Sometimes it's a bit hard to understand, but there is humor in science. In the 1970s, E.M. Southern developed a method for locating a particular sequence of DNA within a complex mixture. This technique came to be known as Southern blotting. In a tongue-in-cheek fashion, those who used a similar method for locating a sequence of RNA named it Northern blotting. It is also known as Northern hybridization, or RNA hybridization.
The procedure for and theory behind Northern blotting is almost identical to that of Southern blotting, except you are working with RNA instead of DNA. Please see the section on Southern blotting for more information.