As we gear up for what promises to be a spectacular Independence Day, let’s dig into what makes this forecast so rare.

Since Philadelphia’s weather office first started keeping July 4th records in 1872, an average birthday for the nation has delivered a high of 85°, a muggy pre-dawn low near 67°, and about a tenth of an inch of rain.

That rain may not sound like much, but rainfall has dampened 39% of our Fourths, and roughly one in four has seen at least a tenth of an inch.

In three summers (1872, 1928 and 2007) the heavens really opened with more than an inch, 1981 holding the crown at 2.08″–a fire-works-soaking deluge that still tops the chart.

The “Good”

Dry, seasonably warm Independence Days–what many of us would call perfect–happen much less often. About six in ten Fourths stay rain-free, and roughly four in ten also keep the afternoon temperature below 90°.

When you fold in a comfortably cool night (low under 70°–a a sign of lower humidity) the odds shrink to a little better than one chance in four. That means tomorrow’s forecast–sunny, mid-80s, low humidity with dew points in the 50s, and not a storm cloud in sight–lands tomorrow’s weather in the “this-doesn’t-happen-very-often” bucket.

The “Bad”

A handful of holidays have turned into washouts. Besides the record-setter in 1981, thunderstorms dumped 1.95″ on the very first year of record in 1872 and 1.13″ in 2007, chasing picnickers under the nearest pavilion and canceling outdoor concerts.

The “Ugly”

Blistering heat: Only twice has the thermometer cracked triple digits–103° in 1966 and a neat 100° in both 1919 and 1999.

Oppressive nights: 36% of all Fourths never cool below 70°, leaving sparklers fizzling in tropical air.

Surprise chill: On the flip side, 1978 only reached 65° beneath gray skies–and yes, spectators watched fireworks in jackets that year.

A few revolutionary footnotes

When the Continental Congress convened on July 4, 1776, Thomas Jefferson’s diary shows a high of 76° under cloudy skies–more spring-like than summer-sweltering.

Legend says horseflies from a nearby stable moved the declaration process along as delegates swatted away, though most signatures actually trickled in weeks later.

Ben Franklin, ever the weather watcher, sprinkled Poor Richard’s Almanack with wry July advice: “Some are weather-wise, some are otherwise.”

So, how rare is tomorrow’s outlook?

Putting it all together – clear skies, morning temps in the 60s, afternoon highs in the 80s, and comfortable 70s for concerts and fireworks – we’re set to enjoy a “Once in a Red, White, and Blue Moon” Independence Day celebration.

Finally, the weather and the fireworks will both sparkle!