Meteorological measurement
Stations for measuring various atmospheric properties are set up all over the world by the meteorological authorities in different countries. The coverage of the surface of the earth
Is not complete and so there is a wealth of information for some areas and a scarcity of it for others.
Measurements are made of temperatures and humidity within louvred boxes, positioned in the open air. The louvres permit the ready circulation of outside air by natural means over the instruments but shield them from rain and sunshine. Since the air movement over any wet-bulb thermometer mounted in such a screened housing is by natural means, the velocity of airflow will be too low (less than 4.5 m s_1) to minimise interference from radiation effectively (see section 2.17) and so, ‘screened’ wet-bulb values will be about degree higher than ‘sling’ wet-bulb values. Whereas wet-bulb values on the psychrometric chart published by the CIBSE are sling values, data taken from meteorological tables are always based on screened values, and due allowance must be made for this when using such data.
At modern meteorological stations hourly measurements are made and recorded of dry — bulb temperature, humidity and other relevant climatic variables. The highest and lowest values of temperature for each month are averaged over a suitably recent, relevant period of years and quoted as the mean daily maximum and mean daily minimum temperatures for a particular month. These values represent typical high and low temperatures in the month.
The extreme high and low values of temperature in each month are also noted, and averages of these are calculated over the number of years for which observations are made. This yields mean monthly maximum and minimum values of dry-bulb temperature which are typical of warm and cold spells of weather. Other records of temperature are kept but these are largely irrelevant to a study of air conditioning. In underdeveloped countries records may not be so extensive but the principles remain the same.
As regards records of relative humidity, the picture is not so comprehensive. Records are not always kept of wet-bulb temperature, but instead, measurements of relative humidity may be made twice daily—one at about 09.00 h and another at about 13.00 h. The readings are all taken at a height of about 1.5 m above ground level. Typical meteorological data are shown in Table 5.2.
Hourly values of dry-bulb and wet-bulb temperatures are recorded at many meteorological stations and the data processed to show their frequency of occurrence. The CIBSE (1999) covers both cold and warm weather and gives information for several places in the UK with data from Heathrow corresponding to London. The percentage of the total hours in the four summer months, June to September, inclusive, is presented in two ways: as a combination of coincident dry-bulb and wet-bulb values and, separately, as a percentage of the total hours in the summer months that quoted dry-bulb and wet-bulb values are not exceeded. Tables 5.3 and 5.4 list some values.
EXAMPLE 5.3
Making use of the data in Tables 5.3 and 5.4 determine the total number of hours in the four summer months that (a) the dry-bulb has exceeded 28°C, (b) the wet-bulb has exceeded 20°C and (c) the combination of dry — and wet-bulb temperatures has been within the limits 26°-28° dry-bulb and 18°-20° wet-bulb, simultaneously.
Answer
The total number of days in the four summer months is 30 + 31 + 31 + 30 = 122 days. The total number of hours is 24 x 122 = 2928. Hence the answers required are:
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Table 5.2 Some typical meteorological data from observations made at Kew (now closed and transferred to Bracknell) during the period 1931 to 1960 but now out of date. Up-to-date observations are similar but warmer. For example, according to Levermore and Keeble (1997) the annual mean temperature at Heathrow in 1960 was 9.95°C but 11,25°C in 1995. They extrapolate a value of 11.7°C for 2010
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Table 5.3 Percentages of the total hours in the months June to September, inclusive, that combinations of dry-bulb and wet-bulb temperatures lie within quoted ranges of values, at Heathrow for the period 1976-95 |
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Wet-bulb temperatures |
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Temperatures |
16°-18° |
18°-20° |
20°-22 |
O CM 1 0 (N CM |
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24°-26° |
1.37 |
1.32 |
0.25 |
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26°-28° |
0.42 |
0.77 |
0.32 |
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28°-30° |
0.10 |
0.31 |
0.25 |
0.01 |
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30o_32o |
0.01 |
0.12 |
0.14 |
0.02 |
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Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999). |
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Table 5.4 Percentages of the total hours in the months June to September, inclusive, that dry-bulb and wet-bulb temperatures exceed the stated value, at Heathrow, for the period 1976-95
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Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999). |
(a) The dry-bulb exceeded 28°C for (0.98/100) x 2928 = 28.7 h.
(b) The wet-bulb exceeded 20°C for (1.09/100) x 2928 = 31.9 h.
(c) The outside state lay within the limits of 26°-28° dry-bulb and 18°-20° wet-bulb for (0.77/100) x 2928 = 22.5 h.
Posted in Engineering Fifth Edition