<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/betsy-satellite.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/betsy-satellite.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/betsy-satellite.jpg?v=ap&w=980&h=551 800w" > Satellite view of Hurricane Betsy in the Atlantic on Sept. 4, 1965. (NOAA)

At a Glance A reanalysis of hurricane seasons from 1961 through 1965 has been completed by NOAA.

Hurricane Betsy was upgraded to a Category 4 at landfall in Louisiana in 1965.

Nine new Atlantic tropical storms and hurricanes were discovered in the five-year period. A hurricane was just upgraded to a Category 4 more than a half-century after it made landfall.

Hurricane Betsy made landfall in southeast Louisiana on Sept. 9, 1965, where for decades it was originally thought to have made landfall as a Category 3 on the Saffir-Simpson Hurricane Wind Scale .

The storm was devastating, no matter its final landfall intensity. Betsy killed 75 people in the U.S. and is regarded as the first hurricane in U.S. history to cause $1 billion in damage .

But a NOAA reanalysis of hurricane seasons from the early 1960s found signs that it was even stronger than first thought.

A review of data for Betsy found that its Louisiana landfall was a Category 4 with maximum sustained winds of 130 mph. The intensity adjustment was based on several factors, like weather observations, cited in NOAA's report .

NOAA's Hurricane Research Division (HRD) has been reanalyzing past hurricane seasons to improve the historical record. Researchers there review original data from past hurricane seasons such as weather observations and reconnaissance data and then assess storms based how we understand hurricanes in the present day.

The HRD reanalysis also discovered nine new Atlantic tropical storms and hurricanes from 1961 to 1965, the most recent time period reanalyzed. Satellite imagery was in its infancy during those years, which made some storms harder to detect in real-time than is possible now.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/betsy-track.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/betsy-track.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/betsy-track.jpg?v=ap&w=980&h=551 800w" > Track of Hurricane Betsy in 1965. The pink parts of the track show where it was classified as a Category 4. (NOAA)

There have been many adjustments to data from other past hurricanes seasons in recent years as well.

HRD found 10 new Atlantic tropical storms after completing its reanalysis for 1956 through 1960 a few years ago.

Three landfalling US. hurricanes also had their intensity adjusted during those years. One of those was Audrey from June 1957 which was downgraded from a Category 4 to a Category 3 at landfall in southwest Louisiana.

Hurricane Andrew in 1992 had its South Florida landfall upgraded from a Category 4 to a Category 5 in a reanalysis released in 2004 .

Hurricane Michael in 2018 was also reclassified from a Category 4 to a Category 5. That intensity adjustment occurred just months after landfall when the National Hurricane Center released its final report last April.

The HRD reanalysis project is still ongoing, but it's already investigated many hurricanes seasons dating as early as 1851.