A cap is a vertical region of stable air that prevents convection from penetrating through this vertical layer. As long as this layer prevents convection, the cap is in place. This cap or capping inversion is also called the Elevated Mixed Layer (EML). ... As the air moves east, it flows over the top of warm and moist air.
1. The EML prevents deep, moist convection until high instability is achieved. In the absence of deep, moist convection, warm, moist air can flow poleward in an unimpeded manner. Daily evapo-transpiration also adds moisture to the boundary layer further enhancing theta-e.
2. The EML tends to keep storms isolated. When deep moist convection occurs in a capped environment, it tends to be in localized areas of enhanced convergence such as along out flow boundaries, dryline and terrain features, or of course along frontal boundaries. Isolated storms tend to be more severe than widespread storms since there is less competition for available warmth and moisture.
3. The EML along the southern edge of the westerlies prevents deep vertical mixing. Deep vertical mixing is a CAPE destroyer. It is very difficult to maintain high mixing ratios when very deep mixing is occurring. The cap provided by the EML confines the moisture to a shallow layer, preventing the mix-out effect. This effect is most apparent in late-spring and summer when the southern edge of the westerlies retreats to 40-45N. The high dewpoints will usually be along the southern edge of the westerlies where the lid is the strongest and where cold fronts stall out. Moisture convergence is also greatest along the southern edge of westerlies, typically just poleward of stalled out fronts or outflow boundaries where evapo-transpiration is at a maximum from vegetation and previous rains. The mixout effect can also occur beneath the strong capping inversion in cases where moisture return is extremely shallow, particularly when strong synoptic-scale disturbances are involved.