Dew point is a good indicator of a muggy dayRecently Eliot posted in the comments...

Do you know of a forecast on the web that tries to predict humidity? I'm baffled that it's so hard to find out whether tomorrow will be muggy - given that it's the most decisive thing to know about the weather other than rain.

Well Gothamist Weather will try it's hand at the answer a la Ask Gothamist style... First off, there are areas of many weather sites that do an hourly weather prediction for temperature, humidity, dew point, wind, and precipitation probability. Of note in this category are, and AccuWeather's hour by hour pages. But it should be noted that humidity is not really a good measure of mugginess. So as we head closer to the often brutal summer months, what should we be looking at in the forecast?

Humidity is the amount of water vapor in the air. The term "Relative Humidity" is often misleading as it refers to the percentage of water vapor in the air in relation to the total amount of water vapor the air can hold at that temperature. The warmer air gets, the more moisture it can hold. So then, if the weather is calling for 45 degrees and 90% relative humidity, it isn't going to feel very humid compared to if the weather is 80 degrees with a relative humidity of 65%. It will feel downright thick.

The more accurate way to measure the mugginess factor is with dewpoint. Remember that the warmer air is, the more moisture it can hold, and the reverse holds true as well. So if we cool the air while maintaining the same moisture in it, eventually the air will not be able to hold the amount of moisture present and it will cause condensation. This is the dewpoint. It is more of an accurate measure of the actual moisture in the air... and how uncomfortable it is outside.

NOAA has a rough guide for dewpoint (in degrees F)

Less than 50Not Muggy
50 - 59slightly muggy
60 - 69moderately muggy
70 - 79very muggy