What makes your microclimate

Your microclimate is the weather your trees actually live in, and it's rarely the weather at the airport. On a clear, still night it comes down to heat: how fast the ground loses it, where the cold air goes once it forms, and what puts heat back. A few degrees either way decides what you can grow, so it's worth knowing how it all works.

How the cold gets made

All night the ground gives off heat as infrared light, aimed at the sky. How much comes back depends on what's overhead. Under clouds or humid air a lot of it comes back. Under a clear, dry sky very little does, so the ground cools fast and chills the thin layer of air touching it. Air is a poor conductor, so the cold stays low. That's why there can be frost on the lawn while the thermometer on the porch reads in the mid 30s.

Cold air is heavier than warm air. At the same pressure, air at 20°F is about 6 percent denser than air at 50°F. So it slides downhill, slowly, along the slopes and down the creek bottoms and draws, and spreads out across the valley floor. Anything in its way backs it up. By morning the floor holds a lake of cold air, coldest at the bottom.

That lake is an inversion. Normally it gets colder as you go up. Inside the pool it gets warmer as you go up, until you reach the top of the cold air. The slopes just above the pool are the thermal belt, and on a still night that's the warmest ground in the airshed. Above the belt it gets colder with height again.

In winter, under high pressure, with long nights, snow on the ground and no wind, a pool can sit for days or weeks. Each long night adds more cold than the low winter sun can take back, and fresh snow throws most of that sunlight right back to the sky. If you've lived through a January inversion in a western valley, you've seen your airshed with your own eyes: the smoke and fog sit right in it and stop at the top. Air quality people use the word airshed for the same reason.

What puts heat back

• Clouds. A cloud deck soaks up the ground's infrared and sends a lot of it back down. A deck that slides in at 2 a.m. can stop the drop right there. Clear nights are the frost nights.
• Moisture. Humid air holds heat in the same way, though not as well as a cloud. And when the air cools to its dew point, water comes out as dew or frost and gives off heat as it does, which slows the fall. Dry air has nothing to slow it down. On a clear, calm night the low often lands near the evening dew point, which is why growers watch it.
• Water freezing. Water gives off heat as it freezes, about 144 BTU for every pound. That's how sprinklers protect blossoms: as long as fresh water keeps freezing on the bud, the bud stays near 32°F. Shut them off too early and evaporation pulls the heat back out, and the buds end up colder than if you'd never started.
• Wind. Any breeze stirs the cold layer into the warmer air above it. A wind machine does nothing else: it pulls down the warm air sitting on top of an inversion. No inversion, no gain.
• Lakes. Pound for pound, water holds about five times as much heat per degree as dry soil, and a deep lake stores a whole summer's worth. Open water just above freezing next to air at 20 below is a heater. Once a lake freezes and snows over, it acts like a field. That's why Flathead Lake and Lake Pend Oreille, which mostly stay open, warm their shores in a cold snap, and lakes that freeze over don't. Big lakes also keep the shore cool in spring, which holds bloom back until most of the frosts are over. That's how the cherry orchards on Flathead Lake's east shore work, and the fruit belts along the Great Lakes.
• Town. Brick, concrete, asphalt and stone soak up the day's sun and give it back all night. Buildings block part of the cold sky, so less heat escapes, and heated buildings leak heat. The EPA puts city nights about 2°F to 5°F warmer than the country around them. In our valley towns the map gives built-up ground up to 2.5°F.
• Trees and walls. A canopy blocks part of the sky the same way, so the ground under trees loses less heat. A south-facing wall soaks up the afternoon sun and gives it back after dark, which is why the south side of the house is often the warmest spot on the lot.

What comes from above

The ground makes cold on any clear night. The coldest nights of a decade need help from above: arctic air, and nothing to push it back out.

