What's the difference between humidity, and relative humidity?

Water is a unique substance. It can exist as a liquid, solid (ice), and gas (water vapor). A primary way water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Winds in the atmosphere then transport the water vapor from one place to another. A major source of water vapor in Kentucky is the Gulf of Mexico. Most of the water vapor in the atmosphere is contained within the first 10,000 feet or so above the earth's surface. Water vapor also is called moisture.

ABSOLUTE HUMIDITY:
Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount of water vapor, the higher the absolute humidity. For example, a maximum of about 30 grams of water vapor can exist in a cubic meter volume of air with a temperature in the middle 80s. SPECIFIC HUMIDITY refers to the weight (amount) of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of air). Absolute and specific humidity are quite similar in concept.

RELATIVE HUMIDITY:
Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

DEWPOINT:
Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare. While dewpoint gives one a quick idea of moisture content in the air, relative humidity does not since the humidity is relative to the air temperature. In other words, relative humidity cannot be determined from knowing the dewpoint alone, the actual air temperature must also be known. If the air is totally saturated at a particular level (e.g., the surface), then the dewpoint temperature is the same as the actual air temperature, and the relative humidity is 100 percent.