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The Stability of the AtmosphereAppendix 11Air has three stability conditions; unstable, neutral, and stable. Stabilityconditions must be considered during the computation of sensible heat flux(H) because they affect the aerodynamic resistance to heat transport (r ah ). InSEBAL, a neutral atmospheric condition is initially assumed and a stabilitycorrection is later applied using the Monin-Obukhov length (L) as the indicator.The three stability conditions for air are illustrated in Figure 11.1. If a dry airmass at 10 o C is suddenly moved upward 100 meters, its temperature willdecrease about 1 o C due to the lapse effect. If the surrounding air temperatureis lower than this air mass, an upward force on the air mass will occur. Thiscondition is called “unstable”. If the surrounding air temperature is the sameas the air mass, no force will occur and the condition is called “neutral”. If thesurrounding air temperature is higher than the air mass, a downward force onthe air mass will occur. This condition is called “stable”.Generally, air temperature decreases by about 0.65 o C when elevationincreases by 100 m under neutral stability conditions. In places where positivesensible heat flux (H) occurs, the change of temperature with elevationbecomes smaller because the air mass is heated by the positive H. In thiscase, vertical air movement is easier and therefore aerodynamic resistancebecomes smaller as H becomes higher. In general, air is in a neutral conditionat a well-watered agricultural field at noon and an unstable condition at a drybare field at noon. The stable condition will most likely occur at night andsometimes in the afternoon in irrigated areas surrounded by desert.91