At this stage we are talking about land based wind power.

Average Wind Speed by State and Month for US:

 Here is one form of the data about average wind speeds for all the states.

Checking David MacKay's Data:

 Does this affirm the data that David MacKay was using?

Oregon Wind Data:

 Oregon wind data

NREL Map Database

In case the link breaks here is the link to the NREL map database.

National Wind Data from NREL:

Here's a large version of the national map. We'll explore how this compares to Britain.

Wind Rose Data for all States:

 In case you ever want this data for your own purposes.....

Wind Speeds at other heights:

Handy access point for wind speeds at other heights

Shepards Flat Wind Farm data and characteristics