Change the weather on one day, 1900-present

Kaze

Banned
Avert the Hindenburg Disaster. Instead of a rainy bad afternoon (one of the theories suggests a lighting discharge), the airship lands safely and continues its long service until Allied bombers in World War Two mistakenly bomb the hanger destroying the Hindenburg. Post war, the company rebuilds its fleet and continues onward as the "luxury liner of air-travel".
 
Bockscar.

I'll throw one out there: In October 1919, an extended period of bad weather allowed White Sox pitcher Red Faber to fully recover from the flu. With Faber healthy and in the rotation, mobster Arnold Rothstein abandoned his scheme to fix the World Series, and the White Sox won five games to two. Infielder Chick Gandil was later released due in part to suspicion that he attempted to broker a deal to fix the Series.

The big news in the offseason was the White Sox trading Shoeless Joe Jackson and $80,000 to the Red Sox for Babe Ruth.

Ruth was eventually traded to the Browns during a contract dispute. White Sox owner Charles Comiskey was a notorious cheapskate and also unloaded Ed Cicotte, Lefty Williams, and Buck Weaver during the 1920s. But his stinginess had one positive effect. In an effort to cut payroll during the Depression, as well as take advantage of the South Side's changing demographics, Comiskey signed Cool Papa Bell, Satchel Paige, and Josh Gibson in 1933. The Pale Hose went on to win the 1933 World Series against the New York Giants, and most teams integrated by 1939, with the Braves, Phillies, and Red Sox as the only holdouts.

So Ruth joins George Sisler, Ken Williams and others, helping the Browns win the 1922 AL pennant and World Series. St. Louis becomes a hardcore AL town, and the Cardinals are always playing second fiddle. By the late 1940s, they've had it in St. Louis and move to Baltimore, becoming the Orioles...
 
About Sarajevo in 1914: all you need to do is have the day get modestly warmer. Naturally aspirated cars had a tendency to vapor lock and be difficult to start in warm weather if one attempted to re-start a hot engine. Have it just warm enough so that 1911 Graf und Stift has trouble starting, yielding a few minutes' delay, and Gavrilo Princip says the hell with it and walks away from his post having accomplished nothing.
 
Something a bit more recent that could be interesting on terms of butterflies: Sep 11th, 2016, NYC: A bit colder and dry weather helps Hillary Clinton to attend the 9/11 memorial ceremony until its end....no overheating situation and pneumonia is discretely treated during the next days...
 
September 11th, 2001: A torrential downpour of rain in the Northeastern United States grounds all flights on the east coast, preventing air travel for much of the day.
Rain wouldn't, but widespread fog could slow operations enough for the hijack teams to be stretched out, giving time 1-3 of the planes to be grounded before the hijackings can commence, or the F-16s intercept.

Personally having been in Philly that morning, I'd like to not associate absolutely perfect weather with that day.
 
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