Late July in St. Louis can be extremely hot (often in the 90s Fahrenheit / 30s Celsius) and extremely humid (often ranging between 60% and 90%). However, almost all buildings are air-conditioned and kept at a temperature (about 70 Fahrenheit / 21 Celsius) that, to some people, feels very cool in contrast to the outside.
As a result, we recommend that you dress in layers. When outside, wear light clothing, but inside, you may wish to add a jacket or sweater. Also recommended is a rain jacket or umbrella in case of inclement weather.