The Atmosphere: Composition and Characteristics

Characteristics & Composition of the Atmosphere

The Earth's atmosphere is an extremely thin sheet of air, made of several layers of gases that envelope the planet. It extends from the surface of the planet up to as far as 10,000 kilometers (6,214 miles) above. After that, the atmosphere blends into space.

The atmosphere is held to the surface of the earth by gravity.

In the picture below, taken from a spacecraft orbiting at 200 miles above the surface, we can see the atmosphere as the thin blue band between the surface and the blackness of space.

Importance of the Atmosphere

The atmosphere is important because of the following reasons:

• It contains oxygen, which is vital for living organisms.
• Ozone found in the atmosphere (which is made up of three oxygen atoms) protects living organisms from harmful UV radiation.

. It has a role in warming the earth and making the temperature tolerable for organisms to live. . The warmth of the earth is credited to the greenhouse gases found in the atmosphere. The greenhouse gases include methane, carbon dioxide, and water vapor. . The gases found in the atmosphere are important for many life processes, such as photosynthesis and respiration. . The atmosphere also plays a key role in the water cycle. The steps of the water cycle, such as precipitation and condensation, take place in the atmosphere.

Characteristics of the Atmosphere

Structure - layers Composition - mixture of gases and dust particles

The Structure (Layers) of the Atmosphere

The atmosphere consists of 5 different layers namely:

• Troposphere

. Stratosphere . Mesosphere

• Thermosphere -> Middle
• Exosphere

ATMOSPHERE OF EARTH EXOSPHERE EXOBASE THERMOSPHERE KARMAN LINE MESOSPHERE STRATOSPHERE OZONE LAYER TROPOSPHERE

Troposphere Layer

The troposphere is the lowest layer of the atmosphere, as it starts from the ground and reaches about 12 kilometers high on average (8km at the poles, 17km at the equator). It is the layer where organisms live. The troposphere contains about 75% of all the air in the atmosphere and almost all of the water vapour which forms cloud and rain, along with dust and ash particles. The photosynthesis and respiration processes depend on the gases found in this layer. In the troposphere, the density of gases as well as temperature decreases when height increases. At the end of the troposphere layer, there is a boundary called tropopause, which separates it from the next layer: the stratosphere.

Stratosphere Layer

The stratosphere reaches from the top of the troposphere, which is called the tropopause, to an altitude of approximately 50 kilometers (30 miles). Here, the water vapour and atmospheric dust diminishes. The air is less dense with lower pressure, and temperatures increase with altitude. A high concentration of ozone, a molecule composed of three atoms of oxygen, makes up the ozone layer of the stratosphere. This ozone absorbs some of the incoming solar radiation, shielding life on Earth from potentially harmful ultraviolet (UV) light, and is responsible for the temperature increase in altitude. At the end of the stratosphere layer, there is a boundary called stratopause which separates it from the next layer: the mesosphere.

Mesosphere Layer

Above the stratopause is the mesosphere, which reaches as far as about 85 kilometers (53 miles) above Earth's surface. Temperatures return to decreasing with altitude in the mesosphere. In fact, the coldest temperatures in the atmosphere are near the top of the mesosphere-about -90℃ (-130°F). The atmosphere is thin here and rarefied (has low pressure) but still thick enough so that meteors burn up as they pass through the mesosphere-creating what we see as "shooting stars." There is also an increase in the percentage of ions (electrically charged particles). The upper boundary of the mesosphere is called the mesopause.

Thermosphere Layer

The thermosphere is located above the mesopause and reaches out to around 600 kilometers (372 miles). Not much is known about the thermosphere except that temperatures increase with altitude continuously. The air is more rarefied (has lower pressure), and solar radiation makes the upper regions of the thermosphere very hot, reaching temperatures as high as 2,000℃ (3,600°F).

Ionization is more intense, resulting to the second name for this layer, ionosphere.

Exosphere Layer

The uppermost layer, that blends with what is considered to be outer space, is the exosphere. The pull of Earth's gravity is so small here that molecules of gas escape into outer space.

Visual Aid - Layers of the Atmosphere

https://youtu.be/KXf39bQH6iE

Composition of the Atmosphere

The air that makes up the atmosphere is for the most part a mixture of gases:

• 78% Nitrogen
• 21% Oxygen
• 0,9% Argon
• 0,04% Carbondioxide
• Other gases (neon, helium, hydrogen, ozone)

This composition remains constant up to the portion at about 100km.

Argon, <1% CO2, 0.04% Other gases, Oxygen, 21% Nitrogen, 78%

Composition of the Atmosphere - Cont'd

The air also contains water vapour that is found concentrated in the lowest layer (troposphere), and which originates mainly from the evaporation of water from oceans and rivers.

In addition, there are also very fine dust particles that come both from the land surface (e.g ashes from volcanic eruptions and wildfires, smooth sand and industrial fumes) and from space. All together they are referred to as atmospheric dust.

Water in the Atmosphere

Earth is often dubbed the "blue planet" due to its plentiful supply of water. Unlike on other bodies in the solar system, liquid water is abundant on earth, and its presence has allowed millions of species to evolve and flourish. About 71% of Earth's surface is covered with water, and a colossal 96.5% of the planet's water supply is found in the oceans.

But water doesn't just stay down below; as a part of the water cycle (also called the hydrologic cycle), it travels up to the atmosphere.

Water in the Atmosphere - Evaporation and Precipitation

Water vapour is present in the air because the heat originating from solar radiation causes the evaporation of water that is found on earth's surfaces; in high parts of seas, and also in lakes, rivers and land. A reasonable amount of it also comes from transpiration of plants. Through evapotranspiration, the water passes through the atmosphere to return to the earth's surface through meteorological precipitations (rain, snow, hail) Water evaporates from the surface of the ocean. When it reaches higher altitudes where the air is cooler, it condenses back into water drops which you can see as clouds. When it cools still further, the drops join and when they become too heavy they fall as rain.

The Hydrologic Cycle

Precipitation 3 Infiltration Snowmelt and Runoff Condensation Evaporation Evapotranspiration Plant Uptake

On a global scale, the amount of water evaporating is about the same as the amount of water delivered to the Earth as precipitation. This does vary geographically, though. Evaporation is more prevalent over the oceans than precipitation, while over the land, precipitation routinely exceeds evaporation.