Effects Of Clouds

Clouds affect flows of radiation by reflecting some, absorbing part and transmitting the remainder (Table 8.5). The consequence of obscuring the Sun is that cloudiness is closely correlated with the solar irradiance of the ground (Section 2.3). Figure 8.17 shows an example from Brazil, illustrating how a fairly simple determination of sunshine duration (Figure 8.10) yields estimates of the radiation, which is much more difficult to measure.

Clouds tend to warm the Earth by radiating sky radiation to the ground, and by preventing the loss of terrestrial radiation to space. Nevertheless, clouds cause cooling on the whole by reflecting solar radiation away (Note 8.K). In the absence of clouds, the current concentration of greenhouse gases (Section 1.3)

January

percentage cloudiness

Figure 8.14 Effect of season on cloudiness over the globe. The units are percentages, e.g. 40 means 4 tenths of cloud (i.e. 40 per cent of the sky is covered with cloud), or 3 oktas.

Table 8.5 Typical fractions of incident solar radiation either reflected, absorbed or transmitted by various kinds of cloud

Cloud

Reflected

Absorbed

Transmitted

Ns, Cb

85

-

-

fair-weather Cu, 450 m thick

50

7

43

St, 100 m thick

40

3

57

St, 600 m thick

50

11

39

Ci

20

1

79

Figure 8.15 The annual mean number of hours of bright sunshine in New Zealand.

Figure 8.15 The annual mean number of hours of bright sunshine in New Zealand.

would make the Earth's surface temperature about 30°C on average, well above the observed average of about 15°C (Section 2.7).

In addition, the moisture which yields cloud tends to imply moist air at screen height also, i.e. a high surface dewpoint Td, which approximates the daily minimum temperature (Section 6.2). Cloud also reduces the daily maximum temperature Tx. As a result, it is associated with a low value of {Tx-Td} as shown in Figure 8.18, and with a reduced daily range of temperature (Section 3.4).

The connection between cloud and rainfall is discussed in the next chapter.

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