Scorching heat gave way to a green-tinged sky on Aug. 21, 1883, when a violent tornado gashed Rochester, then a southeastern Minnesota wheat hub of about 5,000 people.

But Rochester had no hospital. Not yet, anyway.

Injuries from the intense twister prompted Franciscan nuns to lobby for a hospital, and St. Marys opened six years after the storm. Today it's one of two medical campuses (Methodist is the other) that make up Rochester's Mayo Clinic Hospital.

The tornado struck with an enormous roar at 7 p.m. that Tuesday night, sending residents scrambling to their cellars. By the time the storm passed and the stars came out, the sound of "shrieks and groans" punctuated the scene, according to newspaper accounts.

"Yesterday Rochester was one of the most beautiful cities in southern Minnesota, with fine wide streets, shaded with magnificent trees, containing many substantial business structures and dwellings, and a large number of costly houses, and populated by a well-to-do and intelligent class of citizens," according to the Minneapolis Tribune. "Today it presents a picture of indescribable devastation."

The tornado blew off church steeples and the courthouse cupola. It derailed a train, collapsed a railroad bridge and destroyed more than 100 buildings including homes, schools, mills and grain elevators. The death toll reached upwards of 30, with at least 200 injured.

Rochester Mayor Samuel Whitten said one-third of the city was leveled by what meteorological historian Thomas Grazulis since has judged to be an F5 twister, the ranking for the nastiest tornadoes.

A small boy broke his leg and wrist when he was "picked up by the tornado, hurled across the Zumbro River and deposited near the Oak Wood Cemetery, where all the gravestones had been blown flat," according to local accounts.

With no hospital nearby, 40 of the injured spent the first night at the convent of the Sisters of St. Francis. A local physician, Dr. William Worrall Mayo, scurried to aid the wounded at his office, the Buck Hotel, a dance hall and the German Library Association lodge.

Then 64, Mayo had emigrated from England in his 20s, working first as a pharmacist in New York City and making his way west as a tailor and a medical student. After completing his studies in the 1850s he moved to Le Sueur, where he also worked as a farmer, ferryboat operator, justice of the peace and newspaper publisher to supplement his income.

The Union Army rejected Mayo's bid to become a regimental surgeon in 1861, but he landed an appointment a couple of years later to evaluate draftees for the enrollment board in Rochester. He and his wife, Louise, settled there and raised a family — including sons Will and Charlie.

Will graduated from medical school at the University of Michigan in 1883, the same year as the tornado. Charlie was 18 when the storm hit and would eventually receive medical training in Chicago. Both boys had been riding with their dad on medical runs for years.

The Sisters of St. Francis had sheltered in their convent cellar when the funnel cloud neared, while the brothers Mayo ducked into a blacksmith shop. Their father urged them to care for dozens of the injured that night at the convent.

After the storm caught them flat-footed, Mother Alfred Moes began pushing for a hospital in Rochester. "But when she presented the idea to Mayo, he balked at the plan," according to one history of St. Marys.

Back then hospitals were considered a place to die, and the Mayos weren't sure Rochester was large enough to support a pricy new one. The elder Mayo estimated it would cost $40,000 — about $1.2 million in today's dollars.

"Just promise me that you will take charge of our hospital," Mother Alfred told him, "and we will set the building before you at once."

Within four years, the nuns raised the $40,000 to open St. Marys Hospital. It opened in the autumn of 1889, a day before the scheduled grand opening, when Dr. Charlie and Dr. Will jumped the gun and removed a cancerous tumor from a patient's eye. The Mayos welcomed another eight patients that first week.

"For many decades, Rochester residents dated events from the cyclone and headed straight for the basement whenever a storm approached," according to MNopedia.com. "But as long as the Mayo Clinic stands, Rochester will remember their perseverance in the wake of tragedy."

Mother Alfred spent the first months supervising a team of sisters assigned as nurses who shoveled coal, delivered patient meals and lugged water from the basement. She handed over control to younger sisters in 1890 and retired to St. Paul, where she died at 71 in 1899. For years afterwards, the nuns she had led across the Midwest were known as "Al's gals."

Curt Brown's tales about Minnesota's history appear each Sunday. Readers can send him ideas and suggestions at mnhistory@startribune.com. His latest book looks at 1918 Minnesota, when flu, war and fires converged: strib.mn/MN1918.