Atmospheric Layers, Weather, Climate, and Insolation

The Stratosphere and Mesosphere

The stratosphere lies directly above the troposphere, extending from an altitude of $12\,\text{km}$ up to $50\,\text{km}$. This atmospheric layer is free from clouds and other weather disturbances, making it ideal for flying aeroplanes. A critical feature of the stratosphere is the presence of the ozone layer, which shields life on Earth by filtering out harmful short-wave photon radiation and ultraviolet radiation emitted by the Sun. The boundary that marks the upper limit of the stratosphere and separates it from the mesosphere is known as the stratopause.

The mesosphere is the third layer of the atmosphere, situated directly above the stratosphere, and extends from an altitude of $50\,\text{km}$ to $80\,\text{km}$. Within the mesosphere, temperature decreases as altitude increases:

$\text{Altitude} \uparrow\, \implies \text{Temperature} \downarrow$

Temperature decreases with increasing altitude exclusively in two atmospheric layers: the troposphere and the mesosphere. Most meteorites entering the Earth's atmosphere from space burn up upon reaching the mesosphere.

The Thermosphere and Ionosphere

The thermosphere lies above the mesosphere, extending from an altitude of $80\,\text{km}$ up to $700\,\text{km}$. In contrast to the mesosphere, temperature in the thermosphere rises very rapidly with increasing altitude:

$\text{Altitude} \uparrow\, \implies \text{Temperature} \uparrow$

This rapid temperature increase occurs because gas molecules in this layer directly absorb high-energy X-rays and short-wave ultraviolet radiation from the Sun, making the thermosphere the hottest layer in the atmosphere. The thermosphere also contains the Kármán line at an altitude of $100\,\text{km}$.

A major portion of the thermosphere is the ionosphere. The thermosphere and ionosphere facilitate radio communication across the globe by reflecting radio waves transmitted from the Earth back towards its surface.

Atmospheric Phenomena: Auroras

Auroras are vibrant, colourful displays of light seen in the sky near Earth's polar regions. The word "aurora" originates from the Latin language, meaning "dawn" or "morning light," and is named after Aurora, the Roman goddess of dawn.

Auroras are caused by interactions between atmospheric gases and the "solar wind," which consists of electrically charged particles emitted by the Sun. Upon reaching the Earth's atmosphere, these charged particles are directed along magnetic lines toward the magnetic poles. When these electrically charged particles collide with different atmospheric gases, each gas glows with a characteristic colour.

Displays occurring in the Northern Hemisphere are known as the Aurora Borealis, whereas displays occurring in the Southern Hemisphere are known as the Aurora Australis.

The Exosphere

The exosphere is the uppermost layer of the Earth's atmosphere, characterized by extremely thin air. Because Earth's gravitational force is weak at this extreme altitude, light gases such as helium and hydrogen float off into space. Despite its low density, the exosphere plays an essential role in Earth's atmospheric processes and affects overall weather and climate systems.

Atmospheric Pressure and Human Counter-Pressure

The air surrounding the Earth exerts a significant force on human bodies. However, humans do not feel this pressure because air exerts force equally from all directions simultaneously. In response, internal fluids and tissues inside the human body exert an equal counter-pressure outward. Because internal body pressure equals external atmospheric pressure, the two forces balance and cancel out.

Weather versus Climate

Weather refers to the hour-to-hour and day-to-day conditions of the atmosphere within a localized area over a short period. Atmospheric weather conditions directly affect human emotions and behavior: hot or humid weather can cause irritability, whereas pleasant or breezy weather promotes cheerfulness and plans for outings. Weather conditions vary significantly from day to day.

Climate represents the overall pattern and sum total of weather conditions and variations over a large geographical area over an extended period. Climate calculations require collecting and averaging atmospheric data over a duration of at least $30\,\text{years}$ (typically $30\text{--}40\,\text{years}$).

Elements of Weather and Insolation

Five major atmospheric elements determine weather and climate conditions and influence human life on Earth:

1. Temperature
2. Atmospheric Pressure
3. Wind
4. Humidity
5. Precipitation

Atmospheric temperature varies between day and night, as well as across seasons, with summer conditions being hotter than winter conditions. A primary factor governing global temperature distribution is insolation, defined as the amount of incoming solar energy intercepted by the Earth.

Solar energy interception is highest near the equator and decreases progressively toward the poles. Because the poles receive significantly less direct solar energy, insolation and atmospheric temperatures remain substantially lower in polar regions.