Climatic Data - Climate Change: Identification and Projections - page 20

difference between IRBM and PMOD reaches 0.6 Wm -2 , which is a
considerable figure, given the peak amplitudes of the order of 1 Wm -2 .

There are also measurements of spectral distribution IR/UV of total
irradiance, which could have an influence on the climate. In addition, it is
important to briefly highlight that since satellites are not able to measure the
overall radiated flow to better than one percent accuracy while the sun is a
quasi-punctual source, they are even less able to measure with any accuracy
the energy reflected or re-emitted in the visible spectrum or infra-red as this
flow is distributed over the surface of the Earth. The numerical evaluations
of net radiation that can be found (Trenberth, 2009), accurate to a tenth of a
2

Wm − , can never result from direct measurements.

Wolf Number

200

Wolf number annual mean
11 years mean shift

22

150

23

100

24

50

0

1650 1700 1750 1800 1850 1900 1950 2000 2050

Figure 2.7. Sun spots

A second effect of solar activity is the number of sun spots observed,
according to the counting method standardized by the astronomer Rudolf
Wolf in 1855. This number or Wolf index is written: R = k (10 g + f ), where f
is the number of spots visible, brought together into g groups, k denotes a
coefficient (generally <1) depending on the quality of the images. The spots
have been observed since 1610, the date of the invention of the telescope
accredited to Galileo. Between 1650 and 1700, these spots were rare