Chapter 4: Air

The atmosphere is the vast blanket of air that envelops our planet. It is an indispensable component of our environment, providing the oxygen we need to breathe and acting as a protective shield against the sun's harsh radiation. Without the atmosphere, Earth would experience extreme temperatures—scorching hot during the day and freezing cold at night. This gaseous layer is what makes the temperature on Earth suitable for life.

Composition Of The Atmosphere

The air we breathe is a mixture of several different gases. The bulk of the atmosphere is made up of just two primary gases, while several others are present in much smaller quantities. The air also contains tiny dust particles.

The approximate composition of dry air is as follows:

• Nitrogen (N₂): 78%
• Oxygen (O₂): 21%
• Argon (Ar): 0.93%
• Carbon Dioxide (CO₂): 0.03%
• Other gases like Helium, Ozone, and Hydrogen make up the remaining small fraction.

Key Gases and Their Roles

• Nitrogen: As the most abundant gas, nitrogen is essential for plant life. While we inhale it, our bodies don't use it directly. Plants, however, cannot take nitrogen directly from the air either. They rely on special bacteria in the soil and on their roots to "fix" the nitrogen, converting it into a usable form.
• Oxygen: The second most plentiful gas, oxygen is the life-giving gas essential for the respiration of humans and animals. This balance is maintained by green plants, which produce oxygen as a byproduct of photosynthesis. Deforestation disrupts this crucial balance.
• Carbon Dioxide: Although present in a small amount, CO₂ is vital. Green plants use it to produce food during photosynthesis. However, human activities, primarily the burning of fossil fuels (like coal and oil) for industries and transport, have significantly increased the concentration of CO₂ in the atmosphere.

The Greenhouse Effect and Global Warming

Carbon dioxide is a greenhouse gas. It traps heat radiated from the Earth's surface, creating a natural "greenhouse effect" that keeps our planet warm enough for life. However, the excessive release of CO₂ enhances this effect, leading to a rise in the Earth's average temperature, a phenomenon known as global warming. The consequences of global warming are severe, including the melting of polar ice caps, rising sea levels that cause coastal flooding, and drastic climate changes that threaten the survival of many plant and animal species.

Structure Of The Atmosphere

The atmosphere is not uniform; it is divided into five distinct layers, each with unique characteristics. These layers, starting from the Earth's surface and moving upwards, are:

1. Troposphere: This is the lowest and most important layer, extending to an average height of 13 km. It contains the air we breathe and is where nearly all weather phenomena—such as rain, clouds, fog, and hailstorms—occur.
2. Stratosphere: Extending up to a height of 50 km, this layer lies above the troposphere. It is almost completely free of clouds and weather disturbances, making it the ideal layer for commercial aeroplanes to fly in. The stratosphere contains the crucial ozone layer, which absorbs the sun's harmful ultraviolet (UV) radiation.
3. Mesosphere: This is the third layer, reaching up to 80 km. It is in this layer that most meteorites burn up upon entering the Earth's atmosphere, protecting us from impacts.
4. Thermosphere: In this layer, the temperature rises dramatically with increasing altitude. It extends from about 80 km to 400 km. A part of this layer is the ionosphere, which contains electrically charged particles (ions). This sub-layer plays a vital role in modern communication by reflecting radio waves transmitted from Earth back to the surface.
5. Exosphere: This is the uppermost layer of the atmosphere, where it gradually merges with outer space. The air here is extremely thin (low density), and light gases like hydrogen and helium float out into space from this layer.

Weather And Climate

It is important to distinguish between weather and climate.

• Weather refers to the short-term, hour-to-hour or day-to-day condition of the atmosphere at a particular place. It can change dramatically and frequently.
• Climate, on the other hand, describes the average weather conditions of a place over a very long period (typically 30 years or more).

Temperature

Temperature is the measure of how hot or cold the air is. The primary factor influencing the distribution of temperature on Earth is insolation, which is the incoming solar energy from the sun that is intercepted by the Earth. The amount of insolation received is highest at the equator and decreases towards the poles. Consequently, temperature also generally decreases as we move from the equator to the poles.

Temperatures in cities are often significantly higher than in surrounding rural areas. This is because urban materials like concrete and asphalt absorb and retain more heat, and the high-rise buildings trap warm air, creating an "urban heat island" effect.

Air Pressure

Air pressure is the pressure exerted by the weight of the air column on the Earth's surface. We don't feel this immense pressure because it pushes on us from all directions, and our bodies exert a counter-pressure.

• Air pressure is highest at sea level and decreases with altitude.
• Temperature influences air pressure. In high-temperature areas, air gets heated, expands, becomes lighter, and rises, creating a low-pressure area. Low pressure is often associated with cloudy, wet weather.
• In low-temperature areas, air is cold, dense, and heavy. It sinks, creating a high-pressure area. High pressure is usually associated with clear, sunny skies.

Wind

Wind is the movement of air from an area of high pressure to an area of low pressure. Winds are named after the direction from which they blow (e.g., a "westerly" wind blows from the west).

Winds are broadly divided into three types:

1. Permanent Winds: These winds, like the Trade Winds, Westerlies, and Polar Easterlies, blow constantly throughout the year in a particular direction.
2. Seasonal Winds: These winds change their direction with the seasons. The best example is the monsoon winds in India.
3. Local Winds: These winds blow only during a specific time of day or year over a small area. Examples include land and sea breezes, and the hot, dry summer wind in northern India known as the Loo.

Moisture

Humidity is the amount of water vapour (moisture) present in the air. When water evaporates from oceans, rivers, and land, it becomes water vapour. The air's capacity to hold water vapour increases with temperature.

Precipitation occurs when this water vapour rises, cools, and condenses into tiny water droplets, forming clouds. When these droplets become too heavy to stay suspended in the air, they fall to the Earth. The most common form of precipitation is rain. Other forms include snow, sleet, and hail.

Based on their formation mechanism, there are three main types of rainfall:

• Convectional Rainfall: Occurs when the sun heats the ground, causing the air above it to warm up, expand, and rise. As it rises, it cools and its moisture condenses to form rain. This is common in equatorial regions.
• Orographic (or Relief) Rainfall: Happens when moist air is forced to rise over a mountain range. The air cools as it ascends, causing precipitation on the windward side of the mountain. The leeward side receives very little rain and is called the rain shadow area.
• Cyclonic Rainfall: Associated with cyclones or frontal systems where a mass of warm air meets a mass of cold air. The lighter warm air is forced to rise over the denser cold air, leading to condensation and rainfall.