how do i calculate rad.maxlat, rad.maxlon, rad.minlat & rad.,minlon for USA ?

Hello team, 

I want to use animated Radar and satellite images,So I am referring this http://www.wunderground.com/weather/api/d/docs?d=layers/radar-satellite but i am unable to set parameter for US and specify city, please tell me how do i calculate rad.maxlat, rad.maxlon, rad.minlat & rad.minlon for USA ?