Monarchs on Radar?
National Weather Service radar picked up unusual signals northeast of St. Louis last Friday, September 19th. Meteorologists suspected migrating monarchs. I interviewed Laura Kanofsky and Scott Truet who reported* the finding.

Scott Truet: The rationale that the radar returns were being caused by migrating monarchs was based on the following reasoning:

1. It was obvious that the returns were not weather related. Skies were clear that morning.
2. The targets were moving, so it was not some ground-type target caused by anomalous beam propagation.
3. The returns appeared to be biological in nature. This was determined from some of our dual-polarization radar data that indicated that the radar targets were oblate and not steady state, which is a known characteristic of flying insects.
4. The returns were obviously not other type of biological returns that are fairly well known to us. For instance, migrating birds usually show up just after sunrise and just before sunset, usually heading for bodies of water (lakes, rivers, etc), and the returns scatter quickly. Insects often display as very fine lines. The echo mass on the morning of the 19th was large and fairly persistent.

After ruling out the above causes, our office started looking for other things that could be causing these returns. The forecasters actually pulled up the Journey North web site, which led us to believe that we were detecting monarch migration.

Numbers

Journey North: We read in news stories that you witnessed a swarm of monarchs.

Laura Kanofsky: I wish press hadn't stated that because that's not what we saw. In fact, although the radar we were using is very sensitive to insects, we can't tell the numbers. The radar is not seeing individual targets; it's seeing a volume of space and sensing things happening within that volume: it's designed to see things like snowflakes, raindrops, etc. Friday was a clear day, so we were using clear air mode. It's really sensitive to insects when there's no weather around.

Flapping Flight

Journey North: Your original report mentioned flapping wings, but monarchs don't flap much during migration when there are north winds. Like eagles, they spread their wings and soar with the wind. If they had to flap all the way to Mexico, they'd never get there.

Scott Truet: This was an assumption on our part, due to the fact that the dual-pol data was indicating that the radar energy was being returned by a chaotic, non-uniform target. We assumed that flapping wings would cause that, but I'm guessing that soaring monarchs would also cause this; that is, even while soaring their wings are not perfectly stationary, and each butterfly probably exhibits some up and down motion due to air currents.

Temperature

Journey North: It would be valuable to know the temperature at the altitude at which you detected the signal. Monarchs can't fly unless their flight muscles are 55°F. In the morning — at a high altitude — it may not have been warm enough for flight.

Scott Truet: We began to pick up these returns shortly after 9 am at an elevation of approximately 8,000 feet. Returns suggested biological targets at the 6,000 – 8,000 feet level. Temperatures at this elevation were between 50-55 degrees.

I looked at temperature data taken from a 7 am weather balloon launch in Central Illinois, as well as temperature data from aircraft that were taking off and landing from Lambert Field in St. Louis between 7 and 8 am. This is fairly low resolution data, so it's very possible that the temperature of the air was at the 55 degree "threshold" you mentioned.

North Winds at Upper Levels

Scott Truet: I looked at the winds at the 6,000 – 8,000 feet level and they were very light that morning. Our radar was detecting north winds. However, this may have been "contaminated" by the migrating butterflies because the weather balloon data indicated southerly winds at less than 10 kts, as did the aircraft data. Again, this is rather low resolution data, and it's kind of interesting that the level of flight appeared to be just below a level where the wind direction was changing from a southerly direction (in the lower parts of the atmosphere) to the northwest (at higher levels of the atmosphere). Perhaps they were actually in or just above this transition zone.

South Winds at Ground Level

Journey North: You say that upper level winds were northerly at 6,000 - 8,000 feet. At ground level that morning, the wind map (to right) shows southerly winds. If monarchs flew up through south winds to reach upper level north winds, that would be a significant discovery.

Scott Truet: Many of the weather parameters needed for migration (such as temperatures and height above the ground) appear to fit well with what is known about monarch migration, but some others (such as wind) are not as favorable.

Laura Kanofsky: It's important to know where the data for these maps come from. For example, how often are the measurements made and what's the distance between observations points? This wind map comes off the forecast grid, it's primarily intended for forecasting — plus it's a coarse measurement. The map may or may not depict actual wind speed and direction at the time.

Migration Sightings Reported to Journey North

Journey North: Our network of observers did not pick up a flight in the St. Louis region on Friday morning that would indicate a large migration. Our nearest peak migration reports that day came from Champain, IL. Would that be included in your detection?

Scott Truet: The returns we were picking up were over southern Illinois, east of St. Louis and south-southwest, which would be a VERY good match to your peak ground observations near Champaign, IL on that day.

Journey North: Another thing that seems odd is that people haven't reported monarchs southwest of the St. Louis region, the direction the targets seen on radar were apparently moving. Our peak migration map is empty for about 400 miles to the southwest. Typically, the map shows clear rates and patterns of progression, so I would think people would have observed more monarchs.

Rate of Travel

Journey North: Would it be possible to use the animated radar maps to estimate the rate of travel?

Laura Kanofsky: I will try. The animation depicts about an hour in time, so I may be able to estimate very roughly based on the distance covered in the frames.

Anything Else?

Laura Kanofsky: We're going to keep exploring this. It's OK if we come to the conclusion that it's not what we saw.

Journey North: We're open to the possibility that monarchs are doing something that doesn't match conventional wisdom; they're always amazing us!

—Elizabeth Howard for Journey North

* Original report posted on National Weather Service's St. Louis Office Facebook page. September 19, 2014

"Keen observers of our radar data probably noticed some fairly high returns moving south over southern Illinois and central Missouri. High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets. High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape. We think these targets are Monarch butterflies. A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape! NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!"