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CHAPTER 13
Page 326
The purpose of this chapter is to describe Earth’s climate and explore the ways
that human activities may be causing climate change. Two serious issues will be
explored: the hole in the stratospheric ozone layer and global warming, the rise
in average global temperature.
Hurricane Fran caused extensive damage in the United States in early
September 1996. The sixth named storm, fifth hurricane, and fourth major
hurricane of the 1996 Atlantic hurricane season, Fran developed from
tropical near Cape Verde on August 23. Due to nearby Hurricane Edouard, the
depression remained disorganized as it tracked westward, though it eventually
intensified into Tropical Storm Fran on August 27. While heading westnorthwestward, Fran steadily strengthened into a hurricane on August 29, but
weakened back to a tropical storm on the following day. On August 31, Fran
quickly re-intensified into a hurricane. By September 2, Fran began to parallel
the islands of the Bahamas and slowly curved north-northwestward. Fran
peaked as a 120 mph (195 km/h) Category 3 hurricane by early on September 5.
Thereafter, Fran weakened slightly, before it made landfall near Cape
Fear, North Carolina early on September 6. The storm rapidly weakened inland
and was only a tropical depression later that day. Eventually, Fran curved eastnortheastward and transitioned into an extratropical cyclone over Ontario early
on September 9.
In the following picture the clouds are rotating (counter-clockwise, typical in the
Northern Hemisphere). The circular opening in the cloud layer at the center of
the storm is a relatively calm spot known as the eye. Hurricanes form in the
trade wind belt as the result of colliding air masses of different temperatures.
Weather is the state of the atmosphere at a particular place at a
particular moment.
Climate is the long-term prevailing weather conditions at a particular
place based upon records taken.
Factors determining climate:
1.
2.
3.
4.
5.
6.
Latitude
Atmospheric circulation patterns
Oceanic circulation patterns
Local geography
Solar activity
Volcanic activity
The most important is the distance from the equator.
Below are different climates based on the location to the equator.
The first is the island of St. John in the Virgin Islands. The second is
of the Antarctic Peninsula. Why are these so different.
Latitude is the distance from the equator measured in degrees north
or south of the equator. The equator is located at 0 degrees latitude.
The most northerly latitude is the North Pole at 90 degrees north,
where as the most southerly latitude is the South Pole at 90 degrees
south.
Low Latitudes: More solar energy falls on areas near the equator
than on areas closer to the po9les. In regions near the equator night
and day are both about 12 hours long through the year.
High Latitudes: Regions closer to the poles have high latitudes
because the sun is lower in the sky.
Atmospheric Circulation: Cold air sinks because it is denser than
warm ar. As cold air sinks it compresses and warms. Second warm
air rises; it expands and cools as it rises. Third warm air can ho9ld
ore water vapor than cold air can; therefore, when warm air cools,
the water vapor it contains may condense into liquid water to form
rain, snow or fog.
Tornadoes occur almost exclusively in the United States and
southern
Canada. Cold, dry air from the north and warm, moist air from the
south often collide on the flat region of the Great Plains. When this
happens a low pressure area is formed and air masses spiral around
it, creating funnel clouds.
Global circulation patterns
Over the major parts of the Earth's surface there are large-scale
wind circulations present. The global circulation can be described as
the world-wide system of winds by which the necessary transport of
heat from tropical to polar latitudes is accomplished.
In each hemisphere there are three cells (Hadley cell, Ferrel cell and
Polar cell) in which air circulates through the entire depth of the
troposphere. The troposphere is the name given to the vertical
extent of the atmosphere from the surface, right up to between 10
and 15 km high. It is the part of the atmosphere where most of the
weather takes place.
Hadley cell
The largest cells extend from the equator to between 30 and 40
degrees north and south, and are named Hadley cells, after English
meteorologist George Hadley.
Within the Hadley cells, the trade winds blow towards the equator,
then ascend near the equator as a broken line of thunderstorms,
which forms the Inter-Tropical-Convergence Zone (ITCZ). From the
tops of these storms, the air flows towards higher latitudes, where it
sinks to produce high-pressure regions over the subtropical oceans
and the world's hot deserts, such as the Sahara desert in North
Africa.
Ferrel cell
In the middle cells, which are known as the Ferrel cells, air
converges at low altitudes to ascend along the boundaries between
cool polar air and the warm subtropical air that generally occurs
between 60 and 70 degrees north and south. This often occurs
around the latitude of the UK which gives us our unsettled weather.
The circulation within the Ferrel cell is complicated by a return flow
of air at high altitudes towards the tropics, where it joins sinking air
from the Hadley cell.
The Ferrel cell moves in the opposite direction to the two other cells
(Hadley cell and Polar cell) and acts rather like a gear. In this cell
the surface wind would flow from a southerly direction in the
northern hemisphere. However, the spin of the Earth induces an
apparent motion to the right in the northern hemisphere and left in
the southern hemisphere.
Polar cell
The smallest and weakest cells are the Polar cells, which extend
from between 60 and 70 degrees north and south, to the poles. Air in
these cells sinks over the highest latitudes and flows out towards the
lower latitudes at the surface.
The circulation of wind in the atmosphere is driven by the rotation of
the earth and the incoming energy from the sun. Wind circulates in
each hemisphere in three distinct cells which help transport energy.
The circulation cell closest to the equator is called the Hadley cell.
Winds are light at the equator because of the weak horizontal
pressure gradients located there. The warm surface conditions
result in locally low pressure. The warm air rises at the equator
producing clouds and causing instability in the atmosphere. This
instability causes thunderstorms to develop and release large
amounts of latent heat. Latent heat is just energy released by the
storms due to changes from water vapor to liquid water droplets as
the vapor condenses in the clouds, causing the surrounding air to
become more warm and moist, which essentially provides the energy
to drive the Hadley cell.ort energy and heat from the equator to the
poles. The winds are driven by the energy from the sun at the
surface as warm air rises and colder air sinks. The Hadley Cell
encompasses latitudes from the equator to about 30°. At this latitude
surface high pressure causes the air near the ground to diverge.
This forces air to come down from aloft to "fill in" for the air that is
diverging away from the surface high pressure. The air flowing
northward from the equator high up in the atmosphere is warm and
moist compared to the air nearer the poles. This causes a strong
temperature gradient between the two different air masses and a jet
stream results. At the 30° latitudes, this jet is known as the
subtropical jet stream which flows from west to east in both the
Northern and Southern Hemispheres. Clear skies generally prevail
throughout the surface high pressure, which is where many of the
deserts are located in the world.
Changes in the Hadley cell and Walker circulation can result in
dramatic climate variations for many regions. In an El Niño winter,
for example, the presence of the warm water in the eastern Pacific
shifts the position of the subtropical jet, leading to heavy rainfall in
Florida and southern Georgia. You can learn more about how El Niño
and its opposite, La Niña, affects the weather in the Southeast which
allows you to look at differences in climate in different years
depending on the El Niño phase. In a warming climate, the Hadley
cell could increase in length and alter the climate of regions around
30°. For example, many deserts in the northern hemisphere are
located around the 30° latitude, and if the Hadley cell were to
increase in length, that could cause dry conditions to move north of
30°. Ultimately, this would alter the precipitation patterns of many
regions, including the Southeast.
1. Think of a place on Earth where you would like to live or visit
some day. Write a description of the climate of your chosen
location and list various factors that might influence the climate
there.
2. Think about the reasons you chose this particular factors as
contribution to the climate.