Subsidence in the Earth’s atmosphere is most commonly caused by low temperatures: as air cools, it becomes denser and moves towards the ground, just as warm air becomes less dense and moves upwards. Cool subsiding air is subject to adiabatic warming which tends to cause the evaporation of any clouds that might be present. Subsidence generally causes high barometric pressure as more air moves into the same space: the polar highs are areas of almost constant subsidence, as are the horse latitudes, and these areas of subsidence are the sources of much of the world’s prevailing wind. Subsidence also causes many smaller-scale weather phenomena, such as morning fog. An extreme form of subsidence is a downburst, which can result in damage similar to that produced by a tornado. A milder form of subsidence is referred to as downdraft.
Why is this lesson important? Well, this is the main reason why Connecticut remained mostly devoid of thunderstorm activity yesterday afternoon. As the air pushed over eastern New York State and into Western Connecticut, it encountered some colder air and began to sink (albeit, not much cooler). This sinking motion is not conducive for thunderstorm development, but did not negate the fact that the air itself was moist and unstable. Therefore, as it pushed back into a more favorable environment for lift, intense storm updrafts occurred and formed the damaging storms that were witnessed into Massachusetts and Rhode Island yesterday afternoon. This is not a phenomena that is often observed into these areas during the summer months, but it is not all that uncommon given the mesoscale (small scale) nature of New England. Often times, it can be the marine layer ( a layer of drier air in the mid levels of the atmosphere caused by the cooler onshore air off the ocean) that causes the collapse of any thunderstorm activity in the area. However, subsidence was the cause of this negation of storms yesterday into the Connecticut area, but actually caused enhancement of thunderstorm activity once the air began to rise once again.