2. Composition and Structure of the Atmosphere

2.1 Composition of the Atmosphere

The present gaseous composition of the Earth's atmosphere is the result of various processes over the Earth's 4.6 billion years of existence. Most of the gases in the atmosphere were created in the first few 100 million years of Earth history. Gases like carbon dioxide, nitrogen, and water vapor were released from the Earth's molten surface as it cooled to form the solid outer crust. The oxygen in our atmosphere is primarily the result of the development and expansion of life some 3 to 2 billion years ago.

The idea that the atmosphere contained a number of different gases began with the work of 18th century chemists. One of the first gases to be discovered in the Earth's atmosphere was carbon dioxide in 1752. Over time other atmospheric gases were isolated and studied. By 1894, the last commonly occurring atmospheric gas, Argon, was discovered.

As displayed in Table 1.1 below, nitrogen and oxygen make up most of the dry atmosphere. The remaining gases listed constitute less that 1% of the remaining atmosphere. Water vapor content in the atmosphere varies from 0 to 4% by volume near the Earth's surface. Areas of maximum water vapor near the Earth's surface include the tropics, areas over warm oceanic currents, and locations on the western side of continents in the mid-latitudes. Areas of minimum water vapor near the Earth's surface include continental deserts and the cold polar regions.

Table 1.1 : Average composition of the lower atmosphere excluding water vapour.

2.2 Structure of the Atmosphere

The Earth's atmosphere contains several different layers that can be defined according to air temperature, Figure 1.1 displays these layers in an average atmosphere.

According to temperature, the atmosphere contains four different layers (Figure 1.1). The first layer is called the troposphere. The depth of this layer varies from about 8 to 16 kilometers. Greatest depths occur at the tropics where warm temperatures causes vertical expansion of the lower atmosphere. From the tropics to the Earth's polar regions the troposphere becomes gradually thinner. The depth of this layer at the poles is roughly half as thick when compared to the tropics. Average depth of the troposphere is approximately 11 kilometers as displayed in Figure 1.1 .

About 80% of the total mass of the atmosphere is contained in troposphere. It is also the layer where the majority of our weather occurs. Maximum air temperature also occurs near the Earth's surface in this layer. With increasing height, air temperature drops uniformly with altitude at a rate of approximately 6.5° Celsius per 1000 meters. This phenomenon is commonly called the Environmental Lapse Rate. At an average temperature of -56.5° Celsius, the top of the troposphere is reached. At the upper edge of the troposphere is a narrow transition zone known as the tropopause.

Above the tropopause is the stratosphere. This layer extends from an average altitude of 11 to 50 kilometers above the Earth's surface. This stratosphere contains about 19.9% of the total mass found in the atmosphere. Very little weather occurs in the stratosphere. Occasionally, the top portions of thunderstorms breach this layer. The lower portion of the stratosphere is also influenced by the polar jet stream and subtropical jet stream. In the first 9 kilometers of the stratosphere, temperature remains constant with height. A zone with constant temperature in the atmosphere is called an isothermal layer. From an altitude of 20 to 50 kilometers, temperature increases with an increase in altitude. The higher temperatures found in this region of the stratosphere occurs because of a localized concentration of ozone gas molecules. These molecules absorb ultraviolet sunlight creating heat energy that warms the stratosphere. Ozone is primarily found in the atmosphere at varying concentrations between the altitudes of 10 to 50 kilometers. This layer of ozone is also called the ozone layer. The ozone layer is important to organisms at the Earth's surface as it protects them from the harmful effects of the sun's ultraviolet radiation. Without the ozone layer life could not exist on the Earth's surface.

Separating the mesosphere from the stratosphere is transition zone called the stratopause. In the mesosphere, the atmosphere reaches its coldest temperatures (about -90° Celsius) at a height of approximately 80 kilometers. At the top of the mesosphere is another transition zone known as the mesopause.

The last atmospheric layer, as defined by vertical temperature change, has an altitude greater than 80 kilometers, and is called the thermosphere. The thermosphere is the hottest layer in the atmosphere. Heat is generated from the absorption of solar radiation by oxygen molecules. Temperatures in this layer can reach 1300 to 1800° Celsius.

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