A researcher at the Max Planck Institute for Chemistry in Mainz, Germany, has suggested a very simple formula for converting between the relative humidity and the dewpoint temperature without needing a calculator, which makes it much easier to determine expected comfort levels and cloud base altitudes on humid days (Bulletin of the American Meteorological Society, 86, 225-233, 2005).
Meteorologists indicate the amount of moisture in the atmosphere in many different ways. Two of these are the relative humidity and the dewpoint temperature. People generally are most familiar with the relative humidity, and know that when the relative humidity is high, like 90%, the air can begin to feel uncomfortable, especially when it is hot. Meteorologists, on the other hand, tend to prefer the dewpoint temperature, which is a better measure of phenomena such as comfort levels, the altitudes of cumulus cloud bases, and the potential effectiveness of evaporative coolers.