, then adjust the angle slightly and makes another rotation, repeating this process until it completes a full scan of the surrounding sky. The radar operator can adjust the scanning pattern as needed to optimize performance. The volume coverage pattern (VCP) number indicates one of several pre-defined scanning patterns from which the operator can choose.
3. Operation Mode
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. Clear air mode allows meteorologists to see what’s going on in the atmosphere even though no rain is falling, including cold fronts and subtle airmass boundaries. When conditions are right, these boundaries can become the focal point for storm initiation, so being able to see them is extremely important. Clear air 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. For this reason, the NWS will sometimes leave a radar in clear air mode when it’s snowing.
When precipitation begins, the radar is usually switched to precip mode. The radar scans more quickly and covers more tilts in precip mode, allowing for more frequent updates and a better view of vertical storm structure.
You can learn more about volume coverage patterns and operation modes on this NWS web page.