The seasonal change of outside psychrometric state

When up-to-date meteorological information is available for mean monthly maximum and minimum temperatures, mean daily maximum and minimum temperatures, and observations of relative humidity for each month of the year, it is possible to obtain an approximate, overall view of the psychrometric envelope within which the climate of the particular place lies. Remembering that humidity observations are made on a daily basis the method adopted in Example 5.2 can be used to link temperatures with moisture contents. This has to be done when meteorological data is not readily available on an hourly basis in terms of frequency of occurrence of dry — and wet-bulb temperatures.

The frequency of occurrence data for Heathrow, partly shown in Table 5.3, is available more extensively in the CIBSE Guide A2 (1999) and can be used to provide a better psychrometric picture of the climate. Figure 5.4 shows values of coincident dry-bulb and wet-bulb temperatures with their frequencies of occurrence as percentages of the total hours in the four summer months, taken from the data published in the CIBSE Guide A2 (1999). The wet-bulb temperatures recorded at hourly intervals and published in the meteorological data are screen readings. The CIBSE psychrometric chart shows aspirated (sling) values of wet-bulb temperature because these are stable values (see section 2.17), corrected for radiation error. The difference between a screen and a sling wet-bulb value varies according to the humidity and dry-bulb temperature.

At 100% saturation the two wet-bulbs are the same but the difference between them increases as the humidity falls, the sling value always being the lower. For instance, at 22°C dry-bulb, the CIBSE Guide Cl (1988) gives the difference between the screen and the sling values as 0.4K at 50 per cent saturation and 1.3 K at 0 per cent. At 32°C dry-bulb the corresponding differences are 0.4K and 1.5K. The humidity in the UK seldom falls below 30 per cent saturation and for most of the year it exceeds 50 per cent. Hence to obtain an adequate picture of the area on the psychrometric chart within which the climate lies it is reasonable to ignore the difference between sling and screen wet-bulbs. Figure 5.4 does this.

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