1999 Nissan Pathfinder a belt sqreaches when cranked

During the past month (when the weather has gotten warmer), a belt in my engine makes a horrible sqreaching noise when I first crank the vehicle. The noise lasts for about 5 seconds or so. It only does this when it hasn't been running for several hours (like when I crank it first thing in the morning and when I get off of work).
Thank you for your help!
Tracy