Investigation on the relationship between wind direction and angle of air flow at the tower of Mount Washington Observatory, NH, from selected examples

The summit of Mount Washington is the highest in north-eastern United States with its 1917 m a.s.l.; it got famous as the highest wind speed (231 mph) ever observed directly by man was measured there in 1934. The observatory is located on the north-western side of the summit area and thus built into the main wind direction. The summit area can be characterised as being flat but slightly convex. As air flows over the summit, the currents are vertical due to the morphology of the top on the one hand as well as the tower of the observatory on the other hand. To take this constellation into account, the ´Pitot-Static-Anemometer´ which is used for measuring wind speed, has been installed with a downward angle of 7°. If wind comes in from the Northwest, the air meets the instruments directly with a strong vertical upward component (due to the morphology of the summit area). If- on the contrary- wind direction is Southeast, the air has to pass the whole summit area before wind direction and speed are measured by the instruments. Observations with sonic anemometers showed, however, that air current was not upward in this case; it was rather horizontal or even downward. This means that the pipe of the ´Pitot-Static-Anemometer´ which measures the wind speed (relative to the pressure generated by the air that is being pushed into it) cannot be orientated optimally; thus, it partly measures velocities that are too low. The sonic anemometer which is installed on the tower additionally makes it possible to detect the 3-dimensional wind vector, which consists of wind direction and a horizontal and vertical component; based on this data, calculation of the angle of air flow in relation to a horizontal datum line is possible. The aim of this investigation is to determine a potential relationship between wind direction and speed as well as angle of air flow from selected examples; the measurements for different directions of air flow are also qualitatively assessed.