Wind power density (WPD) is a calculation of the effective power of the wind at a particular location.A map showing the distribution of wind power density is a first step in identifying possible locations for wind turbines. In the United States, the National Renewable Energy Laboratory classifies wind power density into ascending classes. The larger the WPD at a location, the higher it is rated by class. Wind power classes 3 (300–400 W/m2 at 50 m altitude) to 7 (800–2000 W/m2 at 50 m altitude) are generally considered suitable for wind power development.There are 625,000 km2 in the contiguous United States that have class 3 or higher wind resources and which are within 10 km of electric transmission lines. If this area is fully utilized for wind power, it would produce power at the average continuous equivalent rate of 734 GWe. For comparison, in 2007 the US consumed electricity at an average rate of 474 GW, from a total generating capacity of 1,088 GW.




WPD = ½ * ρ * V3 (1)
And that air density can be determined to varying degrees of accuracy with the following.
Methods
1.) ρ = 1.225 kg/m3 (constant value based on U.S. Std. Atmosphere, at sea level)
2.) ρ = 1.225 - (1.194 * 10-4) * z (z=the location's elevation above sea level in m.)
3.) If you have pressure and temperature data:
ρ = P / RT (kg/m3)
where P = air pressure (in units of Pascals or Newtons/m2)
R = the specific gas constant (287 J kg-1 Kelvin-1)
T = air temperature in degrees Kelvin (deg. C + 273)
4.) If you have temperature data but not pressure data:
ρ = (Po / RT) * exp(-g*z/RT) (kg/m3)
where Po = std. sea level atmospheric pressure (101,325 Pascals)
g = the gravitational constant (9.8 m/s2); and
z = the region's elevation above sea level (in meters)

Wind Power Density Map
(Numerical Weather Prediction) modeling approach can be use to determining WPD map. Unlike traditional models that merely interpolate observed wind speeds between widely dispersed points, the interaction between the atmosphere and the earth's surface, to create a more robust and accurate wind climatology.

picture above shown map with wind power density at Bolivia, calculated by GIS and Numerical Weather Prediction by 3TIER, by using combination GIS and NWP, we can get data both annual and monthly values of power density and at multiple hub heights: 20, 50, and 80 meters above ground. this method can provide high-resolution data and advanced analysis of the spatial and temporal characteristics of wind resources anywhere in the world. This analysis utilizes the latest mesoscale NWP (Numerical Weather Prediction) models and can be performed even before on-site observational data have been collected.

more information about map of wind power density can be found at 3TIER GIS

Wind Power Density Calculation method and example in detail also available in Oklahoma University, Just Download the document here
Estimates of wind power density are presented as wind class, ranging from 1 to 7. The speeds are average wind speeds over the course of a year,[8] although the frequency distribution of wind speed can provide different power densities for the same average wind speed

The air density, ρ, changes slightly with air temperature and with elevation. The ratings for wind turbines are based on standard conditions of 59° F (15° C) at sea level. A density correction should be made for higher elevations as shown in the Air Density Change with Elevation graph. A correction for temperature is typically not needed for predicting the long-term performance of a wind turbine

Temperature and pressure

The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:

where ρ is the air density, p is absolute pressure, R is the specific gas constant for dry air, and T is absolute temperature.

The specific gas constant for dry air is 287.058 J/(kg·K) in SI units, and 53.35 (ft·lb_f)/(lb_m·R) in United States customary and Imperial units.

Therefore:

• At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of 1.2754 kg/m³.
• At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³.
• At 70 °F and 14.696 psia, dry air has a density of 0.074887 lb_m/ft³.

