text: The trees swayed, the rain battered the domed roof of the headquarters building and, given the lecture he'd just delivered to African leaders from their own pulpit, it might well have been the ghosts of dictators past rattling the rafters.
  He talked about democracy and the "cancer of corruption," but the words which reverberated across the continent were an off-script put-down of leaders who "refuse to step aside when their terms end".
  The audience cheered, but not surprisingly the assembly's seats weren't filled by the continent's presidents.
  Instead it was the AU's commission he addressed - its secretariat.
  That was always the plan according to the White House.
  Journalists and commentators had been salivating at the prospect of America's first black president meeting Africa's oldest.
  Robert Mugabe, who's 91, is the AU chairman, and the speech would have rattled the rafters from the inside if the heads of state had been present.
  "I love my work, but under our constitution I can't run again," President Obama said.
  "I actually think I'm a pretty good president - I think if I ran again I could win."
  That's probably not true - given he's only half as popular back in the US as he is in Kenya, the first stop on a trip hailed as a homecoming.
  "I don't understand why people want to stay so long, especially when they've got a lot of money," he mocked, to the loudest cheer of the day.
  Third-termism is catching. It's already made a comeback in Burundi, caused a government to collapse in Burkina Faso, and appears on the cards in Rwanda, the Democratic Republic of Congo and the Republic of the Congo.
  And the three elder statesmen of the AU have racked up a hundred years in power between them.
  Presidents Teodoro Obiang of Equatorial Guinea (36), Mugabe of Zimbabwe (35), and Paul Biya of Cameroon (33) are the longest standing of those leaders on the continent who aren't so keen on giving up the trappings of office.
  "Sometimes you will hear a leader say 'I'm the only person who can keep the nation together'. If that's true that leader has failed to truly build the nation," President Obama said in the last speech of his Africa trip, and arguably his best.
  He emphasised the defining themes of his visit - the significance of security, particularly the fight against extremism; trade replacing aid; the importance of youth and women, human rights and democracy; and the battle against corruption.
  They were points you could tick off from every speech from his very first in Kenya, where he opened the Global Entrepreneurship Summit emphasising trade and promising investment.
  With him on the trip were American businessmen and at least one billionaire.
  He made sure to visit successful start-up companies, among them one developing a solar lighting system to bring power to the remotest of villages.
  Power Africa is one of his headline programmes, but has so far failed to achieve much.
  Big slow-burning fuses in infrastructure, agriculture, trade and young leadership are what he's staking his legacy on.
  Presidents Clinton and Bush ploughed huge sums into Africa to tackle Aids and malaria - massive health challenges that needed the dollars.
  China is now the largest single investor in Africa and spares the AU a lecture, but President Obama hopes he can create long-lasting change - providing "not the fish, but the fishing rod" as he put it.
  His supporters say his African legacy may take time to build, but will bear fruit, and it's obvious he's keen to spend more time on the continent once he's left office.
  "I'll be honest with you," he told the AU. "I'm looking forward to life after being the president. I won't have this big security detail all the timeâ€¦ and I can visit Africa more often."
  Coming to Kenya while president was an important part of plotting out that future.
  He bounded down the stairs from Air Force One in Nairobi, and the video clips of him dancing at the State Dinner showed the passion he has for his "ancestral home".
  And it was great PR for Kenya.
  Terror attacks have hit the country's reputation hard and the first ever visit by a sitting American president did a great deal to improve that global image.
  There was real Obama-mania on the streets of Nairobi, despite the high security and grumbling over road closures.
  And he didn't pull his punches there either - publicly clashing with President Uhuru Kenyatta over gay rights.
  President Obama's renewed engagement with Africa began late in his presidency, by gathering the continent's leaders in Washington last year, amid accusations he was playing catch-up with China.
  America's commitment to resolving the crisis in South Sudan looks unlikely to succeed in the short term as it becomes increasingly complicated and difficult to undo.
  But it's the long game where President Obama, "Kenya's first American president" as he called himself, will best be judged.
target: As President Obama finished his speech to the African Union (AU) in Addis Ababa last month and stepped off the stage, a tempest whipped up outside.