Instability can be increased by increasing low level temperatures, increasing low level moisture or cooling in the middle levels of the troposphere. Warming temperatures and increasing dewpoints have been covered in previous topics. The cooling temperatures aloft is the main focus of this writing. Cooling temperatures aloft can be described as a cold pool aloft, cold air advection aloft or a lifting mechanism that is cooling the air aloft such as dynamic lifting or evaporative cooling. These processes help weaken the cap aloft and enhance instability. Where the cap weakens the most and where the greatest cooling occurs aloft will be a location that a rising thermal from the surface will have the easiest time breaking the cap. This can be a burst point for initial thunderstorm development. The three ingredients for thunderstorms are lifting, adequate moisture and instability. Cooling aloft will increase instability and make it easier for a rising parcel of air to break the cap and develop into a thunderstorm. Thus, not only are surface conditions important but what is occurring aloft can be very significant also in determining where convection will develop first. Lifting aloft and cooling aloft are important to examine right along with surface temperatures and dewpoints. |