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I wonder if there is a problem with the data collection at that time? From your link, it looks like to drop much lower than 0.33, lowest value on that chart is 0.11, which is massively lower than any previous worldwide value, before jumping up to ~10.

Maximum being 1 or 100?

I believe 100, because lower than 36% humidity is fairly common while lower than 1% is incredibly rare.

The graph and chart indicate it's 0 to 100%, so 0.33 is indeed one third of one percent.

What causes the humidity to be so low? Is the fire lowering the humidity? Or the lower humidity causing the fire?

Wiki Santa Ana winds. They typically bring <10% humidity as they blow towards the pacific ocean. This can happen overnight and you'll wake up with "raisin-eyes".

You have your units mixed up - Iran record is two orders if magnitude lower than in LA (30% vs 0.3%)

That’s not what his link [1] shows. Santa Monica airport appears to have reported a relative humidity of 0.33%