variability as theair massesbring surface air poleward and equatorward over the
planet. For instance, winter satellite imagery frequently shows “the Pineapple
Connection” from Hawaii to the California coast. This huge “conveyor belt” of
vapor brings tropical moisture to become the winter rains and snows. Although
exact numbers are not available, the estimate of the total amount of Earth’s water
vapor is 138 cubic kilometers with an average residence time of 3 to 4 days. This
means that there is an unending sun-fired transfer of water into atmospheric vapor
and that it is soon precipitated out.
Hurricanes
Hurricane is the name for an intense and dangerous type of cyclonic storm known
through most of the tropical regions ofEarth. In the Atlantic Ocean and in the
Pacific Ocean east of the International Dateline the storm is called a hurricane.
InthePacificOceanwestoftheInternational Dateline it is known by the name
typhoon although there are local appellations such as baguio (Philippines) and
willy-willy (Australia). The storm is known as a tropical cyclone or, simply,
cyclone in the Indian Ocean and the western South Pacific Ocean. The name
appears to originate from the Spanish spelling of a word “huracan” used by peo-
ples of the Caribbean basin to denote a “god of evil” but is alternatively translated
as “big wind.”
Hurricanes usually start as tropical waves that are mild disturbances in the trade
wind of the tropical latitudes. These tropical waves pass from east to west at speed
averaging around 20 km/hr with wavelengths somewhat over 2,000 km. The waves
are common, with a hundred or so passing over the North Atlantic during its hur-
ricane season. They are areas of disturbed weather containing rain and thunder-
showers and pass over a location in 3 to 4 days. A few of these storms develop
closed isobars whence they are named tropical depressions. When wind speeds
exceed 17 m/s they become tropical storms and when the wind exceeds 34 m/s
they become hurricanes.
Hurricanes can exist for days at a time. Some hurricanes have lasted for upward
of a month, but hurricane force winds usually inhabit the disturbance for an average
of a few days. Hurricanes weaken as they pass over cool water or over land or
encounter strong, shearing winds at high altitude; any of these conditions disrupts
the latent heat supply on which the hurricane feeds. The strengths of tropical dis-
turbances are rated on the Saffir-Simpson scale. The scale ranges from category 1
through category 5. The category definitions are given in Table 2. The wind defini-
tions are based on sustained winds and not the higher wind gusts.
Hurricanes 177