Transcript atmosphere

Chapter One
Composition and
Structure of the Atmosphere
The atmosphere is a mixture of gas molecules,
microscopically small suspended particles of
solid and liquid, and falling precipitation.
Meteorology is the study of the
atmosphere and the processes
that cause what we refer to as the “weather.”
If we think of the atmosphere as a reservoir for gas,
the gas concentration in the reservoir will remain constant
so long as the input rate is equal to the output rate.
Under such conditions, we say that the concentration of
the gas exists in a steady state.
The average length of time that individual molecules
of a given substance remain in the atmosphere
is called the residence time.
The residence time is found by dividing the mass
of the substance in the atmosphere (in kilograms)
by the rate at which the substance enters and exits
the atmosphere (in kilograms per year).
Atmospheric gases are often categorized as
being permanent or variable,
depending on whether their concentration is stable.
Permanent gases are those that form a
constant proportion of the atmospheric mass.
Permanent Gases of the Atmosphere
Permanent gases account for the greater part of
the atmospheric mass—99.999 percent—and occur
in a constant proportion throughout the
atmosphere’s lowest 80 km (50 mi).
Because of its chemical homogeneity,
this region within 80 km of Earth’s surface
is called the homosphere.
Above the homosphere is the heterosphere,
where lighter gases (such as hydrogen and helium)
become increasingly dominant with increasing altitude.
Because its composition varies with altitude,
the heterosphere contains no truly permanent gases.
Variable gases are those whose distribution
in the atmosphere varies in both time and space.
The most abundant of the variable gases, water vapor,
occupies about one-quarter of 1 percent
of the total mass of the atmosphere.
Most atmospheric water vapor is found
in the lowest 5 km (3 mi) of the atmosphere.
Variable Gases of the Atmosphere
Water is constantly being cycled between the planet
and the atmosphere through the hydrologic cycle.
Water continuously evaporates from both open water
and plant leaves into the atmosphere, where it eventually
condenses to form liquid droplets and ice crystals.
These liquid and solid particles are removed from the
atmosphere by precipitation as rain, snow, sleet, or hail.
Another important variable gas is carbon dioxide (.037%).
Increases in the carbon dioxide content of the atmosphere
may have some important climatic consequences
that could greatly affect human societies.
Carbon dioxide is removed from the atmosphere
by photosynthesis, the process by which
green plants convert light energy to
chemical energy (Box 1-1).
Since the 1950s, the concentration of carbon dioxide
has increased at a rate of about 1.8 ppm per year.
The increase has occurred mainly because of
anthropogenic combustion and deforestation
of large tracts of woodland.
Carbon dioxide increase since the 1950s
Small solid particles and liquid droplets in the air
(excluding cloud droplets and precipitation) are
collectively known as aerosols (Box 1-3).
Aerosols play a major role in the formation
of cloud droplets because virtually all
cloud droplets that form in nature do so on
suspended aerosols called condensation nuclei.
The density of any substance is the amount of mass
of the substance contained in a unit of volume.
At lower altitudes, there is more overlying
atmospheric mass than is the case higher up.
Because air is compressible and subjected to greater
compression at lower elevations, the density of the air
at lower levels is greater than that aloft.
Meteorologists find it convenient to divide the atmosphere
vertically into several distinct layers. Some layers are
distinguished by electrical characteristics,
some by chemical composition,
and some by temperature characteristics.
Together with the change in density with height,
this layering of the atmosphere gives it its structure.
Scientists divide the atmosphere into four layers
based not on chemical composition but rather on
how mean temperature varies with altitude.
The average temperature profile, called the
standard atmosphere, shows the four layers:
troposphere, stratosphere,
mesosphere, and thermosphere.
Temperature profile of the atmosphere
The troposphere is the lowest of the
four temperature layers. The troposphere
is where the vast majority of weather events
occur and is marked by a general pattern
in which temperature decreases with height.
At the top of the troposphere, a transition zone
called the tropopause marks the level at which
temperature ceases to decrease with height.
Despite the strong tendency for temperature
to decrease with altitude in the troposphere,
it is not uncommon for the reverse situation
to occur. Such situations, where temperature
increases with height, are known as inversions.
Above the tropopause is the stratosphere.
Little weather occurs in this region.
