Relative Humidity is just that, "relative". It is a measure of how much moisture is in the air relative to how much it can hold. A better measure of atmospheric moisture is the Dew Point temperature. The Dew Point is he temperature to which a parcel of air must be cooled for dew (clouds or fog) to form. When the air temperature and dew point temperature are equal, the relative humidity is 100%. Knowing that, with an air temperature of 85 degrees, the dew point would have to be 85 degrees for the relative humidity to be 100%. However, in the world of weather, we almost never see dew points above 80 degrees, and I have never witnessed a dew point of 85. So, it's simply not possible to have a temperature of 90 with 100%. Typically in tropical climates, high temperatures climb into the 80's and 90's in the summer. Dew points will be in the upper 70's on the muggiest days. With a temperature of 90 and a dew point of 75, the relative humidity 61%, yet most people would find that kind of day to be stiffling and unbearable. Also, since a very moist atmosphere takes longer to heat than a dry one, temperatures on a very humid day will rarely get above 90, while on dry, hot summer days (or if you live in a desert), highs can rapidly climb above 100 degrees.
If you would like to calculate your own relative humidity, use our humidity calculator at the bottom of the FirstWeather page