137. Read the following passage, then answer the question below.

Humans have harnessed wind for thousands of years, first to sail boats and later to pump water and grind grain. The modern wind turbine, which converts the motion of its blades into electricity, descends from experiments in the late nineteenth century, but only in recent decades has the technology become competitive. Larger blades, taller towers, and improved electronics have lowered the cost of wind electricity so much that, in many regions, new wind farms are now cheaper to build than new coal plants.

Size matters because the power available from the wind increases with the square of the blade length and with the cube of the wind speed. Doubling the speed yields eight times as much energy, which is why turbines are mounted high, where winds are stronger and steadier, and why offshore sites, with their fewer obstacles, are particularly attractive. Offshore farms are costly to build and maintain, however, and they face the hazards of saltwater corrosion and storms.

Wind power also has drawbacks that critics emphasize. The wind does not blow on demand, so grids that depend heavily on it need backup sources, long-distance transmission lines, or storage, such as large batteries. Local residents sometimes object to the sight and sound of turbines, and spinning blades can kill birds and bats, although studies indicate that the number is far smaller than deaths caused by buildings, cats, or vehicles. Engineers are responding: painting one blade of a turbine black has reduced bird collisions in trials, and radar systems can shut turbines down when large flocks approach.

Wind will not supply all the world's electricity, but combined with solar power, whose output often peaks when the wind is calm, it can provide a large share at a falling cost.

Why does the author mention that solar output often peaks when the wind is calm?