Wikipedia defines relative humidity (RH) as "the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature." The definition is correct but hardly understandable.
When RH is 100%, there is balance between evaporation and condensation. The two occur at the same rate, and therefore water on your skin stays on your skin. When RH is a little less than 100%, evaporation will slowly outrun condensation. On a hot day in the South, when you're doing yard work, the production of sweat exceeds evaporation minus condensation; therefore, your sweat tends to drip or to dampen your clothing. When RH is 35% or less, evaporation quickly outruns condensation. When you step out of a swimming pool into air with low RH, your skin dries almost instantly and you will feel chill regardless of the prevailing temperature. I've experienced that in Arizona in summer.
Vapor pressures are difficult to measure, so instead we use a chart that indirectly calculates RH using dewpoint and temperature. Assuming that RH is not 100% already, dewpoint is the lower temperature at which evaporation and condensation balance. This is easily measured. Dewpoints in the southern U.S. during summer are generally in the 70s. I asked a local meteorologist what was the highest dewpoint he had ever seen here, and he said that outside of temporary events like the aftermath of a thunderstorm, the maximum dewpoint is about 82. There may be places in the world where dewpoints routinely are higher; an example is the Arabian peninsula adjacent to a very warm Red Sea or Persian Gulf. But reports in the U.S. of dewpoints in the high 80s or low 90s are very rare and always transient. Why? Because inevitably a thunderstorm will form in such moist air, causing both a drastic drop in temperature and a reduction in water in the air through rainfall. In other words, the atmosphere self-corrects excesses in dewpoint.
In the absence of precipitation, the dewpoint is usually close to the temperature at dawn. Why? Because as the temperature drops slowly overnight, condensation increases. The process of condensation adds heat to the air. In effect, water vapor cushions the fall of overnight temperatures. For example, in Raleigh this morning the temperature bottomed at 78 with a dewpoint of 77… meaning that RH at sunrise was almost 100%.
As the day goes on, however, the temperature rises but the dewpoint doesn't change much, absent a storm or significant inflow of different air such as a cold front or a sea breeze (which occasionally does get this far inland). This means that RH falls during the day. Suppose the dewpoint at 3 pm is still 77 and the temperature is 97. The RH will be 53%. Uncomfortable and potentially dangerous, definitely. But nowhere close to 99/99.
For the temperature and RH both to be 99, the dewpoint would have to be 98. That won't happen! Even a temperature of 90 and an RH of 90 requires a dewpoint of 87, and the likelihood of an 87 dewpoint in Raleigh would be one in a million — and short-lived.
So, while we complain about two weeks straight of moist heat and explain to our European friends (who rarely experience that kind of prolonged moist heat) why we like air conditioning so much and are so willing to accept the CO2 production that most air conditioning requires, let's not be unscientific by quoting exaggerations like 99/99.
