The Physics of Stratification Air density varies with temperature—cold air at -18°C is approximately 15% denser than air at +20°C. This density difference creates buoyancy forces that cause cold air to sink and warm air to rise, regardless of where your refrigeration system discharges cold air. In refrigerated spaces, this natural convection creates distinct temperature layers. Floor-level temperatures typically run 2-5°C colder than ceiling temperatures, but this apparent advantage at floor level is often negated by heat infiltration through poorly insulated floors, especially in vehicles where pavement temperatures can exceed 60°C.
Stratification vs Refrigeration Design
Standard transport refrigeration units discharge cold air horizontally from roof-mounted evaporators, expecting air to travel across the space before returning. However, the Froude number—the ratio of momentum to buoyancy forces—decreases with distance from discharge. When Froude number drops below 1, buoyancy dominates and cold air falls to the floor regardless of the intended trajectory. The result: cold air intended for the rear of a loadbox may fall to the floor within the first 1-2 metres, leaving upper rear zones chronically under-cooled.Research Findings
Studies of refrigerated transport show consistent stratification patterns:- Temperature mapping studies find 8-15°C variations within single freezer units
- Upper rear corners typically represent the warmest zones in roof-discharge systems
- Bottom-layer cargo often shows higher temperatures than mid-layer cargo due to floor heat infiltration overwhelming the cold-air-sinks advantage