What causes a mid-latitude low pressure to strengthen? (1) Evacuation of mass due to the jet stream. The jet stream is a high-speed ribbon of air that creates a large amount of upper level speed shear. This speed shear (through divergence) forces air to rise from the lower levels of the troposphere and forces a low pressure to strengthen. Low pressures coupled with the jet stream become stronger than those not coupled with the jet. (2) Surface convergence. Surface convergence forces the air to rise. Since air coming together at the earth's surface can not go down into the earth's surface, it rises. Rising air creates lower pressure. This is what creates the pressure troughs in association with cold and warm fronts. The rising air near the frontal boundary causes pressures to lower along the front. Convergence at the surface can occur through several ways. Examples include frontal convergence, orographic convergence, frictional convergence, and convergence created by a strong synoptic low-level wind flow (low level jet). (3) Release of latent heat. The release of latent heat causes air within a low pressure to be more buoyant than it otherwise would be. Increased buoyancy leads to an increase of rising air and a lowering of pressure. Low pressures developing in a very dry air mass tend to not develop as well as those lows that have the potential to release latent heat. Well developed mid-latitude cyclones will have all three components mentioned: low level convergence, upper level divergence, and latent heat release. It is important for the upper level divergence to be strong enough to maintain the low level convergence. When upper level divergence is greater than low level convergence, the air will continue to rise. If upper level divergence weakens, the low level convergence will cause the low pressure to fill and weaken (since mass is no longer being evacuated at an efficient rate above the low). |