Air density

The power available in the wind is proportional to air density multiplied by the cube of wind speed. Density is not a constant: it falls with altitude and with temperature, so the same twelve meters per second carries meaningfully less energy on a hot afternoon at elevation than on a cold morning at sea level. A swing of ten percent between summer and winter conditions is ordinary.

Because density enters the equation linearly, the correction is straightforward in principle and frequently skipped in practice. The IEC 61400-12 standard specifies normalizing measured wind speed to a reference density of 1.225 kilograms per cubic meter before a power curve is compared to a warranted one, and a comparison that omits the step will show a seasonal pattern that belongs to the atmosphere rather than to the machine.

This is one of the clearest cases where a model that learns from a turbine's own history has an advantage over a datasheet comparison. If density, temperature, and pressure are among the inputs, the expectation moves with the conditions automatically and the seasonal artifact never appears in the first place.

It also matters for siting comparisons across a portfolio. A high-altitude site will report lower output than a coastal one with identical machines and identical maintenance, and reading that gap as an equipment problem is a mistake that loss attribution exists to prevent.