Ask Ellen: What’s this weird beam on radar?
GRAND RAPIDS, Mich. (WOOD) — Frequent West Michigan radar watchers may have noticed a weird feature that occasionally appears jutting from the Grand Rapids Airport.
This perfectly straight radial beam isn’t always visible but when it appears, its always to the northwest in a straight line.
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How does a radar work?
A radar works by sending a beam of microwave energy out into the atmosphere as it completes a full 360° rotation. Next, the radar stops transmitting and switches into “receive” mode to see if any energy is bounced back by particles in the sky.
For the first sweep, a radar will adjust the beam to transmit out 0.5° above the horizon. Then, for every successive sweep the radar will be sent out at a slightly higher tilt. For example, after the 0.5° beam is sent out and received, the radar will move up to 0.8°, and so on.
As the beam of energy travels out from the radar in a straight line, it will naturally gain height as it gets further from the radar site. In fact, a radar beam shot out at 0.5° will intersect the bottom of a storm closer to the radar, and the middle of a storm further away from the radar. This has to do with the fact that a radar beam travels in a straight line, but the curvature of the earth increases the distance between the energy beam and the ground.
A weather radar is perfectly tuned to detect hydrometers large and small. Occasionally, beam energy can be bounced back by birds, bats, or bugs. Luckily, a weather radar has a way of discerning the shape of an object allowing the output on a map to filter out the riffraff and plot rain, snow, sleet, and hail (with some occasional exceptions).
So, what causes the straight line spiking from the radar?
It’s not a bird, it’s not a plane, it’s a water tower. In West Michigan, most of the radar data comes from the weather radar located at the GR Ford Airport. Unfortunately, a large water tower sits along Patterson, north of 36th Street, just to the northwest of the radar’s location.
The radar beam being broadcast at 0.5° often has not gained enough height off the ground to clear the water tower by that point. However, daily atmospheric conditions influence the behavior of the beam. This means some days the beam might shoot just above the water tower. Many days, the radar beam is blocked leaving a blank spike shooting out to the northwest on single-site radar displays. Many weather apps use radar data collaboratively across the nation and will use math and additional data or scans to “fill in” the gap.
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