Humidity Aloft

The water content of the atmosphere decreases greatly as one ascends in the free air into cooler layers (Figure 6.12). Most of the air's water is within 3 km of the ground. Mixing ratios at 1,500 m, 3,000 m and 5,500 m are typically only two-

TEMPERATURE (°C)

Figure 6.11 The variation of comfort in terms of the new Effective Temperature (ET*). The curved diagonal lines are relative humidity isopleths, just as on a psychrometric chart (Figure 6.6). The straight diagonal lines fanning out from the 100 per cent RH line are ET* isopleths with the value shown on top. Various human comfort zones are shown also. For instance, there is neutral comfort when ET* is 24°C, and conditions are intolerable when ET* exceeds 41°C.

TEMPERATURE (°C)

Figure 6.11 The variation of comfort in terms of the new Effective Temperature (ET*). The curved diagonal lines are relative humidity isopleths, just as on a psychrometric chart (Figure 6.6). The straight diagonal lines fanning out from the 100 per cent RH line are ET* isopleths with the value shown on top. Various human comfort zones are shown also. For instance, there is neutral comfort when ET* is 24°C, and conditions are intolerable when ET* exceeds 41°C.

thirds, one-third, and one-eighth, respectively, of the value at sea-level (Table 6.3). This implies that cloud and rain form chiefly in the lowest layers of the troposphere (Chapters 8 and 9).

The relative humidity of subsiding air becomes low for two reasons. Firstly, it starts from levels where the vapour pressure is less, as just mentioned, and, secondly, the air is warmed by compression as it comes down (Chapter 7).

The pattern of the vertical distribution of moisture (i.e. the humidity profile) can be shown most conveniently by a special graph of the dewpoint or the mixing ratio, called an aerological diagram. This includes a temperature profile also. There are several versions, but the one used in Australia is the skew T—log p diagram (Note 6.I).

It is possible to total the amount in the column of air above a unit area of ground from a knowledge of the moisture content of the air at various levels, shown by an aerological diagram. The sum is the precipitable water (Note 6.B). This is all the atmospheric water vapour, omitting any moisture in the form of cloud droplets or ice crystals. It amounts to about 97 per cent of the atmospheric water mentioned in Figure 6.2. It is the depth of liquid water on the ground that would result from precipitating all the vapour in the air column.

It varies with latitude, as shown by Figure 6.13. There is around 38 mm at the equator, 27 mm at 20°S, 16 mm at 40°S, 8 mm at 60°S, and 2 mm at 80°S. The pattern is more like that of temperature (Figure 3.4) than of evaporation (Figure 4.11).

It also depends on the season, being most in summer, e.g. the average at 30°S is 27 mm in January, but only 18 mm in July. The mean precipitable amount for the whole southern hemisphere is 26 mm in January and 20 mm in July. That seasonal variation is smaller than the range from 19 to 34 mm in the northern hemisphere. The reason is that the north has a greater land area and therefore experiences a wider annual range of temperature (Section 3.3). Higher temperatures in summer lead to more precipitable water, and this increase of water vapour leads to higher temperatures, as water vapour is a greenhouse gas (Section 2.7). So the effect is amplified by a 'positive feedback process', of the kind discussed in the next chapter.

Figure 6.12 Variation of the mixing ratio at Port Hedland (Western Australia) with height and day of the year.

Figure 6.13 Annual mean pattern of precipitable water in units of a millimetre.

Figure 6.12 Variation of the mixing ratio at Port Hedland (Western Australia) with height and day of the year.

Pit 1 tLOn I Wl**. SUMVU WTH SLOT 24 COPYRIGHT - E» -

Pit 1 tLOn I Wl**. SUMVU WTH SLOT 24 COPYRIGHT - E» -

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