Anemometer in pressure of the two lines. The measurement devices can be manometers, pressure transducers, or analog chart recorders. In the tube anemometer the dynamic pressure is actually being measured, although the scale is usually graduated as a velocity scale. If the actual air density differs from the calibration value, due to differing temperature, elevation or barometric pressure, a correction is required to obtain the actual wind speed. Approximately 1.5% (1.6% above 6,000 feet) should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level. At airports, it is essential to
Gibbs measure property. Measures with this property are sometimes called Markov random fields. More strongly, the converse is also true: "any" positive probability distribution (nonzero density everywhere) having the Markov property can be represented as a Gibbs measure for an appropriate energy function. This is the Hammersley–Clifford theorem. What follows is a formal definition for the special case of a random field on a lattice. The idea of a Gibbs measure is, however, much more general than this. The definition of a Gibbs random field on a lattice requires some terminology: We interpret as the contribution to the total energy (the Hamiltonian)