Diurnal variation of humidity
If the relative humidity is known in conjunction with the dry-bulb the moisture content can be established. Meteorological data usually include a measurement of relative humidity in
Suntime in hours
Fig. 5.2 Temperature variations on 17 July 1967 at Wethersfield, Essex.
The afternoon, at a time near that of the maximum temperature, even when the information is not more comprehensively stored. If the time of humidity measurement does not coincide with that of the highest mean daily temperature (about 15.00 h) equation (5.4) may be used to estimate the temperature at the same time as the humidity measurement. This will then establish the moisture content of the outside air at that time. In the absence of weather changes it might be supposed that the moisture content would stay constant throughout the day until the air cooled to its dew point. Although this is not strictly true, the dew point plotted in Figure 5.2 does remain fairly constant, providing a partial verification. In fact, it is difficult to measure dew point with high accuracy, except in the laboratory, and the apparent inconsistency of the values in Figure 5.2 may, perhaps, be attributed to measurement error. During hot weather, vegetation releases a good deal of its moisture into the air, becoming dehydrated in the process. This causes an increase in the moisture content of the air, even though no change of weather has occurred. However, assuming a constant moisture content during most of the day, the relative humidity of the air will rise as the afternoon and evening pass and night falls, because of the drop in dry-bulb temperature. As cooling of the air continues the dew point is reached and since the air is saturated any further fall in temperature will cause dew or mist to form, reducing the moisture content of the atmosphere. With the dawn and sunrise, temperatures increase and the relative humidity of the air falls somewhat, dew evaporating as the morning proceeds. Eventually all the dew disappears and the moisture content is back at its assumed constant value of the day before.
At a meteorological station in the Midlands the mean daily maximum and minimum temperatures are recorded as 20°C (assumed to occur at 1500 h sun-time) and 11.7°C (assumed to occur at 0300 h sun-time), respectively, in August. The mean relative humidity in August is recorded as 65 per cent (at 1300 h sun-time). Plot the typical diurnal variation of temperature and humidity in August.
First determine the mean outside moisture content and dew point in August. Since the maximum dry-bulb temperature occurs at about 1500 h sun-time, a typical temperature can be determined at 1300 h sun-time (the time at which the humidity is measured) by using equation (5.4).
F13 = 20 — [(20 — 11.7)/2][1 — sin(13jt — 9rc)/12]
= 20 — [8.3/2][l — sin(4jc/12)]
= 20-4.15[l — sin 60]
= 20 — 4.15[1 -0.866]
= 20 — 0.56 = 19.4°C
Thus, at 1300 h sun-time the dry-bulb is 19.4°C and the relative humidity is 65 per cent. Remembering that relative humidity is not quite the same as percentage saturation, a typical moisture content is established as 0.009156 kg kg-1 dry air and the corresponding dew point as 12.7°C, by interpolation in CIBSE Guide, Psychrometric Table (1988).
The results are plotted in Figure 5.3 and extended over 24 hours to show how temperature and humidity change and how dew may form.
Sun-time, hours Fig. 5.3 Diurnal variations in temperature and humidity.
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