2025-10-06

Tracking Hong Kong Typhoon Trajectory

Hong Kong receives a few typhoons every year, and the Hong Kong Observatory does a pretty good job of tracking them. Their website offers a page to view the expected trajectory and intensity of a typhoon at various levels of zoom.

Unfortunately, it seems they only provide the latest snapshot of the trajectory / path, and not historical ones. I think the historical prediction is kind of fun to look at and compare it with the actual path.

Downloading images for a timelapse

Since HKO conveniently provides a particular zoom-level of a specific typhoon at a constant URL, I just wrote a simple shell script to grab the image once an hour and save it.

Then, once the typhoon is near the end, I concatenate all the images into a timelapse using ffmpeg to make some fun videos.

Bonus: Deduplication

Depending on the zoom level and the distance from Hong Kong, HKO might update the prediction at a lower frequency (e.g. 2 - 6 hours). In that case the script (as it is) would download duplicate images (or if I forgot to stop it till a week later). Thankfully HKO serves the exact same image, bit-for-bit. So we can do a quick shasum to get rid of the duplicates.

The following works well:

find . -maxdepth 1 -name "*.png" -print0 | xargs -0 shasum | sort | uniq -w 40 | awk '{print "file " $2}' > list.txt
ffmpeg -f concat -safe 0 -r 6 -i list.txt -c:v libx264 -crf 0 -vf "fps=6,format=yuv420p" output.mp4