Natural, hybrid and low energy ventilation
Wind is by far the most signiﬁcant component of the driving force in natural ventilation, particularly in hot seasons. The wind ﬂow over the earth’s surface is a very complex phenomenon that is governed by a number of variables such as: earth’s rotation; temperature differences between oceans and land and polar and tropical air; geographical location and landscape. In meteorology, the following scales are used for describing climatic models: global scale, regional scale, local scale and microclimate scale. The global scale concerns astronomical factors that relate to the size, shape, and self rotation of earth and its elliptic rotation around the sun and this covers a range of thousands of kilometres. These factors create the diurnal and seasonal variations according to latitude, as well continental variations due to the distribution of land and oceans. The regional scale relates to regional climatic features such as geographic landscape (e.g. the inﬂuence of mountains, hills, valleys, etc.), proximity to ocean and location of region with respect to zones of general wind circulation. This scale covers wind ﬂows over hundreds of kilometres. The local scale relates to local geography or water mass (such as hills, valleys, lakes, large rivers, etc.) and urbanization (such as heat islands) and how the local climate, including the wind ﬂow, is affected by these factors. This stretches over a distance of about 10 km. The local climate is of course inﬂuenced by energy balance at the regional scale. The microclimate scale relates to small towns or districts where local features used in their construction could have inﬂuence on the wind ﬂow, such as nature of town planning, presence of artiﬁcial climate modiﬁers (e.g. windbreaks, hedges, etc.), the presence of water, etc. This scale covers a few hundred metres and is greatly inﬂuenced by man’s planning and activities.