☁️Atmospheric Physics Unit 1 – Atmospheric Composition and Structure

Key Concepts and Definitions

• Atmosphere consists of a mixture of gases, aerosols, and other particles surrounding Earth
• Atmospheric composition refers to the types and amounts of gases and particles present in the atmosphere
• Atmospheric structure describes the vertical layers of the atmosphere based on temperature variations (troposphere, stratosphere, mesosphere, thermosphere, exosphere)
• Atmospheric pressure is the force exerted by the weight of the atmosphere on a unit area
  • Decreases with increasing altitude due to less air above
• Atmospheric density is the mass of air per unit volume
  • Also decreases with increasing altitude
• Atmospheric scale height is the vertical distance over which pressure decreases by a factor of $e$ (approximately 7 km)
• Hydrostatic equilibrium is the balance between the upward pressure gradient force and downward gravitational force in the atmosphere

Atmospheric Layers and Their Properties

• Troposphere is the lowest layer extending from Earth's surface to an average height of 12 km
  • Contains 75-80% of the atmosphere's mass and nearly all water vapor and aerosols
  • Characterized by a decrease in temperature with increasing altitude (negative lapse rate)
• Stratosphere extends from the tropopause to about 50 km
  • Contains the ozone layer which absorbs harmful ultraviolet radiation
  • Characterized by an increase in temperature with altitude due to ozone heating
• Mesosphere extends from the stratopause to about 85 km
  • Coldest layer with temperatures decreasing with altitude
  • Noctilucent clouds can form at the top of this layer
• Thermosphere extends from the mesopause to about 500 km
  • Characterized by a rapid increase in temperature with altitude due to absorption of solar radiation by oxygen and nitrogen
  • Ionosphere is a region within the thermosphere containing electrically charged particles
• Exosphere is the outermost layer extending from the thermopause to about 10,000 km
  • Extremely low density where particles can escape Earth's gravitational pull

Chemical Composition of the Atmosphere

• Dry air is composed primarily of nitrogen (78%) and oxygen (21%) by volume
  • Argon (0.93%), carbon dioxide (0.04%), and trace gases make up the remainder
• Water vapor is a highly variable component (0-4% by volume) with higher concentrations in the lower troposphere
• Ozone is a critical trace gas in the stratosphere that absorbs ultraviolet radiation
  • Ozone layer protects life on Earth from harmful UV rays
• Aerosols are solid or liquid particles suspended in the atmosphere
  • Can be natural (dust, sea salt) or anthropogenic (sulfates, nitrates from pollution)
  • Play important roles in cloud formation, radiation balance, and air quality
• Greenhouse gases (carbon dioxide, water vapor, methane, nitrous oxide) absorb and emit infrared radiation
  • Increasing concentrations due to human activities are driving global climate change

Vertical Structure and Temperature Profile

• Temperature varies with altitude in the atmosphere due to different heating and cooling processes
• Troposphere exhibits a negative lapse rate (temperature decrease with height) of about 6.5°C/km
  • Caused by adiabatic cooling as air parcels expand and rise
• Tropopause is the boundary between the troposphere and stratosphere
  • Marked by a temperature inversion where the lapse rate changes sign
• Stratospheric temperature increases with altitude due to ozone heating
  • Positive lapse rate creates a stable layer with little vertical mixing
• Mesosphere exhibits a negative lapse rate with the coldest temperatures at the mesopause (~-90°C)
• Thermospheric temperature increases rapidly with altitude due to absorption of solar UV and X-ray radiation
  • Can reach over 1000°C but feels cold due to extremely low density

