Anemometer have accurate wind data under all conditions, including freezing precipitation. Anemometry is also required in monitoring and controlling the operation of wind turbines, which in cold environments are prone to in-cloud icing. Icing alters the aerodynamics of an anemometer and may entirely block it from operating. Therefore, anemometers used in these applications must be internally heated. Both cup anemometers and sonic anemometers are presently available with heated versions. In order for wind speeds to be comparable from location to location, the effect of the terrain needs to be considered, especially in regard to height. Other considerations are the presence of
Content (measure theory) by This is an outer measure, in other words it has the following properties: The function μ above is an outer measure on the family of all subsets. Therefore it becomes a measure when restricted to the measurable subsets for the outer measure, which are the subsets "E" such that μ("X") = μ("X"∩"E") + μ("X"\"E") for all subsets "X". If the space is locally compact then every open set is measurable for this measure. The measure μ does not necessarily coincide with the content λ on compact sets, However it does if λ is regular in the sense that for