ENV201: Environmental Science - Lecture 4: The Atmosphere Notes

Interaction Between Solar Radiation and the Atmosphere

• Radiation Attenuation: As electromagnetic radiation passes through the mixture of gases comprising the atmosphere, it becomes depleted in certain portions of the spectrum. This process is also referred to as being "attenuated."
• Absorption and Transformation: The depleted portion of the spectrum represents energy that is absorbed by atmospheric gases and transformed into heat.
• Selective Absorption: Atmospheric gases do not absorb all wavelengths equally. Selective absorption occurs because different gases only absorb specific wavelengths.
• Key Absorbing Gases: The interactions between solar radiation and the atmosphere involve several critical gases across different spectrum segments (Visible, Near Infrared, and Thermal Infrared):
  • Methane ($CH_4$)
  • Nitrous oxide ($N_2O$)
  • Oxygen ($O_2$)
  • Ozone ($O_3$)
  • Carbon dioxide ($CO_2$)
  • Water vapour ($H_2O$)

Structure and Layers of the Atmosphere

• Layered Heterogeneity: The atmosphere is not homogenous but consists of several distinct layers based on thermal characteristics:
  • Troposphere: The lowest layer where the vast majority of weather events occur.
  • Stratosphere: The layer above the troposphere, containing the Ozone layer.
  • Mesosphere: The layer above the stratosphere.
  • Thermosphere: The outermost significant thermal layer.
• Atmospheric Boundaries (Pauses): The transitions between these layers are marked by "pauses" where temperature trends change:
  • Tropopause: Boundary between the troposphere and stratosphere.
  • Stratopause: Boundary between the stratosphere and mesosphere.
  • Mesopause: Boundary between the mesosphere and thermosphere.
• The Ozone Layer: Located within the stratosphere, specifically between altitudes of $15\,km$ and $30\,km$.

Properties of the Atmosphere: Temperature, Density, and Pressure

• Temperature Variations: Temperature is not constant throughout the atmosphere and behaves differently in different layers:
  • Troposphere: An increase in altitude is accompanied by a decrease in temperature.
  • Stratosphere: Temperature first remains constant with increasing altitude and then begins to increase as altitude increases.
• Atmospheric Density: Density decreases as altitude increases throughout the entire atmosphere. Air molecules are held near the Earth's surface by gravity, meaning density is greatest at the surface.
• Atmospheric Pressure: Pressure decreases as altitude increases. This is due to the weight of the air above; the more air above a specific level, the greater the compression ("squeezing") on the lower layers. Maximum compression occurs at the surface.
• Standard Atmosphere Model and Exceptions: While the standard model predicts specific temperature decreases with height, local impacts can occur:
  • Temperature Inversions: These occur when air temperature increases with height through a portion of the atmosphere.
  • Radiative Inversions: These develop shortly after sunset as the Earth's surface cools faster than the air above it. These inversions can sometimes reach depths of $100s\,m$ by $21h00$.

Composition of the Atmosphere

• Permanent Gases:
  • Defined as gases that form a constant proportion of the atmosphere.
  • They are relatively stable and possess long residence times.
  • Due to their stability and long residence, they form the bulk of the atmosphere.
• Variable Gases:
  • Gases whose distribution within the atmosphere changes across both time and space.
  • These gases are readily exchanged between the atmosphere and the Earth's surface through various biological and physical processes.

Air Masses and Fronts

• Air Masses: Large bodies of air characterized by uniform properties, specifically temperature and humidity, in any horizontal direction at a given altitude.
  • Formation: They form when the lower atmosphere remains in contact with a large, relatively uniform land or sea area (source region).
  • Acquisition of Characteristics: The longer an air mass remains over its source region, the more it acquires the temperature and moisture properties of the surface below.
• Fronts: Transition zones that form where two air masses with different temperature and humidity characteristics meet.
  • Cold Front: Occurs when cold air replaces warm air over time. Typically, the air behind the front must be at least $3^{\circ}C$ cooler than the air in front of it to be classified as such.
  • Warm Front: Occurs when warm air replaces cold air over time.
  • Occluded Front: Develops when a cold front catches up with a warm front.
• Identifying Fronts on Synoptic Charts:
  • Temperature Changes: Sharp changes over short distances.
  • Moisture Content: Changes in air humidity (cold air holds less water than warm air).
  • Wind Direction: Sudden shifts (e.g., from Northwest ($NW$) to Southwest ($SW$) during a cold front passage).
  • Air Pressure: Sea level pressure typically falls as a front approaches and rises sharply behind it.
  • Clouds and Precipitation: Changes occur as warm, moist air is lifted over denser cold air.
• Case Study (29 August 2013):
  • Cape Town: Cold front passed between $05h00$ and $07h00$. Observations included temperature drops, wind shifts from $NW$ to $SW$, and pressure increases after passage. Gale force winds ($>35\,kt$) were possible.
  • Port Elizabeth (Gqeberha): Cold front passed between $15h00$ and $17h00$. While pressure and temperature followed the standard frontal pattern, the expected wind shift was not observed due to a coastal low that had moved through earlier.

