Storms and Heating the Earth’s Surface
Natural catastrophic events—earthquakes, volcanoes, and intense storms such as
hurricanes and tornadoes—are powerful and often dramatic forces that can
profoundly affect our planet and the living things that inhabit it. An individual
catastrophic event, such as a volcanic eruption, may seem like an isolated occurrence,
but in fact, the cumulative and interrelated effects of such events over billions of
years have helped shape the planet as we know it. These same forces will continue to
alter the earth in the future.
Storms
A thunderstorm is a disturbance
in the earth’s atmosphere that involves
lightning and thunder. Sometimes gusty
surface winds, heavy rain, and hail are
also present. A thunderstorm may form
in a single cloud, called a cumulonimbus
cloud, a cluster of clouds, or a line of clouds that extends in some cases for more than
100km. The birth of a thunderstorm occurs when the unequal heating of the earth’s
surface causes warm, humid air to rise, creating an unstable environment in the
atmosphere. This often happens in late afternoon or early evening, when ground
temperatures are high. Warm air also rises when it encounters high terrain
(mountains) or when masses of cold and warm air collide. These mechanisms can
interact to generate severe thunderstorms and sometimes tornadoes.
A severe thunderstorm can produce large hail (2cm in diameter or larger),
strong winds, flash floods, and tornadoes, as well as lightning and thunder.
Cumulonimbus clouds in a severe thunderstorm can often spawn a rapidly whirling,
funnel-shaped vortex. A vortex is the movement of a liquid or gas in a spiral around a
central axis. The whirling motion of the cloud vortex results from a rapid downdraft
of cold air replacing rising, hot air. Meteorologists have shown that the rotating
vortex begins near the middle height of the thundercloud and gradually works
downward.
When the rapidly rotating cloud becomes visible as a funnel at the base of the
thunder-cloud, scientists call it a funnel cloud. As air rushes in toward the low
pressure of the spinning funnel, it accelerates upward. The funnel cloud stretches
vertically and shrinks horizontally, and the rotational wind speed increases up to
350km per hour. Some meteorologists estimate that rotational wind speeds could
reach 480-500km per hour.
When the funnel
cloud’s circulation comes in
contact with the ground, it
becomes a tornado. A
tornado is a violent
windstorm that spirals
around a rotating column of
air (the vortex) of intense
low pressure and moves in a
narrow path over the land.
Also called a twister, a
tornado can a have a variety
of appearances, including that of a twisting ropelike funnel, a cylindrical funnel, or a
massive black funnel. Small vortices may develop within tornadoes.
The devastation a tornado can cause results from the extremely low pressure of
the tornado in contrast to the higher air pressure in the surrounding area and the
very rapid rate at which the tornado moves. The higher-pressure air outside the
tornado rushes into the funnel from all directions. Much of the damage is caused by
violent winds and flying debris. Depending on the strength of the tornado, such
things as water, debris, buildings, animals, and automobiles can push into the funnel
and be carried for distances.
A hurricane is a massive, rotating storm that originates over tropical oceans
and has sustained winds of more than 119km per hour. Like a tornado, a hurricane
has an interior region of intense low pressure. However, it can affect a much larger
region than a tornado, growing on average to a diameter of 550km and lasting for
days or even weeks. Hurricanes usually develop north of the equator in the Atlantic
and eastern Pacific Oceans in summer and early fall when the Northern Hemisphere
tilts toward the sun. “Hurricane Season” lasts from June through November.
People have different names for this kind of storm in different regions of the
world. For example, when the storm forms north of the equator in the western
Pacific Ocean, it is called a typhoon. When it forms in the Indian Ocean or off the
coast of Australia, it is called a cyclone.
The center of a hurricane is called the eye of the hurricane. Inside the eye, one
would experience the following: rising temperatures, low winds, no rainfall, low
pressure, and a bright sky with a
view of middle and high clouds
above.
Adjacent to the eye of a
hurricane is the eye wall, a ring of
spiraling clouds and thunderstorms
that whirl around the storm’s center
and extend upward to almost 15km
above sea level. Within the eye wall
are the heaviest precipitation and
the strongest winds. These winds,
accompanied by heavy rain, can
generate ocean waves over 10 meters high.
