California's fruit and nut trees need cold, and the valley is losing its fog

Winter chill is an agricultural input as real as water. The tule fog that helped supply it has declined sharply over four decades.

Deciduous fruit and nut trees — almonds, pistachios, cherries, peaches — require a period of winter cold before they will break dormancy and flower properly. Growers account for it in chill hours or chill portions, and it is a hard input: below a variety's requirement, bloom is delayed, uneven and poorly set, and yield falls regardless of how good the rest of the season is.

The Central Valley is one of the most productive agricultural regions on earth partly because it has historically supplied both halves of what those trees need: long hot summers, and winters cold enough to satisfy dormancy.

What the fog was doing

Tule fog — the dense radiation fog that forms on the valley floor under a strong winter inversion — is part of that supply. It is not simply a nuisance for drivers. A valley under fog stays cold all day, because the sun never reaches the surface to break the inversion, and days that never warm are exactly what accumulates chill.

Measured tule fog frequency has fallen substantially over the past four decades. It is one of the clearest and least-disputed climate signals in California, and unlike most it has a direct, quantifiable agricultural cost.

The other end of the state

The Salinas Valley has the opposite relationship with its weather. It grows cool-season vegetables precisely because the marine layer keeps summer afternoons near 71°F when the interior is in the nineties — an ocean that refuses to let the valley warm is the entire agronomic advantage.