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Diurnal Variation
The fourth photograph of this set (Harrison et al, 1988), Figure 5, shows the
diurnal variation for all days and include the effects of cloudiness.
This shows the effect of clouds in reducing the diurnal variation. Note, for
instance, that around the margins of the Sahara Desert in Northern Africa,
the area of high diurnal range shrinks when clouds are present. Clouds
tend to keep daytime temperatures lower and nighttime temperatures higher,
thereby reducing the diurnal range in two ways.
From this you can see that clouds insert a large amount of local variability in the amount of energy the earth radiates to outer space. It also is important to remember that these photographs are averages over many days; if we were to look at a snapshot of a particular day, we would see much more variability from place to place and time to time. For a glimpse at current global cloudiness go to the University of Wisconsin-Madison satellite composite.
