CBSE Class 7 Geography Notes Chapter 4 - Air
CBSE Class 7 Geography Notes Chapter 4 - Air
Composition of the Atmosphere
The Earth's atmosphere is composed of various gases, along with small amounts of suspended solid and liquid particles. The composition of the atmosphere can be broadly categorized as follows:
Nitrogen (N2): Nitrogen is the most abundant gas in the Earth's atmosphere, making up approximately 78% of the total volume. It is a colorless and odorless gas that is chemically stable and relatively unreactive.
Oxygen (O2): Oxygen is the second most abundant gas in the atmosphere, constituting about 21% of the total volume. It is essential for supporting life and is involved in various chemical reactions, including respiration.
Argon (Ar): Argon is an inert gas and makes up about 0.93% of the atmosphere. It is chemically unreactive and serves as a "filler" gas, occupying space without participating in most chemical reactions.
Carbon Dioxide (CO2): Carbon dioxide is a vital greenhouse gas, contributing to the Earth's climate. It exists in trace amounts but has been increasing due to human activities, primarily the burning of fossil fuels. Currently, it constitutes about 0.04% of the atmosphere.
Trace Gases: There are several other gases present in the atmosphere in smaller amounts, often measured in parts per million (ppm) or parts per billion (ppb). These include methane (CH4), ozone (O3), nitrous oxide (N2O), carbon monoxide (CO), and various pollutants.
Water Vapor (H2O): Water vapor is the gaseous form of water and exists in varying concentrations depending on the location and weather conditions. It is an important component for weather and plays a crucial role in the Earth's heat balance.
In addition to gases, the atmosphere also contains suspended particles such as dust, pollen, volcanic ash, and other aerosols. These particles can influence climate, air quality, and visibility.
Structure of the Atmosphere
The Earth's atmosphere is divided into distinct layers based on temperature variations with increasing altitude. From the Earth's surface upward, the atmospheric layers are:
Troposphere: The troposphere is the lowest layer of the atmosphere, extending from the Earth's surface up to an average altitude of about 7 to 17 kilometers (4 to 11 miles). This layer is where weather occurs, and it contains about 75% to 80% of the total mass of the atmosphere. The temperature generally decreases with increasing altitude in the troposphere.
Stratosphere: Above the troposphere lies the stratosphere, which extends from the top of the troposphere (around 7 to 17 kilometers) to approximately 50 kilometers (31 miles) above the Earth's surface. The stratosphere is characterized by a temperature inversion, where the temperature increases with altitude due to the presence of the ozone layer. The ozone layer absorbs a significant portion of the Sun's ultraviolet (UV) radiation.
Mesosphere: The mesosphere is situated above the stratosphere and extends from around 50 kilometers to about 85 kilometers (53 miles) in altitude. In this layer, temperatures decrease with increasing altitude, reaching extremely low temperatures. Meteors often burn up in the mesosphere due to the high atmospheric friction.
Thermosphere: The thermosphere is the layer above the mesosphere, spanning from approximately 85 kilometers to 600 kilometers (372 miles). Despite the high altitudes, the thermosphere experiences extremely high temperatures due to the absorption of intense solar radiation. However, the density of gas particles in this layer is incredibly low, so the heat is not felt as it would be on the Earth's surface.
Exosphere: The exosphere is the outermost layer of the atmosphere, extending from the upper boundary of the thermosphere to the edge of space. It is a region where the atmosphere becomes very thin and merges with the vacuum of space. Gas molecules in the exosphere are sparsely distributed and can escape the Earth's gravitational pull.
It's important to note that the boundaries between these atmospheric layers are not sharply defined but rather gradually transition with altitude. Additionally, there are various sub-layers and features within these layers that contribute to the complex behavior of the Earth's atmosphere.
Weather and Climate
Temperature
Weather refers to the short-term state of the atmosphere, including temperature, humidity, precipitation, wind speed and direction, and atmospheric pressure. It represents the atmospheric conditions experienced over a relatively short period, typically hours to days. Weather conditions can vary greatly from day to day and from one location to another. Weather forecasts provide information about expected weather conditions to help people plan their daily activities.
Temperature is a crucial component of weather and climate. It is a measure of the average kinetic energy of the molecules in the air. Temperature influences various aspects of the atmosphere, such as air density, pressure, and moisture-holding capacity. It is typically measured using a thermometer and expressed in degrees Celsius (°C) or Fahrenheit (°F).
Climate, on the other hand, refers to the long-term average weather patterns observed over a specific region or the entire planet. It encompasses not only temperature but also other elements such as precipitation patterns, wind patterns, and seasonal variations. Climate is characterized by its long-term stability and the statistical analysis of weather data collected over several decades or more.
Climate is influenced by a range of factors, including solar radiation, atmospheric and oceanic circulation patterns, land and water distribution, elevation, and greenhouse gas concentrations. Climate change refers to long-term shifts in average weather patterns over many years or decades, often attributed to human activities such as the burning of fossil fuels, deforestation, and industrial processes.
While weather can change rapidly and is highly variable, climate provides a broader understanding of the long-term patterns and trends that shape the Earth's atmospheric conditions. Understanding both weather and climate is essential for predicting short-term conditions and studying long-term climate change, which have significant implications for human activities, agriculture, ecosystems, and the overall health of the planet.
Air Pressure
Air pressure is the force exerted by the weight of the atmosphere above a given point on the Earth's surface or at a specific altitude due to the gravitational pull on the air molecules in the atmosphere. This force causes the air to be compressed and creates pressure.
Wind
The horizontal flow of air across the surface of the Earth is known as wind. It is primarily brought on by variations in atmospheric pressure, which produce a pressure gradient force that forces air from high-pressure to low-pressure regions.
Wind flow is the term for the passage of air from high-pressure to low-pressure regions. The strength of the wind that results depends on how much pressure exists between two points. Because to the Coriolis effect, which is a result of the Earth's rotation, the direction of the wind is also affected, with the Northern Hemisphere experiencing a rightward deflection and the Southern Hemisphere experiencing a leftward deflection.
Moisture
Water vapour is created as water evaporates from land and various bodies of water. Humidity is the measurement of air moisture. A humid day is one in which there is a lot of water vapor in the air. It becomes increasingly humid as a result of the warmer air's increased ability to hold water vapor.
The water vapour begins to cool as it rises. Water droplets are created when water vapour condenses. Masses of these water droplets make up clouds. These water droplets fall to the ground as precipitation when they are too heavy to float in the air.
Rain is a type of precipitation that falls to the ground as liquid. There are three types of rainfall based on mechanism:
Convectional rainfall occurs when the sun heats the Earth's surface, causing the air near the surface to warm and rise.
Orographic rainfall is associated with the lifting of air masses as they encounter elevated terrain such as mountains. Frontal rainfall occurs along the boundaries (fronts) between air masses with different temperatures and moisture content. When a warm air mass and a cold air mass meet, the less dense warm air rises over the denser cold air, cooling and condensing, forming clouds and precipitation.
Frontal rainfall can be prolonged and widespread, as it is associated with the movement of weather systems.
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