☁️Meteorology Unit 2 – Atmospheric Composition and Structure

Key Concepts and Terminology

• Atmosphere consists of layers with distinct properties (temperature, pressure, density)
• Troposphere is the lowest layer where most weather phenomena occur
  • Contains ~75% of the atmosphere's mass and ~99% of its water vapor and aerosols
• Stratosphere is the second layer characterized by increasing temperature with height due to ozone absorption
• Mesosphere is the third layer where temperature decreases with increasing altitude
• Thermosphere is the uppermost layer with extremely high temperatures due to solar radiation absorption
• Exosphere is the outermost region of the atmosphere that gradually fades into space
• Homosphere and heterosphere describe the atmosphere's composition at different altitudes
• Scale height measures the decrease in atmospheric pressure with altitude

Layers of the Atmosphere

• Troposphere extends from Earth's surface to an average height of ~12 km (varies with latitude and season)
  • Tropopause marks the boundary between the troposphere and stratosphere
• Stratosphere extends from the tropopause to an altitude of ~50 km
  • Stratopause separates the stratosphere from the mesosphere
• Mesosphere spans from the stratopause to an altitude of ~85 km
  • Mesopause is the boundary between the mesosphere and thermosphere
• Thermosphere extends from the mesopause to ~500-1000 km
  • Exosphere is the outermost part of the thermosphere that gradually transitions into space
• Ionosphere is an electrified region within the thermosphere containing charged particles (ions and electrons)
• Magnetosphere is the region around Earth dominated by its magnetic field, extending thousands of kilometers into space
• Karman line at 100 km altitude is considered the beginning of space

Atmospheric Composition

• Atmosphere is a mixture of gases, with nitrogen (78%) and oxygen (21%) being the most abundant
• Trace gases include argon (0.93%), carbon dioxide (0.04%), and others (neon, helium, methane, krypton, hydrogen)
• Water vapor content varies significantly with location and time (0-4%)
  • Plays a crucial role in weather phenomena and heat transfer
• Aerosols are solid or liquid particles suspended in the atmosphere (dust, smoke, salt, pollen)
  • Affect air quality, visibility, and climate by scattering and absorbing radiation
• Ozone is a key component in the stratosphere, absorbing harmful ultraviolet radiation from the sun
• Atmospheric composition is generally uniform up to ~80 km (homosphere) and varies above that (heterosphere)

Temperature Profile

• Temperature varies with altitude in the atmosphere, creating distinct layers
• Troposphere experiences a decrease in temperature with increasing altitude (lapse rate)
  • Average lapse rate is ~6.5°C/km, but varies with location and time
• Stratosphere has an increasing temperature profile due to ozone absorption of ultraviolet radiation
  • Temperature inversion in the stratosphere creates a stable layer that limits vertical mixing
• Mesosphere exhibits a decreasing temperature profile, reaching the coldest temperatures at the mesopause
• Thermosphere has a rapidly increasing temperature profile due to absorption of solar radiation by oxygen and nitrogen
  • Temperatures can exceed 1000°C, but the air is too thin to effectively transfer heat

Pressure and Density Changes

• Atmospheric pressure decreases exponentially with altitude, with the most rapid decrease in the lower atmosphere
  • Pressure at sea level averages ~1013 hPa and drops to ~1 hPa at 50 km
• Density also decreases with altitude, as the air becomes thinner and less dense
  • Density at sea level is ~1.225 kg/m³ and decreases to ~0.001 kg/m³ at 80 km
• Scale height is the distance over which atmospheric pressure decreases by a factor of e (~2.718)
  • Varies with temperature and molecular mass, averaging ~8 km for Earth's atmosphere
• Hydrostatic balance describes the equilibrium between the vertical pressure gradient force and gravity
  • Explains why atmospheric pressure decreases with altitude

Atmospheric Phenomena

• Weather occurs primarily in the troposphere and is driven by energy from the sun
  • Includes phenomena such as clouds, precipitation, winds, and storms
• Jet streams are fast-moving air currents in the upper troposphere that influence weather patterns
• Atmospheric tides are global-scale pressure oscillations caused by solar heating and lunar gravitational pull
• Gravity waves are disturbances that propagate through the atmosphere, often generated by topography or convection
  • Play a role in transferring energy and momentum between atmospheric layers
• Auroras (northern and southern lights) occur in the ionosphere due to collisions between energetic particles and atmospheric gases
• Airglow is a faint emission of light by atmospheric gases, visible primarily at night
• Noctilucent clouds form in the mesosphere at altitudes of ~80 km, visible during summer twilight

Measurement Techniques

• Radiosondes are balloon-borne instruments that measure pressure, temperature, humidity, and wind as they ascend through the atmosphere
• Weather satellites provide global observations of atmospheric conditions using visible, infrared, and microwave sensors
  • Geostationary satellites orbit at ~36,000 km and continuously monitor a fixed area
  • Polar-orbiting satellites circle Earth at lower altitudes (~800 km) and provide detailed observations of the entire globe
• Radar systems detect precipitation, measure wind speed and direction, and track severe weather
• Lidar (light detection and ranging) uses laser pulses to measure atmospheric properties such as temperature, density, and composition
• Aircraft-based measurements provide detailed in-situ observations of atmospheric conditions
• Ground-based instruments include weather stations, air quality monitors, and atmospheric observatories

Environmental Impacts and Climate Change

• Greenhouse gases (carbon dioxide, methane, water vapor) absorb and emit infrared radiation, warming Earth's surface
  • Increasing greenhouse gas concentrations due to human activities are driving global climate change
• Ozone depletion, caused by chlorofluorocarbons (CFCs) and other substances, allows more harmful ultraviolet radiation to reach Earth's surface
  • Montreal Protocol has successfully phased out ozone-depleting substances, and the ozone layer is slowly recovering
• Air pollution, including particulate matter and ground-level ozone, adversely affects human health and the environment
  • Sources include transportation, industrial activities, and biomass burning
• Acid rain results from the reaction of atmospheric pollutants (sulfur dioxide and nitrogen oxides) with water, damaging ecosystems and infrastructure
• Climate change impacts include rising sea levels, more frequent and intense extreme weather events, and shifts in temperature and precipitation patterns
  • Mitigation efforts focus on reducing greenhouse gas emissions, while adaptation strategies help communities prepare for and respond to climate-related risks