Wind speed 3 May 1999 is often quoted as the highest-recorded surface wind speed, although another figure of has also been quoted for the same tornado. Yet another number used by the Center for Severe Weather Research for that measurement is . However, speeds measured by Doppler radar are not considered official records. An anemometer is one of the tools used to measure wind speed. A device consisting of a vertical pillar and three or four concave cups, the anemometer captures the horizontal movement of air particles (wind speed). Another tool used to measure wind velocity includes a GPS combined with pitot
Pre-measure is called "σ"-additivity. Thus, what is missing for a pre-measure to be a measure is that it is not necessarily defined on a sigma-algebra (or a sigma-ring). It turns out that pre-measures give rise quite naturally to outer measures, which are defined for all subsets of the space "X". More precisely, if "μ" is a pre-measure defined on a ring of subsets "R" of the space "X", then the set function "μ" defined by is an outer measure on "X" and the measure "μ" induced by "μ" on the σ-algebra Σ of Carathéodory-measurable sets satisfies formula_4 for formula_5 (in particular,