Cold air is denser than warm air, so on still clear nights it flows downhill like water and collects in valley floors and enclosed basins, forming frost pockets that can be 5-10 degrees Celsius colder than the slopes above. Build homes and tender crops on the slope, not in the valley bottom, and never block the path the cold air takes downhill.
How cold night air drains off slopes into hollows, how to recognize the frost pockets it makes, and how to site buildings and crops around them.
Cold Air Behaves Like a Slow Stream of Water
After sunset the ground loses heat by radiation, most strongly on clear still nights. The air touching it cools, becomes denser and slides downhill under gravity, settling in the lowest ground. Warmer air sits above it. The result is that low ground can be well below freezing while a slope higher up stays above it. The carrier’s figure is that frost pockets can be 5-10 degrees Celsius colder than the slopes above.
Wind and cloud stop it. A windy or overcast night mixes the air and the effect fades; the worst nights for frost pockets are clear and calm.
Signs of a Frost Pocket
- Frost on the ground or on plants in the hollow when the slope is clear.
- Fog or mist lying in the valley at dawn.
- Frost-damaged, late-leafing or stunted trees and shrubs in a hollow.
- Snow that lies longest in low ground.
- Local advice: ask where the first frost falls.
What You Need
- A minimum-reading thermometer, or two ordinary ones — read at dawn.
- Clear, still nights — wait for them; cloudy nights show little.
- A map of the slope and its hollows — see Slope & Drainage Assessment.
Method: Find the Pockets and Place Things Around Them
Step 1: Compare temperatures On a clear, calm night, put thermometers at three heights on the same slope: valley floor, mid-slope, upper slope. Read each at dawn.
Step 2: Map the flow Cold air follows the same lines as water: down gullies, along the lowest ground and toward the downhill outlet. Find where it enters, pools and leaves.
Step 3: Place things off the floor Build living quarters and tender crops on slopes, as the carrier advises, not in valley bottoms.
Step 4: Do not dam the flow A dense hedge, solid wall, embankment or road across a slope can hold cold air back and make a new frost pocket behind it. Leave gaps for it to pass.
Worked Example
Using the carrier’s range: suppose the slope above a hollow has a night minimum of 3 degrees Celsius.
- Hollow minimum = 3 - 5 = -2 degrees Celsius at the mild end.
- Hollow minimum = 3 - 10 = -7 degrees Celsius at the severe end.
- So crops that die at 0 degrees Celsius are safe on the slope and killed in the hollow on the same night. A site comparison that looks only at an average temperature misses this entirely.
Common Mistakes
- Choosing the sheltered hollow — shelter from wind comes with cold, damp air.
- Planting a hedge across the slope below the garden — it traps cold air in the garden.
- Measuring on a windy night — the effect only appears when it is calm.
Safety
Cold, damp and smoke collect in hollows
Smoke from fires also sinks and lingers in low ground on still nights. Keep cooking and heating fires where smoke can drain away, and do not sleep in a hollow with an unvented fire. In freezing weather, cold pockets cause hypothermia in poorly sheltered people and animals.
What’s Next
- Learn next: Agricultural Potential Mapping — keeping tender crops out of pockets
- Combine with: Sun Exposure & Shadows — cold ground is usually shaded ground