Cold Air Accumulation and the Grower's Guide to Frost Protection

Atmospheric stratification and Thermal Boundary lines

Stratified atmosphere is the layering of the atmosphere due to differences in density and temperature.  Density is inverse to temperature- the lower the temperature of the air molecules, the heavier and denser they become. Density is not the same as viscosity, the colder the air molecules, the denser but less viscous they become. Since density is related to temperature, the layers can be identified and separated by temperature differences, as in Fig 2.   It is not possible, during radiation conditions, for a warmer molecule to be below a colder molecule under natural conditions.  The colder air will always be closer to the ground unless modified.   

Visual image: On a radiation night, clear skies, no wind, go to a relatively flat area in an accumulation basin.  Hold a thermometer at 3ft from the ground and record the temperature.  Then raise the thermometer at 1ft increments and see the temperature differences.  

Thermal boundary lines are separation points in the atmosphere due to the differences in the density and temperature of the layered air molecules.  The differences in the characteristics of the air layers can cause differing reactions on either side of a thermal boundary line to forces such as gravity or cold air accumulation.  

A thermal boundary is sometimes referred to a as a ‘ceiling’, and can be used in an absolute or relative context, such as the natural absolute ceiling at the top of the upper inversion layer, or in a relative context such as the artificial ceiling created by heaters where warmed molecules stack. At the absolute ceiling at the top of the upper inversion, the temperature above the thermal boundary begins to drop and the temperature below the thermal boundary also drops.  In this case the thermal boundary is the warmest point; both sides of the line get colder as the distance from the thermal boundary increases.  In the case of an artificial ceiling where heated air molecules will stack, the molecules directly below are the same (or possibly even a slightly higher) temperature as the thermal boundary line. Before the warming with artificial heat takes place the temperature is colder below this point and warmer above- hence no thermal
                                                     
boundary.  This line of demarcation marks the separation point where the temperature rises on both sides of the lines due to the introduction of artificial heat sources.

The term ‘thermal boundary layer’ also refers to an absolute thermal boundary where there is a physical separation such as the point of gravity influence (the point where molecules below are affected by gravity and moving, and those that are above and static), or in a relative context such as the separation of lethal and non – lethal temperature air mass, even though both are moving or non-moving.     

Under radiation conditions, the atmosphere will separate or stratify into layers defined by temperature and density unless some other phenomenon destroys the stratification.  Clouds and wind will destroy stratification, as will surface fog and overhanging trees.  This is the result of the radiation waves being absorbed and reflected back into the ground slowing the ground cooling and inhibiting heat loss into the atmosphere.  Crops that are planted under an overhanging tree will often not suffer frost damage for this reason.  Crops that are planted in areas where the thermal boundary layer can flow more freely and flatten out, such as the first row or two along a wide road will also tend to avoid frost damage as cold air is free to drain off.          

A thermal boundary layer can also be a demarcation line where there is a difference in dynamics of the molecules on either side of the line caused by a difference in temperature and density of the separated molecules.  In fig. 3, the thermal boundary line is defined by the separation of the higher, warmer molecules that are too far from the ground and not dense enough to be affected by gravity, and the colder, heavier molecules in the lower stratus that are affected by gravity.  The molecules above the thermal boundary line (disregarding for turbulence created by the down slope air movement) that are not affected by gravitational pull will remain static until they cool and fall below the TBL.  All the molecules below the TBL are affected by gravity and will flow downhill. The colder, denser molecules closest to the ground are more affected and will move faster than the lighter, warmer molecules. 

Thermal boundary lines can be vertical or horizontal and can exist in a static mass of air such as a build up of cold air over the height of a barrier, or a thermal boundary can exist in a dynamic flow as the separation point between the static and dynamic molecules.