Airplane radiation graph with a geiger counter, MDW to LAX
I also added to the graph a yellow line showing the background rate in my office which is 7.5 microrads/hr. So flying at 38,000ft gives you a rate about 40 times greater than here in my quiet little office in Chicago.
I've previously done this on trips to Los Angeles: Dean W. Armstrong: Geiger Counter
In the past I've also looked at potassium salt, Pentax Takumar lenses, dinosaur bones, and have a live report of radiation rates from my office.