They find that the market is valuing the swaps differently to them - Storm is using 30 years as a reference for his valuations, but the market makers are using 10 or 20 year averages. Storm insists that 30 years is more representative of the climate, but it does not change the market price. The forecast and the shorter averaging periods have made swaps cheap and the company is the seller. The market is buying swaps at 4,800 degree-days. They multiply these two curves by Storm's modeled probabilities to get the two probable payout curves in the same figure.
Storm explains what was unclear earlier.
If the two areas don't sum to zero, then someone is implicitly paying in advance in the amount of the non-zero sum. He smiles smugly as if he has just won a debate. Flash does not seem to notice. They are shocked by the difference in areas above and below the zero-line. Flash is very unhappy about having to give up so much potential upside gain for so little downside protection.