It seems like around here mayflies spawn around the summer solstice. This year there were storms all days except for June 24th. It seemed like a good day for mayflies and there were huge swarms at the Pt. Pelee marsh at sunset. Besides the mayflies near ground level I saw some unusual light brownish clouds. Those could have been pollution but I wondered if there were maybe large clouds of insects.
I remembered reading a story about mayflies showing up on weather radar and I checked that when I got home. Sure enough, something interesting was happening. Click on the image to see the animated GIF of radar that night. (Picasa Web Albums converts GIFs to PNG which loses animation so I hosted via ImageShack.)
Unfortunately, those were not mayflies. I contacted Environment Canada's weather office that night and in the morning I got a response telling me it was anomalous propagation due to a temperature inversion. (I was surprised by how quickly someone responded!) I was also given a link to a page explaining this and other radar interpretation errors, which I probably should have checked first.
Later, it was interesting looking at the errors I had made, such as not being very sceptical when something wasn't quite right. In the start of the video, radar echoes appear well away from marshes. Later echoes on the south shore of Lake Erie seemed more probable, but the lines going inland didn't seem to exactly correspond to rivers. In the morning before I got the response I was surprised by the echoes, but I thought "okay, maybe they'll stay around for a few more hours". Now, one look at the overall video makes me think "these were obviously artifacts".
Here's how I made the animation: I discovered the image URL format using the "open blockable items" dialog of AdBlock for Firefox. I noticed that old images are deleted and so I couldn't get the whole night of images in the morning, so I used the Windows at command to schedule automated downloads via wget. Then I went through the images and duplicated some to replace missing images (apparently some never get put on the website). Finally I assembled them into an animated GIF using gifsicle.