Clouds are observed regularly at weather stations. The extent to which the sky appears covered at the time of observation is expressed in terms of oktas, representing eighths of the sky seen by the observer. Thus 8 oktas of cloud represents a totally overcast sky (Figure 8.9). A cross is drawn through the cloud circle under foggy conditions. The inexperienced often underestimate cloudiness by ignoring the cirrus, or else they overestimate the amount of cumulus because it tends to cover much of the sky near the horizon. Preferably, separate estimates are made of the amounts of cloud in each of the three main layers of the troposphere—low, middle and high (Table 8.2).
Cloudiness above a weather station can be determined indirectly by measuring the duration of bright sunshine. As an example, if eight hours of sunshine were registered in a daytime of twelve hours, there would have been four hours
of cloud. The instrument used is often a Campbell—Stokes bright-sunshine recorder (Figure 8.10), which gives the fraction of time that cloud obscures one place. (A visual observation of cloud in terms of oktas gives the fraction of the sky's area obscured at one time.)
Observations of cloud are plotted on maps with the symbols shown in Figure 8.11.
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