Friday, May 16, 2014

Seeing Traffic on Weather Radar

This morning when I woke up, I grabbed my phone to look at radar. That is my normal routine in this pattern. When I clicked around, I noticed something along Interstate 55. It looked like this:
It did not really look like rain and I was already familiar with seeing this feature in the past. I decided to grab some more images to explain to you what could be seen on radar. It was the traffic flow of Interstate 55. Here is a velocity image of the the same area. It appears the average speed was about 70-75 mph. Seems about right:
So, how does this happen? Well, before I looked at any other data, I knew there had to be a temperature inversion. Typically, the air gets colder as you go up through the atmosphere. When a temperature inversion is in place, there is an area of warmer air aloft. I pulled some data and quickly noticed that while we had mid 40s at the surface, the temperature was about 50° at around 1500':
This temperature inversion impacted the radar beam. Typically, the radar beam would be well above the surface of the Earth. In fact, in a normal situation, the radar beam would be about 4,000 feet above the surface:
So, how did the beam hit the interstate, 56 miles away from the radar site? Well, due to the temperature inversion, the radar beam was bent back toward the ground. This is called radar ducting. Here's a graphic to help explain it. It's not to scale, of course:
The temperature inversion went away pretty fast this morning, but for a short time, we could see the traffic on Interstate 55 and Interstate 155. Pretty cool, huh?


No comments: