Thanks for all the replies and the helpful suggestions. I have put them to the test for my particular casting style. I'm a bit anal when it comes to measurements. So when I wanted to see what I could cast in the wind I used an anemometer(wind speed gauge). 15 mph is marginal for me 20 mph is darn near impossible. On a calm day I can cast all my fly line (100') with a 12' leader on a 10 weight. On a 5 weight I'm good to 80'.
As measured, the wind is somewhat less as you get closer to the surface (ground/water). A 45 degree off of vertical for me gets the fly line between 9'- 10' off the surface, full side arm 90 degree from vertical gets it down to 4'. At those angles a 20 mph wind is 19 and 14 respectively.
My test with a hand tied 20' leader was fine in a calm condition, tended to get blown back at 15 mph and useless at 20 mph. So without a doubt I'll be shortening up on the leader as the wind increases.
I'll do more testing and report back as soon as the rain stops here in Castle Rock, Colorado.
As far as the wind? It's spring and the wind always blows. That is how I came up with the speeds. It has been blowing from 12 mph to 27 mph steady with gusts up to 43.
The next tests will be changing the pitch of the line. So instead of casting to a spot above the horizon I'll angle the cast to shot the line into the sky and stab at the target.
And casting down wind? Like FrankB2 says it really takes the line out. So for all you wind casters may the target be always down wind.