By and large, commodities traders love a disruptive dose of bad weather. Battered hedge fund Amaranth's bets on natural gas might have looked prescient, if only a big hurricane had bothered to show up. Grain traders, however, are thriving on sunshine: too much of it has caused drought in Australia and the US. That has left global wheat inventories at their lowest level (in terms of weeks of consumption) in a decade and pushed Chicago's benchmark soft red wheat (SRW) contract to a 10-year high. There are positive secular trends, too, such as rising Chinese and Indian appetite. Meanwhile, corn prices are benefiting from high oil prices boosting demand for corn-based ethanol, a fuel substitute.

Agricultural commodities investments are fundamentally different from taking a punt on oil or metals. Crops are renewable and it takes less time to grow more of them than, say, digging a copper mine. That means little premium is paid for near-term supplies, which would keep forward prices below those for immediate delivery (known as backwardation). That in turn precludes earning a yield by rolling futures positions forward. In the absence of a positive "roll yield", speculative profits depend more on calling spot prices correctly.