Rainbow pictures are hard to plan for.  Unless you are photographing a waterfall they are completely unpredictable.  This is one of those situations were you should always carry your camera and be ready to take advantage of any opportunity.

That said, desert thunderstorms create rainbow picture opportunities which are uncommon elsewhere.  Destert thunderstorms occur in cells which may be quite small.  It is not uncommon to have heavy rain falling one place and be totally dry a half mile away.  We have actually seen times where the highway went from dry to wet in less than 100 yards. 

When the sun is setting and there is a storm cell east of your position you should get out the camera and start looking around.  There may not be time to select the perfect background, but there should be a good rainbow.  Sometimes they are double and sometimes they will go from horizon to horizon. 

They are tricky to get a good picture of though.  Something about the colors makes them look great to the eye but disappointing on the monitor.  That's probably a place for some Photoshop work.

This website displays only a few of our collection of rainbow pictures.  At Walking Rain Gallery we have more, as well as many pictures of other subjects.  Clicking links on this site will take you to the rainbows category of Walking Rain Gallery.