Atmospheric Pressure and Density

• Atmospheric pressure decreases exponentially with altitude following the barometric formula
  • $P(z) = P_0 \exp(-z/H)$, where $P_0$ is surface pressure, $z$ is altitude, and $H$ is scale height
• Sea-level pressure averages around 1013 hPa (1 atm) but varies with weather systems
  • High pressure systems have sinking air and clear skies
  • Low pressure systems have rising air and stormy weather
• Atmospheric density also decreases exponentially with altitude
  • Density is related to pressure by the ideal gas law: $P = \rho R_s T$
• Scale height is the e-folding distance for pressure and density
  • Varies with temperature (higher scale height in warmer air)
• Hydrostatic balance is the equilibrium between the vertical pressure gradient force and gravity
  • $dP/dz = -\rho g$, where $\rho$ is density and $g$ is gravitational acceleration

Energy Balance and Radiative Transfer

• Earth's climate is driven by the balance between incoming solar radiation and outgoing terrestrial radiation
  • Incoming shortwave radiation is mostly in the visible and near-infrared
  • Outgoing longwave radiation is in the thermal infrared
• Albedo is the fraction of incoming solar radiation reflected back to space
  • Varies with surface type (higher for snow and ice, lower for oceans and forests)
• Greenhouse effect is the trapping of outgoing infrared radiation by atmospheric gases
  • Warms the surface and lower atmosphere compared to a transparent atmosphere
• Radiative transfer equation describes the change in radiation intensity along a path
  • Includes absorption, emission, and scattering processes
• Absorption and emission of radiation by gases depend on their molecular structure and wavelength
  • H2O, CO2, O3 are important absorbers in the thermal infrared
• Scattering of radiation by air molecules and aerosols affects the transmission of light
  • Rayleigh scattering by molecules is stronger at shorter (blue) wavelengths
  • Mie scattering by aerosols is more wavelength-independent

Atmospheric Dynamics and Circulation

• Atmospheric motion is driven by pressure gradients, Coriolis force, and friction
  • Pressure gradient force points from high to low pressure
  • Coriolis force is an apparent force due to Earth's rotation (deflects to the right in the Northern Hemisphere)
• Geostrophic balance is the equilibrium between pressure gradient force and Coriolis force
  • Results in parallel flow along isobars (lines of constant pressure)
• General circulation of the atmosphere is characterized by three main cell types
  • Hadley cells are thermally direct circulations in the tropics (rising near equator, sinking in subtropics)
  • Ferrel cells are thermally indirect circulations in the mid-latitudes (rising in subpolar regions, sinking in subtropics)
  • Polar cells are thermally direct circulations at high latitudes (rising in subpolar regions, sinking over poles)
• Jet streams are narrow bands of strong upper-level winds
  • Polar jet stream is associated with the polar front (boundary between cold polar air and warm subtropical air)
  • Subtropical jet stream is associated with the descending branch of the Hadley cell
• Rossby waves are large-scale meandering patterns in the upper-level flow
  • Caused by the variation of Coriolis force with latitude (beta effect)
  • Play a key role in the development and propagation of weather systems

Measurement Techniques and Instrumentation

• Surface observations provide direct measurements of atmospheric variables at fixed locations
  • Include temperature, pressure, humidity, wind speed and direction, precipitation
  • Automated weather stations and human observers contribute to global observing network
• Radiosondes are balloon-borne instruments that measure vertical profiles of atmospheric properties
  • Launched twice daily at hundreds of sites worldwide
  • Provide crucial input data for weather forecasting models
• Radar and lidar are active remote sensing techniques that use electromagnetic waves to probe the atmosphere
  • Weather radars measure precipitation and wind velocity
  • Doppler lidars measure wind speed and turbulence
• Satellites provide global coverage of atmospheric observations from space
  • Geostationary satellites (GOES) provide continuous imagery over a fixed area
  • Polar-orbiting satellites (NOAA, NASA) provide global coverage with varying overpass times
  • Instruments include radiometers, spectrometers, and sounders for measuring temperature, moisture, and composition
• Aircraft and drones are mobile platforms for atmospheric measurements
  • Research aircraft are equipped with a wide range of in-situ and remote sensing instruments
  • Drones (UAVs) offer flexibility and high-resolution sampling in the lower atmosphere