Portugal - Atlas do Ambiente I.7 - Vento

Summary

The terrestrial atmosphere is a thermal machine. Part of the solar radiation received on Earth changes to kinetic energy which originates the permanent movement of the atmosphere named wind. The wind is particularly the horizontal movement of the air. As a vector the wind is described by its components: direction, representing the point from which the wind blows, and intensity, representing the velocity of the wind. The wind presents frequent variations of direction and velocity. The major or minor roughness of the terrestrial surface provokes, by friction effect, a reduction of the wind velocity. Different instruments are used for the observation of direction and velocity of the wind. So, for direction, are used weather vanes and for velocity, anemometers. These instruments are sited at 10 meters above open and flat terrain. Wind direction should be reported in degrees to the nearest ten degrees measured clockwise from the geographical north, or by reference to the cardinal points. Wind speed should be reported in meters per second (m/s), kilometers per hour (km/h) or knots (mille per hour — m.p.h.), to the nearest unit. With an error of 3 pc, a meter per second may be considered equivalent to two knots. The Chart was based on frequency values for eight directions (N, NE, E, SE, S, SW, W, NW) and speeds, on five classes of velocity (calm, 2 to 5 km/h, 6 to 20 km/h, 21 to 50 km/h, more than 50 km/h) for the 1951-1960 period, and time of observation at 18 UTC, because it is mo