Santa Ana Winds

[lamontcranston]

I've noticed that my fuel economy is down about 3mpg on exactly the same trip since Monday. Here's the thing...

In Southern California the winds almost always blow from west to east. I drive from east to west in the morning when the winds are still, and then home from west to east with the wind at my heels.

Once a year, for about a week, the wind shifts so it blows from northeast to southwest. The temperature jumps up at the same time. We call these the Santa Ana winds.

So, when the temps were about 95 in the afternoon, I was averaging 63mpg round trip (measured by the Prius itself). Since Monday, when the Santa Anas started, the afternoon temps have been about 108.

I remember from learning to fly that higher temps mean thinner air, and I think that combined with a headwind (driving into the wind) are causing the change in MPGs.

What do you all think?

[bwilson4web]

Originally Posted by lamontcranston

. . .

I remember from learning to fly that higher temps mean thinner air, and I think that combined with a headwind (driving into the wind) are causing the change in MPGs.

What do you all think?

Correct. The higher temperature changes the air density and is used to calculate the density altitude. In the high plains, it can be so hot at the ground that light aircraft with a heavy load can not take off.

As for air drag, the primary force at speed, it varies linear with the air density and by the square of the velocity. When fighting a head wind, look for a semi trailer truck and match its pace at a safe distance. You'll still get where your are going but you won't burn as much fuel.

GOOD LUCK!
Bob Wilson