May 17-20, 1780 was reportedly dark to very dark in New England from New York to Massachusetts, Maine, New Hampshire, Vermont, Connecticut & Rhode Island. It reportedly did not extend to Pennsylvania so much. The sun turned to a faint, glowing, clearly-visible circle that was green/blue in the afternoon & deep red to rust in the evening. The sky became a featureless sulfur-yellow to green/brown, then blue-gray.
On May 19, it became incredibly dark by afternoon with lanterns needed with not a big of sunshine. Chickens & native birds went to roost, crickets chirped & frogs croaked. A panic ensued with much of the population either going to church or to the local tavern as the notion of Divine Wrath & end of the world was at hand. There was even greater concern when a black, oily rain, mixed with ash & a strong odor of smoke, began to fall.
Eventually, the blanket of darkness dispersed, but for years afterward, this day was marked by much of the population going to church in prayer & fasting to prevent Divine Wrath. It was not until 1881 that an even remote phenomenon occurred in the same area, though a black, oily, ashy rain was reported to the southwest at Elkhart, Indiana in October 1861. 90% of the sun was obscured for a day in September 1881 & a full 60% of the sun was obscured in October 1871 in that area.
The magnitude of the 1780 event was a combination of massive wildfires in the Great Lakes to Ontario & a warm front with clouds & rain mixing with the wild fire smoke at higher levels of the troposphere. The wind may have been easterly or southeasterly, but the flow aloft was from the west & northwest around upper ridging with hot, dry, windy weather in the Ohio Valley. The 1779-80 was incredibly cold & snowy & regarded as one of the worst winter of record dating back to the 1600s in the eastern & southern U.S. Suddenly, late April to May turned, like a switch, hot, windy & dry with a sudden disappearance of snow cover with the brown trees & ground warming the air rapidly. Native Americans were probably burning anyway, which was commonplace in that time, & fires likely spread with uncontrollable vigor & force. After a pattern of wet growing seasons since 1775, it is likely that great fuel developed & Native American fires likely did not expand over such a wide area. There is also the chance that lightning strikes enhanced or caused the fires of May 1780.
The September 1881 & October 1871 fires were major from Wisconsin to Michigan to Ontario with the smoke plume extending well to the east in a weather pattern very similar to that of 1780. This pattern would feature a warm front in the Northeast with low pressure/upper trough in the Plains, ridging in southern & central U.S. & a howling southwest to west wind down sloping from the Plains & drying & fanning Great Lakes forests & barrens.
There are many, many notes in our area regarding smoky air & skies in early times as prairies, barrens & forests burned by Native Americans & also even some lightning strikes. Even in the southern U.S., massive fires in Mexico & Texas (like May-June 1998) turned the sun blue & the sky yellow, green & gray in May of 1831. Smoky days in weather diaries & notes here & areas to our east are most often noted in March & April & October & November. Following the end of the Ice Age (glacial recedence), the climate turned wetter, but there were still very dry period & even hot, dry periods for 10,000 years. Large expanses of prairie, barrens & open forests were found in our region & elsewhere. Native Americans maintained some of these communities in their burning to encourage growth of grasses that attracted large mammals for grazing & made trekking through woods easier. These fires lasted the longest & burned the farthest in level, often streamless tracks. At the arrival of European settlers, extensions of this burning & relicts of these warmer, dry times were shown in the areas of prairie, savanna & barrens (scrubby fire-scarred oaks, hickories, haw & crabapple thickets) in the area, largely north of I-64 in more level areas. Much of the area was indeed wooded, but some oak savanna & barrens were found in northwestern Vanderburgh, Posey & western Gibson counties with lots of barrens extending into Illinois. An arm of central Illinois-like prairie occurred in northwestern Daviess County, Indiana around the present-day Odon/Elnora exit on I-69 (Indiana 58 interchange).
Fire ecology is driven & bred into the genetics of many plants & plant communities in our region & over the eastern & northern U.S. So, fire, be it through lightning &/or Native American, i.e. Aboriginal burning makes darks days like those in 1780 natural occurrences. Such smoke was connected to conditions in our region & it is likely that our own wildfire smoke blotted the sun to our northeast, east & even southeast in early times dating back to the Ice Age.
Similar surface set-up likely to that of May 1780:
Wet 1779 growing season (massive flooding on Wabash & White Rivers in winter-spring 1779), which led to lush Great Lakes forests, then sudden flash to long-term drought (after really a wet pattern since 1775).