With a cautionary note that it's probably not too helpful to think in terms of "reality", when what we are really discussing is changing perceptions, which are themselves subjective and rooted in the times and the places where the judgements are made, here are some reasons for the changes that have occurred in perceptions of Truman and his presidency.

First, studies of Truman's low popularity ratings at the time of his departure from office show that they correlated significantly with the state of the ongoing war in Korea. Any president running an active war costing numerous lives, and one that (at the time of Truman's departure from office) did not appear to be moving towards a conclusion, is likely to suffer low popularity. That factor has a much lower impact now that the Korean War is, quite literally, history.

A second factor that affected contemporary perceptions of Truman, but which was largely beyond his ability to change, was that the Democrats had been in power for a very long time - two decades, which was long enough for many Americans to feel that it was time for a change in the ruling party. And while Truman could theoretically have run in 1952 (as he had only served one full term as an elected president), there was also a feeling, particularly given Roosevelt's unprecedented four terms, that he should not run again, and potentially serve nearly three full terms himself. It certainly helped that the Republicans fielded a highly credible candidate in Eisenhower, whose popularity and aura of competence were such that the Democrats, too, made an effort to recruit him as their candidate. But all of these issues, which impacted significantly in 1951-52, are pretty much forgotten now, when it comes to assessing Truman.

The factor that you point to, the decision to deploy nuclear weapons in 1945, was actually only beginning to come into play by the time Truman left office, as it became more and more clear how destructive nuclear weapons were and how dangerous it was to live in a nuclear age. At the time the bombs were dropped – and while, yes, some of the scientists involved in the Manhattan programme, who best knew of the destructive power of the new weapons, hoped that they would not be used – the decision was extremely popular with almost everyone (not least with the troops in the Pacific theatre), since it did seem to have ended the war.

Again, it's very important to see this decision in its context. It may seem "unnecessary" to us now, insofar as there were potentially other ways to end the war, we understand how terrible nuclear weapons were and would become, and we know vastly more about the other factors that impacted on the Japanese decision to surrender than anyone - Truman included - did at the time. In the America of 1945, however, pretty much everyone agreed that the alternative to using atomic weapons was an invasion of Japan that was expected to cost 500,000 or more American lives. No US president could expect to survive, politically speaking, the taking of a decision that did not prioritise the lives of his country's troops over enemy lives, especially in the middle of a war that America had not started. That is why, from all the evidence we have, it seems Truman scarcely agonised over the decision at all. We might differ, and think we would have chosen differently, but it's a moral judgement, made with the benefit of knowledge Truman did not have – and while that's a common enough thing for people to do, professional historians quite rightly deplore this sort of historical second-guessing.

With regard to other reasons for Truman's increasing popularity since his time in office, I would highlight the following:

• He refused to let Korea become a nuclear conflict, when it very easily could have become one. Here he benefits from the reverse of the effect I noted above: the decision was unpopular at the time, because it did cost American lives, but from our more distant and more uninvolved perspective, it can be seen as an extremely important one. If Korea had been nuclear, then almost certainly other conflicts since the 1950s would have been nuclear too - use of nuclear weapons risked being normalised. Looked at in hindsight, this was a critical decision that has benefitted all of us alive today. Probably this is the single biggest "tick" historians looking back at the 50s give Truman.

• Truman's decision to fire General MacArthur for insubordination was not that popular at the time - MacArthur still clung to his carefully cultivated image as an effective war hero. Looking back, many now give Truman credit for upholding the principle of civilian oversight of the military - and MacArthur's reputation has taken a serious battering since it peaked with Inchon, so removing him seems a less controversial and more defensible move than it did at the time.

• At the time Truman left the presidency, his administration was seen as prone to cronyism and corruption. Looking back, especially with the benefit of the Nixon administration for comparison, historians tend to view the relatively limited extent of these problems in Truman's time vastly more indulgently.

• Truman's supporters view him as an effective, inclusive leader who did a good job in selecting and marshalling a talented team of subordinates that included George Marshall, Dean Acheson, Jimmy Burns and George Kennan, all still pretty recognisable names today. He was noted for letting the men he chose to run departments of state do their jobs, while backing up the decisions they made and never criticising them in public. His abilities in this respect were such that – almost uniquely in modern presidential history - none of the members of his various cabinets ever went into print as criticising him or his behaviour to them; Acheson was on record as stating that Truman was "so fair!" This contrasts positively with the records of most of the presidents who came before and after him.

• [EDIT] Many historians give Truman considerable credit for beginning – however tentatively – the process of desegregation in the US by pushing through desegregation of the armed forces. This was a divisive policy at the time, but one few would dissent with now, so here again Truman benefits from appearing to be a president with a "modern" take on issues that are still important to us today.

• One final factor, which I personally consider to be pretty important when it comes to understanding how the sort of people who vote in polls that evaluate various presidencies make up their minds, was Truman's attitude to the office of the president itself.

He revered it and (with the exception of writing a set of memoirs that he, as a keen amateur historian, thought would have significant public value, as well as being profitable) he refused to make money out of the fact that he had been president once he left office - turning down numerous lucrative directorships and subjecting himself to considerable real poverty as a result. Many historians have found this admirable, and again the contrast between him and modern presidents who use the office to enrich themselves, and make many millions in fees after they leave it, seems increasingly stark. Truman was really the last "everyman" president America has had, and - in a time when the political class is increasingly distrusted and despised - that counts for a great deal when it comes to perceptions of the Truman presidency.

Who portrays him in such a positive light?

As a Canadian I cant remember one overly positive view.