Iowa City is at its best in the long, hot, green Iowa summer and in early autumn, when the campus is back in session and the temperature drops enough to make sitting out comfortable again. Mid-September through October is the peak — football Saturdays, the Iowa City Book Festival in early October, the colours on the campus trees.
Winters are seriously cold (lows of −15°C are routine for weeks at a time) and best avoided unless you have business at the university. Spring is short and unsettled but pretty; high summer is hot and humid with the occasional severe-weather warning.