People call cyclonic storms by many names; hurricanes, typhoons, tropical cyclones, tornadoes, nor'easters or plain old cyclones, depending on the part of the world in which they occur. All cyclones have physical characteristics in common. Some create disasters that kill thousands and cripple economies. Some are chased by meteorological adventurers with video cameras for just a few miles before they dissipate. As our planet becomes more densely populated, more of these storms are causing more damage to life and property.
Cyclones are low pressure areas that draw air into their centers in a counter-clockwise pattern in the Northern Hemisphere, or a clockwise pattern in the southern atmosphere, forming the familiar comma-shaped vortex. The growth of cyclones and the storms that they become depend on latitude, temperature and the depth of the low pressure at the center to determine whether the cyclone will have a cold or warm core.
We think of cyclones as warm weather storms. Cold-core cyclones, though, occur in higher latitudes and draw energy from the Arctic and Antarctic regions. Polar vortexes are huge cyclones in northern latitudes that measure more than 600 miles across near the poles. These cold-core cyclones can cause blizzards on land or savage storms at sea. As their strength grows and wanes, smaller Polar lows spin off and drift into the middle latitudes to become extra-tropical. Warm-core cyclones form in the middle and low latitudes. Tropical cyclones, called hurricanes in the Western Hemisphere and typhoons in the Eastern Hemisphere, are the largest of these cyclones. Monsoons and mesocyclones are warm-core cyclones, the former requiring a reliable source of water and the latter capable of spawning tornadoes.
Extra-tropical lows can become sub-tropical and then tropical lows given the right direction and source of moisture. Nor'easters are dying sub-tropical cyclones or extra-tropical cyclones picking up steam. The name nor'easter comes from the direction of the wind as it batters the shore from the northeast, coming from the side of the storm where it has the best source of moisture. Most cyclones develop their strongest quadrant just past the best supply of moisture. As a tropical cyclone spins its way north or over land, it will lose strength but it may spin off smaller mesocyclonic areas causing severe storms, water spouts and tornadoes.
Each type of cyclone puts on its stormiest face when climatological conditions are most favorable. The greatest risks from polar lows are the snow storm and "arctic outbreak" following a long period of pleasant winter weather. Hurricanes and typhoons form when the ocean water warms and the sun rises in the sky in late spring through the cooling autumn. Monsoons require strong winds and a warming sea or ocean. Every place on earth has a period of time when the risk factors add up to an environment for a cyclone to become a disaster.
Hurricanes can form outside of the tropics. Tornadoes can occur in the city--or at night. Much of the mythology about these cyclonic storms is based on historical experience. The cooler temperatures of the middle latitudes tend to put the damper on hurricane growth and urban heat islands do disrupt the wind shear necessary to form tornadoes. There are enough examples, though, of the hurricanes that turn around and become killers and of tornado outbreaks that destroyed main thoroughfares, to be prepared when the conditions that cause dangerous storms are present.