In Module 2 we briefly discussed extratropical storms (Northeasters and fronts) and their destructive power. Here we learned an important distinction: Although extra-tropical storms are typically smaller and less powerful than tropical cyclones, they are much more frequent, so their cumulative impact on a coastline can be more significant. We will now take a closer look at these types of storms, also referred to as “Cold Core” storms.
Extratropical storms, otherwise thought of as “cold core” storms, are generally produced outside of tropical regions. In contrast to tropical storms produced by an uplift of warm moist air masses fueled primarily by evaporation of warm waters, extratropical storms are formed when cold air masses interact with warm air masses on land or at sea. As these bodies of cold air collide with warm air bodies, discontinuities (that is, weather fronts) form. As the fronts mature and strengthen, the denser, drier cold air masses move underneath the more buoyant warm air masses and help force the warm air to rise. As a result of this rising air leaving the surface, low-pressure systems, called cyclones, develop and draw air (i.e., wind) into the low-pressure center. In the northern hemisphere, the winds of these cyclonic systems deflect to the right as a result of the Coriolis Effect. The opposite is true in the southern hemisphere. As a result, cyclones have a counterclockwise rotation in the northern hemisphere and clockwise rotation in the southern hemisphere. As the warm air rises in the atmosphere, it cools and releases its potential energy as sensible heat, thereby raising the temperature, making the air more buoyant, and consequently fueling a more intense upward movement of air, which further lowers the pressure at the surface, intensifying the storm. At the same time, the water vapor in the rising air condenses, turning into cloud droplets and the subsequently becoming precipitation. Through these positive feedbacks, temperature, pressure, and humidity (moisture) gradients between lower and upper-level layers of the atmosphere help fuel the continued development of high winds and extreme types and volumes of precipitation. Most extratropical storm systems are neither large nor intense and produce modest winds and precipitation totals, but when conditions are right, they become quite powerful and extraordinary in their scale and impact. Blizzards, nor’easters, and thunderstorms formed along frontal boundaries are examples of powerful extratropical storm systems.