• Arctic air. Arctic air out of Canada is cold, dry, dense and shallow, often shallower than the mountains are tall. It slides south along the east side of the Rockies and piles up against them. The Continental Divide holds a lot of it back, which is a big part of why the valleys west of the Divide usually run milder in a cold snap than the plains do. When it gets deep enough it spills over the low passes and pours out of the canyons on the west side as a hard, cold east wind, and then the west side gets it too.
• Chinooks. A west wind crossing the mountains rises, cools and drops its snow on the west slopes. Coming down the east side it warms by compression at about 5.5°F for every 1,000 feet. On the way up, once it was making snow, it only cooled about 3.5°F per 1,000 feet, because water gives off heat as it condenses. So it lands on the plains warmer and drier than it started. That's a chinook. They blow along the Rocky Mountain Front, reach roughly 100 miles out onto the plains, and can scour a cold pool out in minutes. In January 1972, Loma, Montana went from 54 below to 49 above in about a day, a 103 degree swing. On January 22, 1943, Spearfish, South Dakota went from 4 below to 45 above in two minutes. Chinooks break a cold snap, but they're hard on fruit trees too: a week of warm wind in late winter can pull buds and bark out of their hardiness right before the next arctic front.
• High-pressure lids. A big ridge of high pressure pushes air down from high up, and sinking air warms by the same compression. That sets a warm lid over the valleys. The warm air is right there, a few thousand feet up, and it can't get down, so the pool under it keeps getting colder. On a valley floor the worst cold isn't only cold blowing in. It's warmth that never arrives.
• Mountains and rain shadows. Air forced up over a range drops its moisture on the windward side. The lee side gets drier air and clearer skies, so it loses more heat on calm nights and swings harder between day and night. Valleys that still get some Pacific moisture in winter get more cloud, and cloud slows the cooling.
• Snow cover. Snow is a blanket for the soil and a radiator for the air. It keeps the ground's heat from reaching the air and gives off infrared almost perfectly, so the air over snow gets colder. Roots under a foot of snow stay safe. Wood above the snow takes the full cold, and on a young tree you'll often see the winter injury stop right at the old snow line.
• Height. Above the inversion the air cools with height, on average about 3.6°F per 1,000 feet. On arctic nights it cools a lot less than that, which is why the free-air cooling in our model is so gentle.

Your yard is an airshed too

The same physics works at the scale of a lot. Cold air flowing downhill stops at the first thing that dams it: a solid fence, a tight hedge, a house, a road fill, a berm. The uphill side of that dam turns into a frost pocket. Leave gaps at the bottom of fences and hedges on the downhill side, or put the windbreak uphill of your trees so it turns the cold air around them. Plant on the slope, not at the foot of it. A tree a little way up a slope can come through a frost that takes the one at the bottom. North slopes hold bloom back a few days, which can save an apricot crop. The south and southwest sides of trunks take sunscald in late winter, when afternoon sun warms the bark and it freezes hard at sunset. White trunk paint keeps the bark cool.

What missing heat does to what you can grow

Winter. Hardy wood survives by controlling where the ice forms. Many hardy plants let water move out of their cells into the spaces between them, where it freezes without hurting anything while the cells dry down. Fruit buds and the living wood of many fruit trees use a different trick called deep supercooling: they hold their water liquid well below 32°F. That works down to a hard limit near 40 below, about as cold as pure water can get without freezing (and the one spot where Fahrenheit and Celsius read the same). Past that the water inside the cells freezes, and the cells die. Plants that rely on supercooling tend to stop at about that line on the map.

Hardiness also changes over the season. A tree hardens off in fall as the days shorten and the first frosts come, reaches its peak in midwinter, and loses it in warm spells. The same tree that comes through 30 below in January can be killed by far less cold in early November, or after a week of warm wind in March.

Spring. Blossoms have almost no hardiness at all. These are Washington State University's numbers for full bloom, the temperatures that kill about 10 percent and about 90 percent of the blossoms:

Source: Washington State University bud hardiness research, as compiled by Michigan State University Extension.

On a still night in May a frost pocket can run several degrees colder than the bench above it, and at bloom a few degrees is the difference between a crop and nothing. Frost pockets also shorten the season at both ends, so late apples, pears and grapes may not ripen there even when the trees themselves are fine.

The whole system. An airshed is a system. Energy comes in as sunlight, as warm air from somewhere else, and as heat given off when water vapor condenses and water freezes. It goes out as infrared to the sky and as cold air draining in off the slopes. It gets stored in soil, lakes, buildings and the trees themselves. The wild plants already show where the balance lands. In Vermont, a 2024 U.S. Forest Service study found that cold-air pools turn the forest upside down: spruce and fir on the low slopes, where you'd expect them up high, and hardwoods above them. An orchard is one more part of that system, and every tree you plant is a bet on the coldest night your spot will hand it.

How the maps read your microclimate

We split the drivers in two. The big ones from above (arctic air, chinooks, clouds, high-pressure lids) are already in the weather station records: thirty winters of them, blended from the eight closest stations the way our cold hardiness calculator does it. The ones that change from one side of a road to the other come from our microclimate model, one parameter per driver: height above the cold pool for the thermal belt, drainage channels for the cold air's creeks, side-valley bottoms and hollows for the little pools, slope for ground that sheds its cold, town for thermal mass and blocked sky, tree cover for canopy, open water and big water for stored heat, and wet ground and hay ground for open, damp fields that lose heat fast. The table further down has every weight, and the write-up on each valley's map covers which of these drivers matter most there.