Thanks to all for the upvotes, just a little more of an explanation for ya. It’s not a perfect science, and I’m not a meteorologist- so I’ll do my best. In the morning humidity is typically higher from overnight. As temperatures rise, humidity drops. When it’s below 20% and combined with dry conditions and wind, it becomes very conducive to fire spread. So the 4pm burn law was created to skip the worst of it. Usually it starts to raise before that time, but it’s a compromise as I’m sure when the law was created there was some push back. Today the humidity doesn’t dip super low, but here’s an example of it raising throughout the afternoon:

https://preview.redd.it/gl4vrvwdtppa1.jpeg?width=1202&format=pjpg&auto=webp&v=enabled&s=7a0e206e4f15b0937e2a66d243be1043a28fa7c9