WASHINGTON — Global warming is going to steal away some of those postcard-perfect weather days in the future, according to a first-of-its-kind projection of nice weather.On average, Earth will have 10 fewer days of mild and mostly dry weather by the end of the century, the researchers estimate. Some places will get more days perfect for picnics or outdoor weddings, while other places will lose a lot. Rio de Janeiro, Miami and much of Africa are big losers, while Europe and Seattle will gain nicer weather."It's the type of weather where you can go outside and do something fun," said study lead author Karin van der Wiel, a meteorology researcher at Princeton University and the National Oceanic and Atmospheric Administration .