Introduction

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Introduction Climate and the Environment A special, arid climate prevails in Arriyadh and its surrounding region. In the summer months from June to September, the thermometer rises to an average of 43 °C. In the winter, from December to February, the temperature can fall up to 10 °C below zero. In addition, precipitation is minimal: the average annual rainfall is 99mm and there about 47 cloudy days on average, with only occasional rain. During the summer there may be rare thunderstorms with heavy downfalls. Such extreme climatic conditions place equally extreme demands on those plants, which manage to grow in Arriyadh. _e Reference Manual is aimed at the use of plants, which survive these harsh, climatic conditions as well as possible.

Climatic improvement An improvement of the micro-climate in arid regions should be a major goal in all landscape projects. _e sun and heat in summer can be merciless. Dryness, too, leads to all sorts of health problems. Experience has shown that what arid regions need more than anything else to improve the climate is trees. And, of course, the aesthetic e_ect of tree planting cannot be emphasized enough. Trees are green and soothe the eye! Trees provide enclosure, give directional guidance, reduce glare, _lter dust and stabilize the soil. Trees not only provide shade upon hard surfaces, reducing re_ection and making walking and sitting more comfortable, evapo-transpiration via their leaves also increases air humidity, so that when light breezes pass under the canopies of trees, there is a cooling e_ect, too.

Urban climate of Riyadh the urban climate of Arriyadh varies according to location and the prevailing micro-climate. Generally the city has a desert climate, but within the built-up area, there are many di_erent situations: • Wadi Hanifah – this normally dry river course, to the west of the city, temporarily contains large amounts of water a_er rainfall, but nowadays there is almost a constant _ow of water, making the air more humid than that of the surrounding urban area. Because of evaporation the valley is therefore a source of cooler air, which is bene_cial to nearby neighbourhoods. • Inner city of Arriyadh – extreme heat, dust, solarisation, desiccation, re_ection of sun and heat from hard surfaces are characteristic of the public open spaces of the downtown area. In addition frequently visited or heavily tra_cked spaces such as Kindi Plaza in the Diplomatic Quarter or roadside planting on King Fahd Road are under high pressure of use. • Public Parks with their own micro-climates – like Salam Park and the parks of the King Abdulaziz Historical Centre in Murabba – are systems in themselves, with large areas of grass lawn, water features and therefore su_cient evaporation for the plants to grow well in this more humid environment. Dense tree planting and irrigation produce a micro-climate, which provides protection and optimal conditions for plant growth. • Courtyards between buildings are protected locations, which offer a variety of situations for planting. Generally a wider range of habitats is possible for plants and there is a good supply of irrigation and higher air humidity. _ere are also sunny and shady locations within a small space. • Private farms in the suburbs or on the edge of the built-up area are more exposed to the harsh desert conditions. _ere are extreme


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