In the lowest part of the stratosphere,
the temperature remains relatively constant
up to a height of about 20 km (12 mi).
From there to the top of the stratosphere
(called the stratopause), the temperature
increases with altitude. In the upper stratosphere,
heating is almost exclusively the result of
ultraviolet radiation being absorbed by ozone.
Ozone is the form of oxygen in which three O atoms
are joined to form a single molecule.
The small amount of it that exists in the
the stratosphere is absolutely essential to life on Earth
because it absorbs lethal ultraviolet radiation from the sun.
Near Earth’s surface it is a major component
of air pollution, causing irritation to lungs and eyes
and damage to vegetation (Box 1-2).
The red areas reveal the “ozone hole” over Antarctica
Of the 0.1 percent of the atmosphere not contained
in the troposphere and stratosphere,
99.9 percent exists in the mesosphere
which extends to a height of about 80 km (50 mi).
Temperature in the mesosphere decreases with altitude.
Above the mesosphere is the thermosphere,
where temperature increases with altitude
to values in excess of 1,500 C. The temperature
of the air is an expression of its kinetic energy,
which is related to the speed at which its molecules move.
An additional layer, called the
ionosphere, can be defined based
on its electrical properties.
This layer, which extends from the
upper mesosphere into the
thermosphere, contains large
numbers of electrically charged
particles called ions.
The ionosphere is important for
reflecting AM radio waves back
toward Earth and is responsible for
the aurora borealis
and the aurora australis.
It is generally believed that Earth was
formed perhaps 4.5 billion years ago.
If an atmosphere formed with Earth,
it must have consisted of the gases
most abundant in the early solar system
including large amounts of hydrogen and helium,
the two lightest elements. If molecules move with
sufficient speed, known as their escape velocity,
they can overcome gravity and leave the atmosphere.
Light gases are more likely to achieve escape velocity;
thus, the hydrogen and helium were most readily lost.
Over time a new, secondary atmosphere formed,
made up of gases released from Earth’s interior
by volcanic eruptions—a process called outgassing.
The gases spewed out during volcanic events are
predominantly water vapor and carbon dioxide,
with lesser amounts of sulfur dioxide,
nitrogen, and other gases.
The transformation to an atmosphere high in oxygen
depended on the advent of primitive, anaerobic bacteria
about 3.5 billion years ago. These primitive life-forms
were the first in a long line of organisms that removed
carbon dioxide from the air and replaced it with oxygen.
Ultimately, plant and later animal material sank to
the ocean floor, where the organic carbon
was locked away in sediments.
Atmospheric pressure is one of the most fundamental
of weather characteristics. Air tends to blow away from
regions of high pressure toward areas of lower pressure.
The horizontal variation in air pressure generates winds.
Air tends to rise in areas of low surface pressure
and sink in zones of high pressure.
Rising motions favor the formation of clouds,
while sinking motions promote clear skies.
Atmospheric pressure is routinely plotted on maps by the
use of lines called isobars. Each isobar connects points
having equal air pressure with the pressure being
expressed in units of millibars (mb) in the United States
and kilopascals (kPa) in Canada.
A surface weather map
Information regarding wind
speed and direction can be
obtained on weather maps
by looking at the
station models,
which contain symbols
and numbers giving
weather information for
particular locations.
Station model symbols
Temperature is one of the most obvious
weather components and varies from
place to place systematically. Major changes
in temperature often occur due to the
presence of fronts, fairly narrow
boundary zones separating
relatively warm and cold air.
Cold fronts are shown as a
blue line with triangles
while warm fronts are depicted by
a red line with semicircles.
Relative humidity is one of several ways of expressing the
amount of water vapor in the air. It indicates the amount
of water vapor present relative to the maximum possible;
thus, it is usually reported as a percentage. Another index
called the dew point temperature is often preferred.
The higher the dew point, the greater the amount of
water vapor in the air.
Weather forecasters routinely employ
state-of-the-art computer hardware and software systems
that perform millions of calculations, based on input data
and displayed at work stations employing the
Advanced Weather Interactive Processing System
(AWIPS), which allows forecasters to display maps of
current weather conditions, computer models,
satellite and radar images, as well as forecast
advisories and discussions from other weather facilities.
AWIPS graphical display monitors
The next chapter examines
solar radiation and the seasons.