Why does underside of the lid of my toilet bowl get covered with extreme condensation???

Ok, so all day long, everyday I have to wipe down the underside of the lid of my toilet bowl due to extreme condensation. I have replaced every part in my tank thinking this problem would go away... but nothing. Please somebody help, this is driving me nuts!!!
Update: A little more info... I am in phoenix arizona and the other toilet in the house does not ever do this.
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