#WeatherWednesday – CAP versus CAPE

Happy #WeatherWednesday guys and gals! Have you always wondered what the meteorologists on TV or weather websites really mean when they say the atmosphere is stable or unstable? We will discuss what that means and how “the cap” or “CAPE” relate to these descriptors. A stable atmosphere is characterized by ¬†air that is warmer aloft than the theoretical temperature of an air parcel that is vertically displaced from the surface. This warmer air can be a variety of thicknesses and magnitudes. In this context, the stability is stronger as this temperature differential between the atmosphere and the surface parcel increases and/or the as thickness that differential exists grows. This layer is called the cap. Colder air is denser than warm air and will SINK in the fluid atmosphere. Warm air RISES. Since you need air displacing upward to trigger thunderstorms, a cap will prohibit vertical motion and storms unless a “lifting mechanism” is strong enough to force air to erode the cap due to momentum and cooling effects of evaporation. So what’s the opposite of stability and a cap? It is instability, or CAPE (Convective Available Potential Energy). This is the potential energy for a surface air parcel to accelerate **opposing gravity** upward until it reaches stability again. Here is where the environment is colder than the theoretical surface parcel temperature allowing the relatively warmer air to continue rising...

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