I will jump to the chase :) A dollar has no intrinsic value. Its value is determined by supply and demand. The supply is determined by the reserve rate and the demand is determined by the GDP and interest rate.
Your analogy to units of temperature fails because those are measurements. Your analogy would be more appropriate to the interest rate determining the value.
My analogy was to clarify the intent of my question...I already knew that the dollar was fiat money, just as I knew that a degree Celsius is a measure of temperature. What I wanted to know is how the value of the dollar is determined. In short, my question was 'What's a dollar?' but not in the 'it's a unit of fiat money' sense.