[johnta1]

And watch out for Probird.... he can drive

X2!!!!

[grandville455]

Originally Posted by 455firebird1969

This stuff really isn’t that hard once you get your head wrapped around it.

82 degrees at 37%= 61 grains per pound. DA @ 1700’

73 degrees at 57%= 73 grains per pound. DA @ 1700’

Which scenario above would produce the fastest ET? I’ll take the middle of the day, hot temperature low humidity weather any day of the week. It happens every day, as the temperature warms, water vapor goes down, then every night it cools off, and water vapor rises. In the summer it usually reaches Dew Point (saturation) and can’t hold any more, so it condenses on any cooler surface. If the surface is warm, it will not condense. That’s why your trunk and roof will have water condensation, before your hood will.

Have you ever wondered why, When your AC in your car is on, and it’s blowing some of the cold air into the defrost duct, because you didn’t adjust the levers right, and water condenses where the defroster duct is hitting the inside of the windshield. Have you ever considered the physics behind that? The water condenses on the outside of your windshield, because the cooler air on the inside is cooling the glass. When the humid on the outside of the glass hits the cool glass, temperature drops, and because cool air can’t hold water like warm air, it turns into liquid immediately, hence the condensation only where the defrost duct is blowing cold air. (At least that is my observation in the South).

Farmers park their tractors under open air sheds because it they shield against this phenomenon. Sublimation or Chinook winds are a result of this basic function in the atmosphere. (Google it if you want to learn about it) the bottom line is, Cold air can’t hold water vapor like Warm air.

If you log weather each pass, a change of 12 grains per pound would probably predict the increased ET of .04 that you saw.
Make a chart that takes all important parameters into consideration, density altitude #1, water grains in #2, and then compare it to past performance at each parameter, average it over several similar passes and put your ET in the chart. This will give to a “go to” off the trailer prediction, no time trials required however; track conditions vary, making each time trial important.

The atmosphere is a living, breathing, constantly changing animal. Learn how it changes, and use a hand held weather meter (and chart) and help you to predict your ET.

I agree ,I've been beginning to see that water grains effects the car more then DA, I've always said you can have the exact same 2 DA'S, but how they are arrived at will make the difference in ET

[johnta1]

Why I said this:

It's not just cooler air though.
The humidity or grains of water are more which hurts ET.
Also another reason DA is not usually good for ET comparison.

Accurate weather data such as barometric pressure, temperature, etc. can help a lot.