The Central Valley of California is famously hot in the summer, especially compared to the coastal regions and Bay Area where so many people live. But it’s not just hot. It’s hot and dry.
Dry air feels cooler to humans than damp air. Humans are “liquid cooled”, meaning that we sweat. Dry air causes sweat to evaporate, lowering the temperature of your skin and thus your entire body. If it’s too dry — which can happen here when we have a dry wind on a hot day — you can get overheated while still feeling cool.
Dry air also cools down more quickly than moist air. Most of the heat in the Central Valley comes from the sun, and once it sets, it cools off fairly quickly. It’s not uncommon for our farm to be 95 degrees at 6 p.m. on a summer day, yet by 6 a.m. it’s the same temperature as Berkeley or even San Francisco — 35 or even 40 degrees cooler in just 12 hours.
That means we don’t actually work very many hours in the heat. Like most farms in the Central Valley, we start work at or before sunrise, and try to get as much as possible done by noon. Most days, we end up only working a few hours in the heat, and generally quit long before the temperatures max out.
This is impossible to do in places where it’s hot and humid. It stays hot all night, and even if it’s a bit cooler in the morning, the humidity can make working unpleasant.
Heat combined with humidity is far more dangerous to humans than dry heat. If the air is too moist for your sweat to evaporate, humans’ cooling system stops working completely. Those conditions, which essentially never occur in California, are very dangerous.
Unlike humans, many plants like hot and humid conditions, but so do all kinds of mold and mildew, including many destructive plant diseases that we rarely have to deal with here. Growing crops organically in those conditions is difficult bordering on impossible.
We received almost a quarter inch of rain on Sunday, and ever since, it’s been dramatically more humid here than I can ever remember in June. Even on hot days, it’s not uncommon to work until lunchtime without breaking a sweat. But the last two days, we were sweating by 7:30.
Unfortunately, we’re anticipating a three-day heat wave later this week, and it will still be unusually humid. We’ll be sending folks home early as no one is used to the humidity and even resting in the shade will not provide much relief.
Hot days and warm nights are guaranteed to speed up the growth and ripening of our summer crops — tomatoes, watermelons, sweet corn — so we welcome them. But we could do without the humidity.