Composition and Structure of the Atmosphere

Weather is the state of the atmosphere at a given time and place. A complete description of the weather includes the amount and type of clouds. Rain, snow, thunderstorms, lightning, and even dust storms are part of the weather. Measurements of temperature, pressure, wind speed and direction, and the amount of moisture in the air are also included in a description of the weather. Weather is studied and predicted by scientists called meteorologists.

The science of meteorology is the study of the entire atmosphere, including its weather.To understand and predict the weather, meteorologists must first understand how the atmosphere heats and cools, how clouds form and produce rain, and what makes the wind blow. Meteorologists also study subjects not obviously related to weather. Such subjects include the composition of the atmosphere, the atmospheres of other planets, and the causes of past and present climates.

Observing the Weather Much can be learned about the weather without numerous instruments.Direct observation tells whether it is cloudy or raining. Wind direction and speed can be estimated with little practice. It is easy to tell whether the air is warm or cold. Even humidity and pressure have observable effects.

Certain clouds come with fair weather and other clouds foretell rain. Farmers, sailors, and others dependent on weather become quite skilled at predicting weather from watching the clouds. For example, in the verse Mackerel scales and mares’ tailsThe temperature cannot be guessed accurately, but people feel heat and cold. In the winter, people feel colder when the wind is blowing. The actual temperature can be converted into the temperature the body feels by using a chart for the windchill factor. This adjusted temperature describes the danger of frostbite.

In the winter, weather broadcasts often include temperatures adjusted for windchill. Air pressure cannot be felt, but changes in the air pressure can. The most common effect of pressure change is the “popping” of the ears.Ear popping occurs ding the takeoff or landing of an airplane, a ride in a rapid elevator, or even a ride in a car over a high mountain pass. In all cases, the ears pop because the air pressure falls with height.

To try to predict weather, you need to observe the clouds, wind, temperature, humidity, air pressure, and precipitation over a period of time. Look for patterns in how these factors change relative to one another. For example, is one sequence of events usually followed by rain? Composition of the Atmosphere Earth’s lower atmosphere is a mixture of many gases called air. The two main gases in air are nitrogen and oxygen.Together, they form about 99 percent to dry air by volume. The remaining 1 percent is mostly argon and carbon dioxide.

The atmosphere also contains tiny amounts of helium, hydrogen, neon, ozone, krypton, and other gases. * Nitrogen- 78% * Oxygen- 21% * Argon- almost 1% * Carbon Dioxide- . 03% * All Other Gases- . 01% The air this out quickly at altitudes high above Earth’s surface. Its composition (by percent) remains the same, however, to an altitude of about 80 kilometers. Above this level the air is so thin that it would be considered a vacuum at sea level.

Also, above this level the atmosphere changes to layers of different gases.A layer of oxygen reaches to about 1000 kilometers. Above it is a layer of helium to about 2400 kilometers. Above this is a layer of hydrogen that thins out into space.

Gas molecules at the bottom of the atmosphere are squeezed together by the gases above them. As a result, 99 percent of the atmosphere’s weight is found within about 32 kilometers of Earth’s surface. Half the atmosphere’s weight is within 5. 5 kilometers. Air always contains some water vapor.

Water vapor enters the air by evaporation from the oceans and from water or plants on land. The amount of water vapor varies with location, season, and time of day.Most of the water vapor is near the surface, and the percentage decreases with height. Air also contains ozone, which is a form of oxygen fast that contains 3 oxygen atoms, instead of the normal 2.

Ozone forms when ultraviolet rays from the sun act on oxygen in the upper atmosphere. It is concentrated at heights of about 10 to 50 kilometers in a region called the ozone layer. Ozone is important because it absorbs 99 percent of the harmful ultraviolet rays. If the atmosphere has less ozone, more ultraviolet rays reach Earth’s surface, causing more sunburns, skin cancer, and plant damage.Ozone thinning results from the release of gases called chlorofluorocarbons (CFC’s) into the atmosphere.

These gases contain chlorine, fluorine, and carbon. CFC’s are sued as coolants in air conditioners, to clean electronic components, and in making foam products. The chlorine atoms from the CFC’s break down ozone in the presence of sunlight. Ozone measurements since the late 1970’s show a hole in the ozone layer over Antarctica.

From the later 1970’s to the early 1990’s, the hold grew larger and more intense. In 1990, the ozone decreased by 50 percent.Extreme thinning of the ozone layer in Antarctica results from the isolated climate and extremely cold winters and early springs. The cold allows clouds to form in the atmosphere containing the ozone layer. Ice particles on these clouds provide places for ozone destruction. The ozone hole is filled when warmer winds from the north mix in ozone-rich air.

A similar hole occurs over the Arctic. Winter and spring ozone values over the northern middle latitudes decreased 6 to 8 percent from 1979 to 1990. Smaller decreases have occurred at lower latitudes.Dust, another part of air, includes tiny grains of rock, dirt, pollen, salt crystals from sea spray, soot from fires, chemicals from factories, and bacteria.

Dust helps form fog and rain. Water vapor condenses around some dust grains, forming tiny water droplets. Structure of the Atmosphere Scientists divide the atmosphere into four layers that are based on temperature changes. The layer closest to the earth is called the troposphere. The troposphere starts at Earth’s surface.

Its thickness depends on the latitude. At the equator the troposphere is about 18 kilometers thick; at the poles it is only about 8 kilometers thick.The gases of the troposphere are essential to life on Earth. Earth’s weather occurs in the troposphere.

Temperatures gradually decrease with altitude in the troposphere. The top of the troposphere is called the tropopause. There, the decrease in temperature stops. At the poles, the tropopause temperature is about -55 degrees Celsius. The second layer is the stratosphere. It reaches from the tropopause to a height of about 50 kilometers from Earth.

The stratosphere is clear and dry. It has strong, steady winds and few weather changes. Because of its steady weather conditions, jet aircraft fly in the stratosphere.The lower part of the stratosphere is as cold as the tropopause.

Then it warms up steadily to its top, or stratopause. The absorbing or absorption of sunlight by ozone is what makes the stratosphere’s temperatures increase with height. The ozonosphere is therefore located in the area of the stratosphere. The third and fourth layers are the mesosphere, in which temperatures drop again, and the thermosphere, in which temperatures rise again. The top of the thermosphere is around 500 kilometers from Earth. In the thermosphere, nitrogen and oxygen atoms absorb solar energy, causing the temperature to rise.

At heights between about 65 and 500 kilometers above Earth, the air is highly ionized. The ions are formed when ultraviolet rays from the sun knock electrons off oxygen atoms. This part to the atmosphere is called the ionosphere. It stretches from the lower mesosphere to the top of the thermosphere. The ions and electrons are concentrated in layers at four different levels. Each layer reflects radio waves of different wavelengths.

The ionosphere therefore reflects radio waves back to Earth, which greatly increases the area in which they can be received.The ionosphere does not reflect the waves used to transmit television. These waves, however, can be picked up and rebroadcast by special satellites orbiting high above Earth. The ionosphere is affected by solar events. Eruptions on the sun send out large amounts of short-wave radiation, which disrupts radio communication. The solar eruptions also send out ionized particles.

Since they are electrically charged, these particles are deflected by Earth’s magnetic field to the North and South Poles. At the poles, the ionized particles interact with air molecules to form auroras, colored displays of light in the nighttime sky.