Other Method

While air temperature data is not too hard to come by (and is relatively cheap to
measure even if you don't have data already), air pressure data can be tougher to find. If air
pressure data for your region is unobtainable, you can estimate density as just a function of site
elevation and temperature with the following expression.
ρ = (Po / RT) exp(-g*z/RT) (kg/m3)
where Po = standard sea level atmospheric pressure (101,325 Pascals) [or you can use a sealevel
adjusted pressure reading from a nearby weather station];
g = the gravitational constant (9.8 m/s2); and
z = the region's elevation above sea level (in meters)

Altitude

Standard Atmosphere: p₀=101325 Pa, T₀=288.15 K, ρ₀=1.225 kg/m³

To calculate the density of air as a function of altitude, one requires additional parameters. They are listed below, along with their values according to the International Standard Atmosphere, using theuniversal gas constant instead of the specific one:

• sea level standard atmospheric pressure p₀ = 101325 Pa
• sea level standard temperature T₀ = 288.15 K
• Earth-surface gravitational acceleration g = 9.80665 m/s².
• temperature lapse rate L = 0.0065 K/m
• universal gas constant R = 8.31447 J/(mol·K)
• molar mass of dry air M = 0.0289644 kg/mol

Temperature at altitude h meters above sea level is given by the following formula (only valid inside thetroposphere):

The pressure at altitude h is given by:

Density can then be calculated according to a molar form of the original formula:

Detail About estimating and calculating air density can be found in PDF tutorial by Oklahoma University, Just Download Here

those tutorial provide 3 different methods to calculate air density, although the best methods is using elevation or altitude theory

basic calculation that needs to be done before calculating more detail when designing wind tubines is to calculate the power requirements and power that can be generated from wind turbine currently in design, some things that need to be considered in the calculation of wind power is
1) The power output of a wind generator is proportional to the area swept by the rotor - i.e. double the swept area and the power output will also double.
2) The power output of a wind generator is proportional to the cube of the wind speed - i.e. double the wind speed and the power output will increase by a factor of eight (2 x 2 x 2)

picture above shows the direction and also a component of the wind turbine through which the wind well or not,

The Power of Wind

Wind is made up of moving air molecules which have mass - though not a lot. Any moving object with mass carries kinetic energy in an amount which is given by the equation:

Kinetic Energy = 0.5 x Mass x Velocity²

where the mass is measured in kg, the velocity in m/s, and the energy is given in joules.

Air has a known density (around 1.23 kg/m³ at sea level), so the mass of air hitting our wind turbine (which sweeps a known area) each second is given by the following equation:

Mass/sec (kg/s) = Velocity (m/s) x Area (m²) x Density (kg/m³)

And therefore, the power (i.e. energy per second) in the wind hitting a wind turbine with a certain swept area is given by simply inserting the mass per second calculation into the standard kinetic energy equation given above resulting in the following vital equation:

Power = 0.5 x Swept Area x Air Density x Velocity³

where Power is given in Watts (i.e. joules/second), the Swept area in square metres, the Air density in kilograms per cubic metre, and the Velocity in metres per second.


However, there’s no way to harvest ALL of this available energy and turn it into electricity. In 1919 a gentleman named Betz calculated that there’s a limit to how much power a turbine blade can extract from the wind. Beyond the Betz Limit of 59.26% energy extraction, more and more air tends to go around the turbine rather than through it, with air pooling up in front. So 59.26% is the absolute maximum that can be extracted from the available power.
Simply put, if you capture 100% of the energy available in the wind, you stop the wind.  Obviously, the wind will stop flowing through such a turbine.  The opposite of that is that if you don't capture any energy in the wind, you don't need a turbine.  The wind is able to flow around any major obstruction.  The Betz limit says that essentially, if you capture 59.6% of the energy in the wind, that is the best compromise between stopping the air and forcing it to go around your machine.  You need to maintain the flow of air, that's the compromise any wind machine must make whether it is a horizontal axis (traditional style turbine) or vertical axis turbine, with many blades or few, or any such combination.  It's covered by the Betz limit.

More Detail in PDF Tutorial also available from Oklahoma University just Download Here

The design of blades attached with the rotor also contributes towards an effective wind turbine design. Apart from the shape and weight of these blades, it is also important to consider the material used for manufacturing them. As far as number of blades is concerned, two or three-blade wind turbines are the most popular ones in the industry.