🌦️Atmospheric Science Unit 2 – Atmospheric Composition and Structure

Key Components of Earth's Atmosphere

• Consists primarily of nitrogen (78%) and oxygen (21%), with the remaining 1% composed of trace gases such as argon, carbon dioxide, and water vapor
• Atmospheric composition plays a crucial role in sustaining life on Earth by providing oxygen for respiration and carbon dioxide for photosynthesis
• Greenhouse gases, including water vapor, carbon dioxide, and methane, trap heat and regulate Earth's surface temperature
• Ozone layer in the stratosphere absorbs harmful ultraviolet (UV) radiation from the sun, protecting life on Earth
• Atmospheric pressure decreases with increasing altitude due to the decreasing weight of the air column above
• Atmospheric density also decreases with increasing altitude, as the air becomes thinner and less compressed
• Atmospheric circulation patterns, driven by uneven heating of Earth's surface, redistribute heat and moisture around the planet

Layers of the Atmosphere

• Troposphere extends from Earth's surface to an average height of 12 km (7.5 miles) and contains approximately 75% of the atmosphere's mass
  • Most weather phenomena occur in the troposphere, including cloud formation, precipitation, and storms
  • Temperature decreases with increasing altitude in the troposphere at a rate of about 6.5°C per kilometer (3.6°F per 1,000 feet), known as the lapse rate
• Stratosphere extends from the top of the troposphere to an altitude of about 50 km (31 miles)
  • Contains the ozone layer, which absorbs harmful UV radiation from the sun
  • Temperature increases with altitude in the stratosphere due to the absorption of UV radiation by ozone
• Mesosphere extends from the top of the stratosphere to an altitude of about 85 km (53 miles)
  • Coldest layer of the atmosphere, with temperatures reaching as low as -90°C (-130°F) at the mesopause
  • Meteors often burn up in the mesosphere due to friction with the air
• Thermosphere extends from the top of the mesosphere to an altitude of about 600 km (373 miles)
  • Temperature increases rapidly with altitude in the thermosphere, reaching up to 2,000°C (3,632°F) due to the absorption of high-energy solar radiation by oxygen and nitrogen molecules
  • Aurora borealis and aurora australis (northern and southern lights) occur in the thermosphere when charged particles from the sun interact with Earth's magnetic field
• Exosphere is the outermost layer of the atmosphere, extending from the top of the thermosphere to an indefinite height
  • Extremely low density, with atoms and molecules escaping into space
  • Transition zone between Earth's atmosphere and outer space

Atmospheric Pressure and Density

• Atmospheric pressure is the force exerted by the weight of the air column above a given point on Earth's surface
  • Measured using a barometer, with the standard atmospheric pressure at sea level being 1013.25 millibars (mb) or 29.92 inches of mercury (inHg)
  • Pressure decreases with increasing altitude at a rate of about 1 mb per 10 meters (33 feet) near sea level
• Atmospheric density is the mass of air per unit volume and decreases with increasing altitude
  • At higher altitudes, air molecules are farther apart, resulting in lower density and pressure
  • Density is affected by temperature, with warmer air being less dense than cooler air at the same pressure
• Pressure gradients, which are differences in atmospheric pressure between two points, drive atmospheric circulation and wind patterns
  • Wind flows from areas of high pressure to areas of low pressure, with greater pressure differences resulting in stronger winds
• Atmospheric pressure and density play a crucial role in weather patterns, as well as the lift generated by aircraft wings and the performance of engines

Temperature Variations in the Atmosphere

• Temperature in the atmosphere varies with altitude, latitude, and season, as well as due to local factors such as land cover and proximity to water bodies
• Vertical temperature profile of the atmosphere is characterized by the lapse rate, which is the rate at which temperature decreases with increasing altitude
  • Environmental lapse rate (ELR) varies depending on the moisture content and stability of the air, with an average value of 6.5°C per kilometer (3.6°F per 1,000 feet) in the troposphere
  • Dry adiabatic lapse rate (DALR) is the rate at which a parcel of unsaturated air cools as it rises and expands, with a value of 9.8°C per kilometer (5.4°F per 1,000 feet)
• Latitudinal temperature variations are primarily driven by the uneven distribution of solar radiation, with more energy received at the equator than at the poles
  • Hadley, Ferrel, and Polar cells in the atmosphere redistribute heat from the equator to the poles, moderating temperature differences
• Seasonal temperature variations are caused by Earth's tilt and its orbit around the sun, which affect the angle and duration of solar radiation received at a given location
• Urban heat islands are areas of higher temperature within cities compared to surrounding rural areas, due to the absorption and re-emission of heat by buildings and paved surfaces

Atmospheric Chemistry and Gases

• Atmospheric chemistry involves the study of chemical reactions and processes that occur in the atmosphere, including the formation and destruction of gases and aerosols
• Nitrogen (N2) and oxygen (O2) are the most abundant gases in the atmosphere, making up 78% and 21% of the atmosphere by volume, respectively
  • Nitrogen is largely inert, but can be converted into biologically available forms through nitrogen fixation by bacteria or lightning
  • Oxygen is essential for respiration and combustion, and is replenished by photosynthesis in plants and algae
• Carbon dioxide (CO2) is a trace gas that makes up about 0.04% of the atmosphere, but plays a crucial role in regulating Earth's temperature as a greenhouse gas
  • Atmospheric CO2 concentrations have increased significantly since the Industrial Revolution due to human activities such as fossil fuel combustion and deforestation
• Water vapor (H2O) is the most abundant greenhouse gas and plays a key role in the hydrologic cycle, transporting heat and moisture through the atmosphere
  • Atmospheric water vapor content varies widely depending on location and weather conditions, and can range from near zero in cold, dry air to about 4% in warm, humid air
• Ozone (O3) is a trace gas that is important for absorbing harmful UV radiation in the stratosphere, but can also be a pollutant in the troposphere when formed by reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight
• Other important atmospheric gases include methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs), which are potent greenhouse gases and can also contribute to air pollution and ozone depletion

