Figure 98 comprises four vertical bar charts labelled 'Northern Europe', 'Southern Europe', 'Sahara Desert' and 'Tropical, South East Asia'. The charts have the same structure as follows. The horizontal axis is divided into the months of the year starting with January on the left and ending with December on the right. The vertical axis, labelled 'kWh m superscript -2 day superscript -1', has 6 tick marks. Reading from the base to the top, the tick marks are labelled 0, 2, 4, 6, 8 and 10. Each column is divided into two sections. The lower section of the column is identified as 'diffuse' and the upper section of the column is identified as 'direct'. I shall now give you the approximate heights of the various columns by identifying the month, the diffuse height in brackets followed by the total height. Northern Europe: January (0.6) 0.8, February (0.8) 1.2, March (1.2) 2.2, April (1.9) 3.4, May (2.3) 4.3, June (2.5) 4.6, July (2.3) 4.3, August (2.0) 3.7, September (1.5) 2.8, October (0.9) 1.5, November (0.5) 0.8, December (0.3) 0.5. Southern Europe: January (0.9) 2.5, February (1.2) 3.4, March (1.4) 4.6, April (1.9) 5.9, May (2.0) 6.3, June (2.0) 6.8, July (2.0) 6.8, August (1.9) 6.2, September (1.5) 4.9, October (1.2) 3.7, November (0.9) 2.5, December (0.8) 2.2. Sahara Desert: January (0.9) 4.8, February (0.5) 6.5, March (1.2) 6.8, April (1.5) 7.4, May (1.7) 7.5, June (1.7) 7.7, July (1.7) 7.7, August (1.7) 7.2, September (1.4) 6.6, October (1.2) 5.7, November (0.9) 4.9, December (1.1) 4.0. Tropical, South East Asia: January (1.5) 2.6, February (1.9) 3.7, March (2.0) 5.5, April (2.3) 4.0, May (2.5) 4.6, June (2.5) 4.2, July (2.5) 4.6, August (2.5) 4, September (2.2) 4.3, October (1.9) 3.5, November (1.5) 3.7, December (1.2) 4.2. I will now describe the patterns created by these figures. Northern Europe, Southern Europe and to a lesser extent Sahara Desert have seasonal variation of total sunlight peaking in the May to July period. By contrast Tropical, South East Europe is fairly constant across the whole year. Northern Europe and Tropical South East Asia have greater proportions of diffuse sunlight whilst Sahara Desert has the greatest proportion of direct sunlight. The diffuse sunlight pattern tends to follow the overall pattern but is less pronounced which means that the proportions of diffuse sunlight are much greater at the lower total levels.

 5.9 Why photovoltaics?