Last updated on September 1st, 2023 at 11:41 pm
Climate (from Greek: klima) is defined by the Oxford Dictionary as a region with certain conditions of temperature, dryness, wind, light, etc.
A somewhat more scientific definition is integration in time of the physical states of the atmospheric environment, characteristic of a certain geographical location.
As the weather is the momentary state of the atmospheric environment at a certain location, the climate could be defined as the integration in time of weather conditions.
Tropical climates are those where heat is the dominant problem, where, for the greater part of the year buildings serve to keep the occupants cool, rather than warm, where the annual mean temperature is not less than 20° C.
Before tropical climates can be examined in detail, we must survey the factors shaping the climates, on a global scale.
What do you mean by solar radiation? Explain in terms of its quality & quantity
The earth receives almost all its energy from the sun in the form of radiation, thus the sun is the dominating influence on climates.
The spectrum of solar radiation extends from 290 to 2300 nm (nanometer =10 m). According to human means of perception we can distinguish:
a. ultra-violet radiation, 290 to 380 nm, producing photochemical effects, bleaching, sunburn, etc.
b. visible light, 380 (violet) to 700 nm (red).
c. short infra-red radiation, 700 to 2300 nm, radiant heat with some photo-chemical effects.
The spectral energy distribution varies with altitude, due to the filtering effect of the atmosphere.
Some of the shorter wavelengths are absorbed by the atmosphere and reradiated at much longer wavelengths, .e.g. long infra-red, up- to 10000 nm.
As the luminous efficiency of energy radiation depends on its spectral composition, there is no constant relationship between radiation intensity and its lighting effect.
However, as general guidance, the value of 100 lumens/watt can be used for solar radiation. This would give illumination of 100 lux for every W/m² intensity or 100 000 lux per kW/m²
The intensity of radiation reaching the upper surface of the atmosphere is taken as the solar constant: 1395 W/m², but it may actually vary ± 2% due to variations in the output of the sun itself and it varies ± 3.5% due to changes in the earth-sun distance.
The earth moves around the sun in a slightly elliptical orbit. One revolution is completed in 365 days, 5 hours, 48 minutes, and 46 seconds.
This orbit results from the gravitational pull of the sun and the centrifugal force due to the earth’s inertia and momentum. At aphelion, the solar distance is 152 million km and at perihelion is 147 million km.
How temperature is measured and its data is used for various purposes?
The temperature of the air is measured in degrees Celsius (C), most often with a mercury thermometer.
The dry-bulb or ‘true air temperature ‘ is a value taken in the shade, the thermometer being mounted inside a louvered wooden box, known as the ‘Stevenson screen’ at a height of 1.20 t0 1.80m above the ground.
Readings can be taken at specified times of the day, or if a maximum-minimum thermometer is used one reading daily can give the momentary temperature as well as the maximum and minimum temperature reached in the past 24 hours.
Alternatively, a thermograph can be used, which is based on a bimetallic thermometer and gives a continuous graphic recording of temperature variations.
All these readings would produce an unmanageable mass of data, thus some simplification is necessary.
As a broad description, monthly mean temperatures can be given for each of the 12 months. The average is taken between each day’s maximum and minimum and then the average of the 30 days is found (and possibly as many years’ average for the same month)
To give an indication of diurnal variations, this can be supplemented by monthly mean maximum temperatures. These will establish the average of 30 day’s maximum temperatures. These will establish the monthly mean range of temperatures.
It may be useful to indicate the highest and lowest temperatures ever recorded for each month, i.e. the monthly extreme maxima and minima, to establish the monthly extreme i.e. the range of temperatures.
These five values for each of the 12 months would give a reasonably accurate picture of the temperature condition, on which the design work can be based.
Write a short note on Humidity measurement & Humidity data
a) Humidity Measurement The humidity of air can be described as absolute
humidity (AH), i. e. the amount of moisture actually present in unit mass or unit volume of air, in terms of gramme per kilogramme (g/kg) or gramme per cubic meter (g/m³)
The relative humidity (RH) is, however, a much more useful form of expression, as it gives a direct indication of evaporation potential.
The amount of moisture the air can hold (the saturation-point humidity: SH) depends on its temperature.
The relative humidity is the ratio of the actual amount of moisture present, to the amount of moisture the air could hold at the given temperature expressed as a percentage:
RH=AH/SH x100%
Humidity is usually measured with the wet and dry bulb hygrometer. This consists of two ordinary mercury thermometers mounted side by side.
The Tropical Monsoon and Tropical Marine climate
The first one measures the air (dry-bulb) temperature (DBT). The bulb of the second one is covered with a gauze or wick and is kept wet.
Moisture evaporating gives a cooling effect, thus the reading of the wet-bulb temperature (WBT) will be less than the DBT.
As in dry air the evaporation is faster, the cooling is more pronounced and the difference between the two readings (the ‘wet-bulb depression) is greater.
In the case of 100% RH the two readings will be identical, as a function of the relative humidity. Having made the two readings, the corresponding RH can be found from the psychometric chart from a table or a special slide rule.
b) Humidity Data To give an indication of prevailing humidity conditions, it is sufficient to establish the monthly mean maximum (the average of 30 day’s maximum) and the monthly mean minimum relative humidity values for each of the 12 months.
This is only possible, where continuous hygrograph recordings are available.
Where these are not available, readings are made just before sunrise, e.g. at 6.00 hours (which is likely to be the maximum value), and at 15.11 hours (which is near the minimum value).
As the early morning values are fairly high in any climate, the afternoon values are much more characteristic of a given location. They are often used alone, as a brief indication of humidity conditions.
How wind velocity is measured using different instruments? Also explain the usefulness of wind data.
Wind velocity imeasured by a cup-type or propeller anemometer, or by a pitot tube (similar to the air-speed meters of airplanes), and its direction is measured by a wind vane. An anemograph can produce continuous recordings of wind velocity and directional changes.
Free wind velocities are normally recorded in the open flat country at a height of 10 m.
Measurements in urban areas are often taken at a height of between 10 and 20 m to avoid obstructions. Velocities near the ground are a good deal lower than free wind speed.
Direction can be grouped into eight or sixteen categories: the four cardinal (N.,E.,S. and W.) and four semi-cardinal compass points (NNE, SE., SII, and SSW. WSW.,WNW. and NNW.)
Velocities are measured in meters per second (m/s), but much data can still be found in obsolete units such as ft/min, mph or knot(nautical mile per hour).A ‘wind-force scale developed by Beaufort in 1806, based on visual observation, is still in use in spite of its completely unscientific nature.
The designer must try to determine whether there is a prevailing direction of winds, whether predictable daily or seasonal shifts occur and whether there is a recognizable pattern of daily or seasonal velocities. It is also important for him to periods in months.
All observatories record the occurrence of storms, hurricanes, typhoons or tornadoes it is customary to tabulate winds according to their direction and velocity categories, in terms of their frequency of occurrence, over a significant time, generally 25 to 50 years. Several methods of diagrammatic representation have evolved.