Understanding Earth’s Atmosphere

Understanding Earth’s Atmosphere

Overview

The envelope of air that surrounds the Earth is known as the atmosphere.

• The atmosphere extends to about 1000 km from the surface of the Earth. But 99% of the total mass of the atmosphere is found within 32 km.

• This is because the atmosphere is held by the gravitational pull of the Earth.

Composition of the Atmosphere

• Carbon dioxide is present in small quantities in the atmosphere.

• It is an important constituent of air because it can absorb heat and thus keep the atmosphere warm, thereby balancing the heat of the Earth.

Water Vapour

Water vapour is the most significant component of the atmosphere as far as its effect on weather is concerned, although its quantity varies considerably from practically none (0) to up to about 4% by volume.

• Water vapour is the source of all clouds and precipitation (rain, hailstorm, etc).

• Water vapour, like carbon dioxide, can absorb heat energy.

• It also regulates the hydrological cycle.

Dust Particles

Dust intercepts and reflects incoming insolation.

• The polluted particles present in the air not only absorb a larger amount of insolation but also greatly absorb the terrestrial radiation.

• Dust in the atmosphere contributes to the red and orange colour of sunrise and sunset.

Layers of the Atmosphere

There are five distinct layers of the atmosphere:

A. Troposphere

B. Stratosphere

C. Mesosphere

D. Thermosphere

E. Exosphere

Troposphere

• This is the first layer of the atmosphere.

• According to Encyclopedia Britannica, it extends to a height of 18 km at the equator and 8 km at the poles.

• In this layer, temperature decreases with height.

• This is because the density of air decreases with height, and so the heat absorbed is less.

• It contains more than 90% of the gases in the atmosphere.

• Since most of the water vapour forms clouds in this layer, all weather changes occur in the troposphere ('tropo' means 'change').

• The height at which the temperature stops decreasing is called the tropopause.

• Here, the temperature may be as low as -58°C.

Stratosphere

• This is the second layer of the atmosphere.

• It extends from the tropopause to about 50 km.

• Temperature increases due to the absorption of ultraviolet radiation from the Sun by ozone present in this layer.

• The temperature slowly increases to 4°C.

• This layer is free from clouds and associated weather phenomena. Hence, it provides ideal flying conditions for large jet planes.

• At about 50 km, the temperature begins to fall again. This marks the end of the stratosphere, called the stratopause.

Mesosphere

• Above the stratosphere lies the mesosphere.

• The mesosphere extends to a height of 80 km.

• Here, the temperature decreases again, falling as low as -90°C.

• The end of this layer is known as the mesopause.

Thermosphere

• The thermosphere lies above the mesosphere.

• This layer, according to NASA, extends to a height of about 513 km.

• In this layer, temperature rises dramatically, reaching up to 4500°F or 2482.22°C.

• This increase in temperature is because the gas molecules in this layer absorb the X-rays and ultraviolet radiation of the Sun.

• This results in the breakup of gas molecules into positively and negatively charged particles or ions.

• Thus, this layer is also known as the ionosphere.

• The electrically charged gas molecules of the thermosphere reflect radio waves from the Earth into space.
  Thus, this layer also helps in long-distance communications.

• The thermosphere also protects us from meteors and obsolete satellites, because its high temperature burns up nearly all the debris coming towards the Earth.

Exosphere

• This is the outermost layer of our atmosphere, which lies above the thermosphere.

• The exosphere extends beyond the thermosphere up to 960 km.

• It gradually merges with interplanetary space.

• The temperatures in this layer range from about 300°C to 1650°C.

• This layer contains only traces of gases like oxygen, nitrogen, argon, and helium because the lack of gravity allows the gas molecules to escape easily into space.

How does the Sun create energy?

Hydrogen and helium are the predominant gases that constitute the Sun.

• The proportion of hydrogen to helium is 3:1.

• The core of the Sun acts like a gigantic nuclear reactor and converts a huge quantity of hydrogen into helium.

• In this process of nuclear fusion, the Sun releases a tremendous amount of energy in all directions.

• The Sun radiates energy (both heat and light) in all directions.

• Because of its small size about the Sun, the Earth intercepts only a small part of the Sun's radiant energy.

• Solar radiation is the primary source of heat and light to the Earth.

Insolation

• The incoming solar radiation (energy intercepted by the Earth) is known as insolation, and it is received in the form of short waves.

Terrestrial Radiation

The Sun's energy absorbed by the Earth's surface when radiated out into space is called terrestrial radiation.

• It is long-wave electromagnetic radiation originating from Earth and its atmosphere.

• It is the radiation emitted by naturally radioactive materials on Earth, including uranium, thorium, and radon.

[Source: The American Heritage (R) Science Dictionary]

Weather and Climate

Weather is the description of the atmospheric conditions of a particular place at a particular time for a short period.

• Climate is the composite or integrated picture of the weather conditions over a long period.

• Climatic data is based on calculated averages of data recorded over 35 years.

• The classical period is 30 years, as defined by WMO.

Humidity

It refers to the content of water vapour present in the air. Though its amount in the atmosphere is as low as 4% in volume, it plays a very important role in the determination of the weather and climate of a place.

Humidity Capacity

• The capacity of a given volume of air at a certain temperature to retain the maximum amount of moisture content.

Saturated Air

• The air has a moisture content equal to its humidity capacity.

Dew Point

• The temperature at which the air becomes saturated is called the Dew Point.

Forms of Humidity

Humidity is expressed in three forms:

Absolute Humidity

• The total weight of moisture content per volume of air at a definite temperature is called absolute humidity.

• It is expressed in gm/m³.

Specific Humidity

• It is defined as the mass of water vapour in grams contained in a kilogram of air.

• It represents the actual quality of moisture present in a definite air.

• It is expressed in gm/kg.

Relative Humidity

• Relative humidity is defined as a ratio of the amount of water vapour present in the air having definite volume and temperature (absolute humidity) to the maximum amount the air can hold (humidity capacity).

• It is generally expressed in percentage.

• When the relative humidity is 100%, the process of condensation starts, which causes precipitation due to gravity of the Earth's gravity.

Atmospheric Pressure

Atmospheric pressure is the pressure at any point on the surface of the Earth due to the weight of the column of air above that point.

• Standard sea level pressure is 76 cm or 29.92 inches on this scale (Mercury Barometer).

• Another pressure unit used by meteorologists in drawing weather charts is millibars (mb).

• One bar is divided into 1000 millibars.

• Millibars are now known as hectopascals.

Pressure Measuring Instruments

Final Thoughts

The atmosphere is a vital shield that sustains life on Earth by providing essential gases, regulating temperature, and protecting us from harmful solar radiation. Its different layers, each with unique characteristics, play a crucial role in weather patterns, climate control, and communication systems. Components like water vapour, dust particles, and gases such as carbon dioxide not only influence weather but also maintain the Earth's energy balance.

The Sun, through nuclear fusion, serves as the primary energy source, driving all atmospheric processes. Understanding concepts like insolation, terrestrial radiation, humidity, and atmospheric pressure helps us appreciate the delicate balance of our environment.

As climate change and pollution threaten this balance, it becomes increasingly important to study and protect our atmosphere for the well-being of future generations.