2.2 Climatic factors.
The main natural elements that define the climate, are
• solar radiation,
Their characteristics and relevance for construction depend largely on the geographical location, but also on the topography, altitude and properties of the earth’s surface and its coverage.
The earth receives almost all its thermal energy from the sun in the form of radiation. Thus the sun is the dominant factor that influences climate.
220.127.116.11 Solar radiation
The spectrum of solar radiation extends from ultra violet through visible light, to infrared radiation. The latter is the main medium of energy, in the form of heat.
The solar energy from the sun is always constant. How much heat is received at a given point on earth depends on
• the angle of incidence
At an angle of 30°, a given area (a) only receives half the amount of solar rays it would at an angle of 90°.
The distance (d) that solar rays have to pass through the atmosphere at an angle of 30° is double that if the angle were 90°.
This increased distance reduces the energy received on the earth surface considerably, especially if the atmosphere is humid or dusty.
The angle of incidence changes not only in the course of the day, but also with the seasons. This is due to the earth’s path around the sun.
The amount of energy received on a given surface varies during the course of the day, depending on the angle of incidence of the sun. The graph below illustrates a typical amount of energy received by a south facing and inclined solar collector surface. The total energy received during this day amounts to about 5 kWh/m².
Major factors influencing the amount of solar energy received are the weather and the pollution content of the atmosphere.
Similar to the energy gain during daytime, nighttime heat loss by radiation to the sky is also greatly dependent on atmospheric conditions. A clear sky allows maximum, a thick cloud cover minimal heat loss.
(see Chapter 2.4.1)
Length of the day. According to the geographic latitude, the length of the day and hence the duration of sunshine varies during the year.
18.104.22.168 The sun’s path
While designing buildings anywhere in the world, the sun’s path must be considered as an important factor.
The position of the sun depends upon
• the geographic location (latitude)
It can be determined most easily, and for our purposes sufficiently exactly with the help of the diagrams given in Appendix 5.3
How to read the diagram:
• Select the diagram for the latitude of the building site.
22.214.171.124 The geometry of shadows
Knowing the sun’s position, the geometry of shadows on buildings, facades and shading devices can be derived.
Detailed methods, with the help of the sun-path diagram and a shadow angle protractor, are found in the literature.
If these tools are not at hand, the following simple geometrical method can be used:
1) Draw a plan of the building part, e.g. a window with overhang, and enter the direction of the solar radiation with the help of the azimuth.
2) Draw section A-A parallel to the direction of the solar radiation and enter the solar altitude angle.
3) From the plan and section derive the elevation with the shadow picture.
4) Drawing the normal section B-B provides the shadow angle. This is always bigger than the solar altitude angle except in the case where the direction of the sun is at right-angles to the building elevation. In this case the two angles are identical.
This simple method, analogously applied, provides information about the shading performance of any shape of shading devices or building components, and also shading by surrounding buildings. It provides a basis for planning the orientation and grouping of buildings, and for the design of shading devices and openings.
(Relevance to planning and construction see Chapter 3.)
The reasons for the development of winds are manifold and vastly complex. The main reason, however, is the uneven distribution of solar radiation over the globe. It results in differing surface heating and temperatures. This causes differences in air pressure and, as a consequence, the development of winds.
Typical main winds
The prevailing air pressure pattern on earth is fairly regular. Together with the rotation of the earth, the main winds can be determined.
These main winds are overlaid with secondary winds, mainly of thermic origin.
The daily variations in heating and cooling of land and water surfaces (seas, lakes), of mountainous and flat land areas, and of bare and land covered with vegetation, cause regular wind patterns in certain areas, such as sea winds or valley winds in the daytime, and land winds or mountain winds at nighttime.
Valley and mountain winds; Sea and land winds
Strong solar radiation also causes irregular local thermic winds. This is due to air that is heated near the ground and rises from time to time in the form of bubbles.
Influence of topography
Topography influences wind characteristics. Valley bottoms are generally wind protected areas whereas elevated locations receive more and stronger winds.
The monsoon wind is a result of seasonal differences in the heating up of land and sea areas and is of great importance to a large area in the tropics.
Characteristics of winds
Depending on the origin of the wind, its quality differs. It can be dry or humid, clean, dusty or sandy, hot or cool compared to the prevailing temperature, constant or irregular. Its speed too can vary.
Accordingly, wind can either be utilized for improvements to the indoor climate of buildings or measures must be taken to protect against it.
Wind can have a disastrous effect in the form of storms. Due to climatic constraints, certain zones on the globe are prone to storms. In these areas buildings require special structural protection. In most other zones, however, storms occasionally occur as well, but probable with less intensity. As a consequence, adequate protection is also required there. (see Chapter 3.1.4)
2.2.3 Humidity and precipitation
A major factor in climatic characteristics is water. It occurs as rain, hail, snow, clouds and vapour.
Vapour is water in the form of gas, absorbed by air. Depending on the temperature, the absorption capacity of the air varies.
The curve shows the maximum absorption capacity in relation to the air temperature. This represents 100% relative humidity.
Relative humidity is defined as
humidity at saturation point (g/m³) . 100 / effective humidity (g/m³) = ... %
Fog, clouds and precipitation
Air temperature fluctuates considerably during the day and night, and with it the saturation point. Because the absolute humidity remains constant, the relative humidity changes. If, however, the absolute humidity exceeds the saturation point, the surplus water condenses and occurs in the form of fog, clouds, dew or precipitation. The same can be observed when air rises and thus cools down. Strong thermic upwinds result in cumulus clouds; winds crossing mountains create clouds and precipitation.
Due to topography, distribution of water bodies and winds, the types and quantity of precipitation varies strongly.
Not only the quantity, but also the types and seasonal distribution of precipitation are manifold. For example, in monsoon areas rainfall is concentrated over a certain period of the year and can be extremely intense and long-lasting. In warm-humid regions it can occur over the whole year with short downpours almost every day.
These differences in precipitation patterns are reflected in construction details and building types, at least traditionally. This can be illustrated by typical building types for different regions (Relevance for planning and constructions see Chapter 3)
Thermal capacity of water
Water has an extremely high thermal capacity, and can thus store and emit large quantities of thermal energy. For instance, the temperature of 1m³ earth increases five times more than that of 1m³ water, when putting in the same quantity of heat energy.
This explains the temperature-regulating effect of large water bodies such as seas or lakes, resulting in the typical maritime climate on the one hand and the continental climate on the other.
A similar balancing effect is caused by a thick vegetation cover such as a forest, partly because it contains large quantities of water.
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