This is generally because Wunderground API is returning invalid (or missing) data for your location. For example, if humidity and temperature data are missing, the weather script cannot proceed with the calculation and will return a default 100%. The easiest way to fix this is to identify a nearby personal weather station (PWS) and directly use the PWS as your location. In Edit Options -> Location, when you open the location map, the blue landmarks are your nearby PWS stations. You can choose one of them as your location.