Origins of Extreme Weather
This page covers the following weather related phenomenon: severe thunderstorms, tornadoes, tropical cyclonic storms (hurricanes, typhoons, and cyclones), nor'easters, drought, and the causes and effects of El Niño.
Water and heat
Water
has one of the highest heat capacities of all known substances. This means
that it takes a lot of heat to raise the temperature of water by just one
degree. Water thus absorbs a tremendous amount of heat from solar radiation,
and furthermore, because solar radiation can penetrate water easily, large
amounts of solar energy are stored in the world's oceans.
Further energy is absorbed by water vapor as the latent heat of vaporization, which is the heat required to evaporate water or change it from a liquid to a vapor. This latent heat of vaporization is given up to the atmosphere when water condenses to form liquid water as rain. If the rain changes to a solid in the form of snow or ice, it also releases a quantity of heat known as the latent heat of fusion.
Thus, both liquid water and water vapor are important in absorbing heat from solar radiation and transporting and redistributing this heat around the planet.
Air masses
Due
to general atmospheric circulation patterns, air masses containing differing
amounts of heat and moisture move into and across North America. Polar air
masses, containing little moisture and low temperatures move downward from
the poles. Air masses that form over water are generally moist, and those
that form over the tropical oceans are both moist and warm. Because of the
Coreolis effect due to the Earth's rotation, air masses generally move from
west to east. But, because of the differences in moisture and heat, the collision
of these air masses can cause instability in the atmosphere.
Fronts and mid-latitude cyclones
Different
air masses with different temperatures and moisture content, in general,
do not mix when they run into each other, but instead are separated from
each other along boundaries called fronts.
When cold air moving up from the poles encounters warm moist air moving down from the tropics, a cold front develops and the warm moist air rises above the cold front. This rising moist air cools as it rises causing the condensation of water vapor to form rain or snow. Note that the cold air masses tend to circulate around a low pressure center in a counterclockwise fashion in the northern hemisphere and clockwise in the southern hemisphere. Such circulation around a low pressure center is called a mid-latitude cyclone.
When warm air moving northward meets the cooler air to the north, a warm front forms. As the warm air rises along a gently inclined warm front, clouds tend to form, and can also cause rain, but rain is less likely because the warm front is not as steep as a cold front. If the rapidly moving cold front overtakes the warm front, an occluded front forms, trapping warm air above a layer of cold and cool air. Mid-latitude cyclones and their associated fronts are responsible for such severe weather conditions as thunderstorms, snow storms and associated hail, lightening, and occasional tornadoes.
