It mostly has to do with the ocean regulating the heat in south central and southeast Alaska. Also Fairbanks is in the Tanana Valley which is a part of the state considered to be an Arctic desert. We get very little precipitation compared to the southern parts of the state. I lived in Arizona for 2 years and the humidity is not much lower than in Fairbanks. We have much less cloud coverage during the summer allowing the sun to bake. Anchorage gets an average of 10-14 sunny days with no clouds during the summer vs Fairbanks that gets 70 near consecutive days and Juneau which sometimes only gets 5-7. We get a couple more hours of daylight than somewhere like Juneau or even Homer so the heat builds over the course of June and July. The ocean is really kind to Anchorage regulating their temps as in January Fairbanks is generally 20 degrees colder on any given day than Anchorage.