Energy Balance and Radiation

• Earth's energy balance is determined by the amount of incoming solar radiation absorbed by the planet and the amount of outgoing longwave radiation emitted back to space
  • Incoming solar radiation has an average intensity of 1,361 W/m² at the top of the atmosphere, known as the solar constant
  • Approximately 30% of incoming solar radiation is reflected back to space by clouds, aerosols, and Earth's surface (albedo), while the remaining 70% is absorbed by the atmosphere and surface
• Greenhouse effect is the process by which atmospheric gases, primarily water vapor, carbon dioxide, and methane, absorb and re-emit longwave radiation emitted by Earth's surface, warming the planet
  • Without the greenhouse effect, Earth's average surface temperature would be about -18°C (0°F), instead of the current 15°C (59°F)
• Radiative forcing is a measure of the change in Earth's energy balance due to a specific factor, such as increasing greenhouse gas concentrations or changes in solar activity
  • Positive radiative forcing (e.g., increasing greenhouse gases) leads to warming, while negative radiative forcing (e.g., volcanic eruptions) leads to cooling
• Milankovitch cycles are long-term variations in Earth's orbit and axis tilt that affect the amount and distribution of solar radiation received by the planet, contributing to glacial-interglacial cycles on timescales of tens to hundreds of thousands of years
• Urban heat islands and land use changes can alter local energy balances by affecting surface albedo, evapotranspiration, and the absorption and re-emission of heat by buildings and infrastructure

Atmospheric Phenomena and Weather

• Weather refers to the day-to-day state of the atmosphere at a given location, characterized by temperature, humidity, precipitation, wind, and other variables
  • Weather patterns are driven by atmospheric circulation, which is influenced by factors such as the Coriolis effect, pressure gradients, and the uneven heating of Earth's surface
• Clouds form when air becomes saturated with water vapor and condensation occurs around tiny particles called cloud condensation nuclei (CCN)
  • Cloud types are classified based on their shape, altitude, and composition, and can indicate specific weather conditions (e.g., cumulonimbus clouds are associated with thunderstorms)
• Precipitation occurs when water droplets or ice crystals in clouds grow large enough to fall to the ground under the influence of gravity
  • Forms of precipitation include rain, snow, sleet, and hail, depending on the temperature and moisture content of the air
• Thunderstorms develop when warm, moist air rises rapidly, leading to the formation of tall cumulonimbus clouds, strong updrafts and downdrafts, lightning, and thunder
  • Severe thunderstorms can produce damaging winds, large hail, and tornadoes
• Hurricanes are large, rotating tropical cyclones that form over warm ocean waters and can bring intense winds, heavy rainfall, and storm surges to coastal areas
  • Hurricanes are fueled by the condensation of water vapor, which releases latent heat and drives the storm's circulation
• Atmospheric rivers are narrow corridors of concentrated moisture in the atmosphere that can transport large amounts of water vapor over long distances, often leading to heavy precipitation events when they make landfall

Environmental Impacts and Climate Change

• Climate change refers to long-term shifts in global or regional climate patterns, primarily driven by human activities that increase atmospheric greenhouse gas concentrations
  • Burning of fossil fuels, deforestation, and land use changes are major contributors to the rapid increase in atmospheric carbon dioxide levels since the Industrial Revolution
  • Global average surface temperatures have increased by approximately 1.1°C (2.0°F) since pre-industrial times, with the rate of warming accelerating in recent decades
• Rising global temperatures lead to a range of environmental impacts, including:
  • Sea level rise due to the thermal expansion of ocean water and the melting of land-based ice sheets and glaciers
  • More frequent and intense heatwaves, droughts, and wildfires in many regions
  • Changes in precipitation patterns, with some areas experiencing more frequent and severe flooding, while others face increased water scarcity
  • Shifts in the geographic ranges and phenology of plant and animal species, with implications for biodiversity and ecosystem functioning
• Ocean acidification is another consequence of increasing atmospheric CO2 levels, as the oceans absorb about 30% of the CO2 emitted by human activities
  • As CO2 dissolves in seawater, it forms carbonic acid, lowering the ocean's pH and making it more difficult for marine organisms like corals and shellfish to build their calcium carbonate structures
• Stratospheric ozone depletion, caused by the release of ozone-depleting substances such as CFCs, allows more harmful UV radiation to reach Earth's surface
  • The Montreal Protocol, an international agreement to phase out the production and consumption of ozone-depleting substances, has helped to slow and reverse ozone depletion
• Mitigating climate change and its impacts requires a combination of reducing greenhouse gas emissions, adapting to the changes already underway, and developing sustainable practices in energy production, transportation, agriculture, and land use