Caveat

One complication here is that political dynamics are a moving target. If someone is at least 40 years old, and attended a Fourth of July event without rain before they were 18, that means they attended the event either while, or before, Ronald Reagan was president. One wonders how this correlation might vary depending on what year the person attended the event, or what year the person turned forty.

It'd also be fascinating to know how the probability of good weather on July 4 varies with geographical region of the country.

Or families who of fervent republican persuasion are more likely to be fair weather 4th July flag waving gay celebrants. ;)