The highest temperature of Shanghai occurs in July or August. For ten days or so in these two months, the temperature can reach 35 degrees centigrade.
One thing to mention is what is called the Mei-Yu Season which in English is the 'Plum Rain Season'. This lasts for about a month commencing in early summer when the plums ripen along the middle and lower reaches of the Yangtze River. During this period from mid-June to early July it is difficult to tell whether it will be wet or dry and the rainfall will often equal twenty-five per cent of the city's annual total! From late August and through the first twenty days of September, typhoons can bring heavy rain in their wake. You should be sure to bring an umbrella. Of course, the umbrella can also be used as a protection from the strong ultraviolet radiation on the sunny days. You will need long and short sleeved shirts in summer.