To design naturally ventilated buildings and/or comfortable outdoor environments, the first step is to obtain reliable wind information, such as wind speed and direction. For example, the National Renewable Energy Laboratory derived a set of typical meteorological weather data for 229 stations throughout the United
Figure 2 Diurnal and nocturnal air movements near a large body of water (Source: adapted from Moore 1993)
States and its territories (Marion and Urban 1995). The database provides hourly wind speed and directions that can be used directly in natural ventilation and outdoor comfort design. Rather than accounting for every hour in a full reference year, a designer should analyze the data and divide it into eight directions (N, NE, E, SE, S, SW, W, and NW) for several wind speeds (e.g., Beaufort number <2, 3-4, 5-6, >7, where Beaufort number classifies wind as 0 for calm to 12 for hurricane). The weather data can also be used to determine the percentage of wind for each direction and speed combination (32 in total). The total number can be reduced, eliminating those with very low probabilities.
For countries where typical meteorological weather data are not available, wind roses can be used. Figure 1 shows a part of the wind rose map for northeastern United States in January. The wind roses give the wind direction and percentage. The number inside the wind rose stands for the percentage of calm period. NOAA (1983) uses another figure to provide the monthly average wind speed over a year.
Note that the wind data from weather databases, wind roses, or a weather station is for an open terrain. Numerous factors could have a significant impact on the local climactic conditions. For example, a large water body, such as a lake, can create a local wind from the water body to the land during the day and a local wind from the land to water during the night (Figure 2). This is because water has a higher effective thermal mass than that of land. Under the sun, the surface of land is heated much faster than water. The warmer air above the land goes up due to the buoyancy effect, creating an air pressure differential from the water to land. During the night, land cools faster than water due to thermal radiation. It is again the thermal buoyancy in the air that forms a land-to-water wind. Other factors include valleys, mountain ranges, and even large building blocks.
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