Weather Information Systems
How do people find out that a thunderstorm, tornado, hurricane, or other
severe weather is threatening? The National Weather Service collects data from
different sources. These sources include satellites, Doppler weather radar, and air
craft. Special aircraft, for example, fly into hurricanes. They send the data to weather
centers, such as the National Hurricane Center in Miami, Florida and other offices.
The centers and offices send the data, watches, and warnings to weather reporters,
weather information companies, and research centers, which transmit the
information directly to the public on radio and television and over the internet.
Besides explaining what weather is occurring, weather reporters advise of
precautions to take when storms are about to occur or are in progress.
Several weather information systems exist that can provide data and warnings
to people in the event of tornadoes, hurricanes, and other weather-related events.
The National Oceanic and Atmospheric Administration (NOAA) produces broadcasts
for the national weather alerting system 24 hours a day. Severe watches and
warnings, times that the warnings are in effect, areas affected by the warnings, and
tips on what to do during a weather emergency are often transmitted. Alerts are
automatically activated for emergency broadcasts.
Heating Earth’s Surfaces
The transfer of heat energy is a major factor in the formation of weather. For
example, the movement of heat in the atmosphere causes temperature to change,
winds to blow, storms to develop, and rain to fall. Where does all this heat energy in
the atmosphere come from? Almost all of it originates as energy from the sun, our
solar energy, which travels through space in the form of electromagnetic waves. The
direct transfer of heat energy by electromagnetic waves is called radiation.
Radiation transfers heat from one object to another without the space between the
objects necessarily being heated. Heat can also be transferred by conduction and
convection.
Transfer of heat by Radiation
Solar energy is made up of visible light (43%) and infrared radiation, or heat
(49%), plus a small amount of ultraviolet radiation (7%). Gamma rays, x-rays,
microwaves, TV waves, and radio waves make
up the remaining 1%. As the sun’s rays pass
through the earth’s atmosphere, dust, clouds,
and the earth’s surfaces absorb some of the
energy from these electromagnetic waves and
reflect some back into the atmosphere. In fact,
most of the radiation from the sun does not
reach the earth at all, but passes on out into
space. Of the small amount that does reach the earth, about half of it is absorbed by
land and water. The rest of it is reflected back into space or absorbed by the
atmosphere.
All objects (soil, sand, rocks, vegetation, water, snow, & ice) absorb solar
energy and convert it to heat, which increases the internal temperature of these
objects. Earth surfaces cool as heat is given off, or radiated, from the earth to the
surrounding air. The radiated energy is absorbed mainly by greenhouse gases
(water vapor & carbon dioxide) and clouds in the atmosphere. Much of the energy is
re-radiated back to the earth. This is called the greenhouse effect. If energy were
not absorbed and radiated, the overall surface temperature of the earth would not
remain in equilibrium.
Not all surfaces are equally effective in absorbing or giving off radiation. How
well something absorbs and radiates energy depend on its characteristics, such as
composition, color, texture, moisture, temperature, and specific heat. A black road is
a good absorber of visible radiation. The road material converts energy from the sun
into heat, and the temperature of the road increases. It will also cool quickly by
radiating some of the heat back into the atmosphere at night. In contrast, a surface
that is white, shiny, or smooth (snow or ice) reflects a great deal of sunlight and
absorbs very little.
Modeling Heat Transfer by Radiation
During the investigation “Heating and Cooling”,
you used a lamp to heat equal volumes of soil and
water. An ordinary light bulb is a source of radiant
energy. This energy transferred from the lamp to the
beakers through the process of radiation. The glass
beakers and the materials within them absorb the
lamp’s radiant energy and become warm.
In examining the temperature data, you
observed that the soil’s temperature rises quickly
when it is under the lamp for 10 minutes, while the
water’s temperature rises gradually. However, once
you removed the heat source, the water stayed at its
final temperature for most of the 10-minute cooling
period, while the soil loses its heat energy quickly. In
applying this information to the earth, you can
recognize that land heats quickly during the day and loses the heat energy at night,
while water heats slowly during the day and holds the heat at night. (This is
particularly true in the summer months.)
Modeling Heat Transfer by Conduction and Convection
The process by which heat energy transfers from one material to another
through direct contact is called conduction. In the investigation, energy was
transferred from the beakers of soil and water to the thermometers by conduction.
During convection, heat energy transfers as a result of the circulating motion of fluid,
usually a gas or a liquid. Convection currents in gases and liquids are often caused by
variations in density due to temperature changes.