The short answer
Warm rather than hot. Night-time lows in July and August average about 21 °C, after days of about 37 °C, so evenings are the best part of a summer day. Rooftops, the square and late dinners come into their own. Inside, thick-walled riads hold cooler air, and most riad bedrooms are air-conditioned.
In more detail
Our climate table gives lows of about 18 °C in June, 21 °C in July and August and 19 °C in September. The city's rhythm follows: people stay in through the afternoon and come out after sunset, which in midsummer is at about 19:40. Jemaa el-Fnaa stays busy until about midnight.
Early evening can still feel hot, because walls and paving give back the day's heat. By late evening it is comfortable, and a rooftop with a breeze is the place to be. You will not need a jacket in the city.
Sleeping is easier than you might fear. Traditional riads are built with thick walls, small outside windows and a shaded courtyard to stay cool, and our summer guide notes that most riads also air-condition the bedrooms. Ask before you book if air conditioning matters to you. Out in the Agafay, summer nights are mild and pleasant after the heat of the day.