Official air temperatures are measured at several levels in the atmosphere. The ones at high altitudes above ground level may be taken by airplanes or weather balloons. But the ones we are most interested in are taken here on the surface of the earth where we live. This means official measurements are made with a thermometer usually housed in a well ventilated white wooden box anywhere from 4 ½ to 6 feet off the ground. When we talk about how hot or cold it is or forecast how hot or cold it is going to be, what we really mean is how hot or cold it is or will be in a white wooden box at about eye level. And that works fine for us, because most of us spend our lives within that first 6 feet of air above the ground.
But what about plants? Especially young sprouting plants spend their lives in the first four inches of air above the ground, not the first 6 feet. They couldn’t care less what the temperature is way up several feet above their heads; they care about the temperature within about the length of your thumb above ground, not the height of your head. There can be plenty of difference between the two.
The reason the temperature is different near the ground is because earth warms up faster and cools off faster than air in response to sunlight. In the daytime the sun shines right through the air without warming it up much, but when it hits the ground, the ground warms up sooner and gets warmer than the air. So during daytime hours, the air that plants live in is usually warmer than the air you live in. Your feet may have noticed this when you walk on a blacktop parking lot or dark dirt road without shoes on and get burned.
At night, the opposite happens. Under certain conditions, when the ground can radiate its heat effectively into the atmosphere, the ground cools off faster than the air after the sun goes down. You may have noticed this when camping out on the ground on a warm night and you feel cold anyway because the ground is chilly and you don’t have a good camping mat. Well, plants don’t have a good sleeping mat either, so they feel colder temperatures at night than you do 5 or 6 feet up. The difference may only be a few degrees, but on chilly nights in spring, summer, or fall in Teton Valley, those few degrees can be the difference between freezing and not freezing. Our forecasts for the night really mean what we think the temperature will be 6 feet or so above the ground. At night it will probably be colder than that down where your plants live and may frost them even when your thermometer says it isn’t that cold. And during the day, it will be warmer down there to make your radishes bolt when your thermometer says it wasn’t that hot.
This is just another challenge that growers here in the mountains face and prepare for the best we can. But hey, if living here was easy, everyone would be doing it, right?