My story is post-apocalyptic - takes place when they have put society back together again but in a new way. I'm flexible on when this takes place - could be 5 years later or 100 years later.
For backstory, I need an asteroid big enough to cause turmoil (environmental & societal) for years & preferably some permanent significant changes in the environment/landscape, but not big enough for global extinction.
After some research, I like the idea of a fragmented asteroid, with several very small pieces and one comparatively large piece, impacting the earth.
My current working theory if for the largest piece to be a 3-4 kilometers, iron core asteroid striking the continental shelf off Florida at a 90 degree angle with a velocity of 72 km/s at a water depth of about 500 ft, but I have trouble wrapping my brain around the answers from the Impact Earth! tool and I can't find a clear answer on how severe or how long the impact winter would last under these conditions.
I've been reading several other answered questions on here, and I did find this answered question which was very helpful,
but I'm not entirely sure where Kromey gets: "average ground temps will drop by around 13C (23F, if I did the math right)." The Wikipedia page on Impact Winter says the regolith "lowers the mean global temperature up to about 20 K (20 °C) after a year". Which doesn't make sense to me since 20 Celsius = 293.15 Kelvin. When I convert celsius to farenheit, 20C=68F and 13C=55.4..... so neither the Wikipedia nor Kromey numbers make sense to me. - Thanks to Twelfth I have recalled my high school science lessons and figured this part out! :)
Logically it would seem that the more particulate blocking the sunlight, the lower the temperature and vice versa so it would seem the details of the impact (size, location, etc) should affect the severity of the impact winter, but I can't clearly identify those details in either the Wikipedia or Kromey scenarios. In short... help!
A follow up to that issue is that I am reading different accounts of how long the impact winter would last. I've read several that say 1 year, but others that say 5 years and still more that say thousands of years...
So again, it makes sense to me that this would also be dependent on how much particulate matter initially went into the atmosphere (taking into account water vapor, fire soot, impact debris, etc). Yet I can't find any clear way to figure out what the details of the asteroid should be to effect an impact winter of X severity lasting X years... if that makes sense. If someone can help me figure this out, I would be very appreciative!
ETA - So my question really comes down to: Do the details of the asteroid impact effect the severity & length of the impact winter?