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Answer: The answer all depends on how much water vapor is in the air already, as indicated by the dew point. Water can evaporate as long as the temperature of the air above it or surrounding it (in the case of suspended drops) has a dew point lower than the temperature of the water. If the water and air are at the same temperature, then as long as the temperature is higher than the dew point, water can evaporate. On the other hand, water vapor will condense into liquid water if the temperature of the air containing the water vapor is cooled to the dew point or below, or can condense onto an adjacent surface if that surface is cooled to the dew point or below.
Nov. 11, 2009 | Tags: general meteorology, humidity/dew point

Answer: That all depends on what you mean by "just 'the humidity'." In a general sense, humidity is any measure of the amount of water vapor present in a given space. For example, there are quantities called "specific humidity" (a ratio of mass of water vapor to mass of moist air in a given volume), "absolute humidity" (mass of water vapor per unit volume, in other words a concentration value or the density of water vapor), and as you noted, "relative humidity." Relative humidity (RH) is a ratio of the actual amount of water vapor in a given volume of air to the amount of water vapor that would be required to saturate the air. The saturation value depends on the temperature, as it takes a greater amount of water vapor to reach saturation at higher temperatures and vice versa. When saturation is reached, addition of further water vapor, or any cooling of the air, will result in condensation of some of the water vapor into liquid water. So, an RH of 70%, for example, means that for the given temperature, the air contains 70% of the water vapor that would be required to saturate it. Adding water vapor at the same temperature would raise the RH and vice versa, while increasing the temperature while holding the amount of water vapor constant would lower the RH and vice versa. In many weather reports, if you just hear the word "humidity," it is RH that is being discussed, but there may be exceptions.
Oct. 30, 2009 | Tags: humidity/dew point

Answer: Relative humidity (RH) is a fraction expressing how close the air in question is to being saturated with water vapor. When air is saturated, any additional water vapor or any lowering of temperature, will result in some of the vapor condensing into liquid water. So, 50% RH means that the air contains about half the water vapor required for saturation, 95% means that it is almost saturated, and so on. RH depends on both the amount of water vapor in the air and the temperature, so it can change even if the amount of water vapor stays the same, or it can remain the same even if the amount of water vapor changes. Dew point is the temperature at which air would reach saturation (that is, RH would climb to 100%) if you cooled it to that level. Dew point depends only on how much water vapor is in the air, so that a higher dew point means more moisture and vice versa. For some additional discussion and examples, see www.wral.com/weather/blogpost/3423262/.
Sep. 8, 2009 | Tags: humidity/dew point

Answer: That reading you recalled of 77 degrees is oppressively humid in itself for this part of the world, and a level that is rarely reached. However, the State Climate Office carried out a records search that shows the Raleigh-Durham airport has recorded dew points of 80 degrees or higher on eight days since 1948. The only two days to reach above 80 were July 24, 1965, when the dew point held at 81 for three hours late in the day, and on August 10, 2007 when there was one observation, shortly after a brief shower on a very hot day, with the dew point spiking to a super-steamy 82 degrees.
Aug. 22, 2009 | Tags: humidity/dew point, records/extremes

Answer: The answer depends on exactly what you're asking with the question and what assumptions we make in answering. If, for example, we keep the temperature constant, and you are asking how much the actual water vapor in the air decreases when relative humidity drops from 69% to 61%, the answer is about 11.6 percent. It's worth noting, however, that you could also keep the amount of water vapor exactly the same, and decrease the relative humidity by simply increasing the temperature a few degrees. For example, if you start with air at 70 degrees and 69% RH, you can reduce the RH to 61% by warming the air to about 73.9, without changing the amount of water vapor.
Aug. 9, 2009 | Tags: humidity/dew point

Answer: Dew will form on a surface if the temperature of that surface falls to the dew point or below. In the case of your deck, the portion of the deck that faces upward to open sky will lose energy by way of upward directed infrared radiation with relatively little energy returned from the sky. The covered portion of the deck also loses heat radiatively, but this is balanced by downward radiation emitted by the roof so that the temperature of the covered portion stays a little above the dew point and the wood there remains dry.
Jun. 19, 2009 | Tags: general meteorology, humidity/dew point