You have probably heard the famous saying: “the bad weather always misses Youngstown”. It’s funny because I have heard this same statement now in Alabama, Colorado, Tennessee, etc.
This narrative can be difficult to disprove occasionally because it appears that precipitation goes around big cities on radar in some cases. This phenomenon is called a “radar hole” and it can cause quite the confusion if not understood correctly.
However, fear not, for I will explain this phenomenon to you and then you can be the radar expert in your friend group.
Early Sunday morning, snow appears to be falling in all of central Indiana except for in Indianapolis. Take a look at the image below:
Now, this looks crazy. It appears that there is some type of force field over Indianapolis that is preventing the snow from reaching the city. This image could make you start believing in all different manners of conspiracy theories if you did not know any better. You are going to know better, though.
If you are to understand what a radar hole is, then first you must understand how radar works. Radars emit electromagnetic pulses that bounce off of precipitation producing clouds and return to the radar.
The distance of the precipitation from the radar center is calculated and then the radar data are put on a map for reference.
If the precipitation is occurring close to the radar, then you can assume that the radar is detecting precipitation close to the ground. However, this is not always the case. In fact, the elevation at which radars show precipitation increases with increasing distance from the radar center. This is due to the fact that the Earth’s surface is curved.
Therefore, the radar is actually sampling different parts of the atmosphere at different distances from the radar center.
Let’s take this back to the case from Indianapolis. This morning, there was no precipitation reaching the ground at the radar center. However, 25 miles away, at an elevation of 1,500 feet above the ground there was precipitation picked up by the radar.
Snow was probably occurring over the radar at this time, but dry air between the clouds and the ground was causing the snow to evaporate before it hit the ground. Therefore, there was no snow showing up on radar.
This phenomenon is called “virga” and it is very common at the onset of wintry precipitation events.
Eventually, the atmosphere begins to saturate and the “radar hole” will close up as the precipitation gets closer to ground level. This is exactly what happened over Indianapolis Sunday morning!