Generally speaking, air at high altitudes is cooler than the air at ground level. However, Northern California’s cool ocean currents, its mountains, and its proximity to high atmospheric pressure zones all contribute to the frequent development of temperature inversions.
When cold air settles beneath warmer air it can grow stagnant, freezing the ground, and more importantly to the local economy, the grapes.
From the day the grape buds begin to swell in March, until the danger of frost has passed in early June, grape growers live in a state of constant alert, waking frequently throughout the night to check the thermometer.
When temperatures drop into the thirties, wind machines are used to stir the atmosphere and break up the inversion layer. Originally built from old plane propellers, the sole purpose of the wind machine is to bring warmer air down to ground level and decrease the chance of crop loss.
For a grape growing family, the change of just one or two degrees in air temperature can mean the difference between a good year and bankruptcy.