(WKBN) – Monday, there were areas of light rain/drizzle in parts of the Valley, but there was no precipitation on the radar. If you pay attention to the weather around here, then you might have noticed that this phenomenon occurs routinely.
If radars are able to detect all precipitation, how is there rain with no returns on radar? Well, the answer provides some insight into how radars work and even proves that the Earth is round.
National Weather Service (NWS) radar transmits an electromagnetic signal and then “listens” for a return.
In the above example, the radar pulse interacts with the thunderstorm cloud. The distance from the radar and the intensity of the storm can then be determined by the information returned to the radar.
Radar is an extremely powerful tool, but there are some issues when interpreting the radar. One of the main issues is that the elevation of the radar beam increases in height with increasing distance from the radar.
Therefore, the radar data comes from higher in the sky at distances far from the radar. This happens because as the radar beam travels through the air, the Earth curves under it because the Earth is round.
The National Weather Service can transmit at different elevation angles, but the lowest for most radars is 0.5 degrees relative to the ground. The graph below shows the changes in elevation of the radar beam with increasing distance (disregard the arrows as they are not relevant to this story). .
(Graph courtesy of the National Oceanic and Atmospheric Administration (NOAA).
Notice that the center of the radar beam at 50 miles from the radar is over 5,000 feet. This increases to 10,000 feet at 100 miles from the radar. This obviously has an effect on what the radar is seeing at a given time.
There are scenarios where a radar beam can overshoot the precipitation. This can especially happen if the precipitation is confined to the lower atmosphere. This is the exact scenario that occurred Monday.
There are two NWS radars that cover our viewing area: Cleveland and Pittsburgh. The Cleveland radar is 64 miles away from Youngstown, while the Pittsburgh radar is 45 miles away. Therefore, the height of the center of the radar beam over Youngstown is 4,920 feet for the Cleveland radar and 3,280 feet for the Pittsburgh radar.
If there is precipitation occurring below these heights, then the radar will not pick it up. This happens often in our area because lake effect precipitation from Lake Erie forms at low levels in the atmosphere.
This is why our meteorologists always ask for reports from our viewers. You can help us immensely if there is precipitation flying under the radar!