Temp/Humidity affect on MPG ?
 

What affect does warmer temps and higher humidity have on MPG ?
My MPG has jumped 2 MPG from norm avg two tanks ago and 4 MPG this last tank.
I live on the central east coast of FL & the temp/humidity has increased somewhat over the past couple of weeks...

Higher temps decrease air density and creates less aerodynamic drag. This means you should get better MPGs at highway speeds. Air density is also decreased as you increase elevation.

I would think that increased humidity would increase density though, and increase aerodynamic drag.

Perhaps your fuel station has switched to a summer blend? This might be a better explanation for the increased fuel economy. Way too many variables involved to say conclusively what is responsible for your better MPGs.

MPG's increase with temperature, humidity, and altitude. As mentioned above, air density decreases with increasing temperature. Since the water molecule (molecular weight 18) is lighter than molecules of nitrogen (28), oxygen (32), or Carbon dioxide (44), as the percentage of water vapor increases in the atmosphere, its density decreases. Also water vapor displaces oxygen that your engine would otherwise be burning, reducing its power output and forcing you to open up the throttle (for reduced pumping losses) more to get the same amount of power output. You also get reduced pumping losses as the air density decreases with increasing temperature and/or altitude.

I guess I was thinking of humidity like fog, which I believe is denser than air since it collects in low lying areas. When it's evaporated into the air I suppose it would be less dense since it condenses at higher altitudes. Makes sense to me.

Fog collects in low lying areas because it is a result of a cold air drainage cooling air below its dewpoint, thus visualizing the cold air flow. Moisture condenses at higher altitudes when it is cooled below its dewpoint, often by anabatic winds moving up a slope, expanding as the pressure drops, and cooling as it expands.

I have detailed records from over four years of hypermiling. These are averages for my car and driving style and route, but can be extrapolated to others.

Temperature:
55 mpg at 20F, 80 mpg at 105F

Humidity (mornings only, counted as humid when there's dew or fog):
humid mornings 68 mpg, dry mornings 63 mpg