Lapse Rates and Atmospheric Stability

• Lapse Rate Definition: The rate of decrease in temperature with height within a given layer of the atmosphere.
• Environmental Lapse Rate (ELR): The vertical change in temperature through still (non-rising) air. This represents the actual temperature of the surrounding environment.
• Dry Adiabatic Lapse Rate (DALR):
  • The rate of cooling for an unsaturated ("dry") parcel of air as it rises.
  • The rate is approximately $1.0^{\circ}C\,per\,100\,m$.
  • Sinking air warms at this same rate.
• Wet Adiabatic Lapse Rate (WALR):
  • The rate of cooling for a saturated parcel of air as it rises.
  • The rate is approximately $0.5^{\circ}C\,per\,100\,m$.
  • The $WALR$ is lower than the $DALR$ because latent heat is released during condensation. As water vapour condenses to form clouds, it releases heat into the air parcel, which offsets some of the cooling caused by expansion.
• Atmospheric Stability Assessment: Stability is determined by comparing the lapse rate of a specific air parcel (DALR or WALR) to the ELR.
  • Stable Atmosphere: Occurs when the $WALR > ELR$. If a parcel is lifted, it remains cooler and denser than the surrounding environment and thus sinks back to its original level. It is strongly resistant to change.
  • Unstable Atmosphere: Occurs when the $ELR > DALR$. A rising air parcel remains warmer and less dense than its surroundings, causing it to continue rising and accelerating away from its initial position. This condition allows for cloud formation.
  • Conditionally Unstable Atmosphere: Occurs when $DALR > ELR > WALR$. The stability depends on whether the air is saturated. An unsaturated parcel will be stable (it cools faster than the environment and stays denser), but if forced to rise (e.g., over a mountain) and it becomes saturated, it will then cool slower than the environment ($ELR > WALR$) and become unstable, continuing to rise.

Example Questions & Discussion

• Question 1: Within the standard atmosphere model, at what altitude will the air temperature be $-30^{\circ}C$?
  • (a) Approximately $0\,km$
  • (b) Approximately $7\,km$
  • (c) Approximately $10\,km$
  • (d) Approximately $15\,km$
• Question 2: In the standard atmosphere model, at what elevation would the air pressure be $100\,hPa$?
  • (a) Approximately $25\,km$
  • (b) Approximately $15\,km$
  • (c) Approximately $10\,km$
  • (d) Approximately $5\,km$
• Question 3: Based on the standard atmosphere models, at which altitude ($5\,km$ or $15\,km$) would the density of the air be the highest? Why?
• Question 4: In which hypothetical scenario is the atmosphere unstable (Explain Choice)?
  • (a) $ELR$ is $4^{\circ}C\,per\,100\,m$ and $DALR$ is $1.0^{\circ}C\,per\,100\,m$.
  • (b) $ELR$ is $0.4^{\circ}C\,per\,100\,m$ and $WALR$ is $0.5^{\circ}C\,per\,100\,m$.
  • (c) $ELR$ is $0.8^{\circ}C\,per\,100\,m$ and $DALR$ is $1.0^{\circ}C\,per\,100\,m$.
  • (d) None of the above.
• Question 5: A parcel of dry air is forced to rise over a mountain. ELR is $2^{\circ}C\,per\,100\,m$ and $DALR$ is $1.5^{\circ}C\,per\,100\,m$. Which statement is appropriate?
  • (a) Parcel sinks back to original position.
  • (b) Parcel continues to rise into atmosphere.
  • (c) Scenario describes conditionally unstable atmosphere.
  • (d) Scenario describes stable atmosphere.
• Question 6: In which scenario is an imaginary planet's atmosphere stable (Explain Choice)?
  • (a) $ELR$ is $1.4^{\circ}C\,per\,100\,m$ and $WALR$ is $0.25^{\circ}C\,per\,100\,m$.
  • (b) $WALR$ is $0.15^{\circ}C\,per\,100\,m$ and $ELR$ is $0.02^{\circ}C\,per\,100\,m$.
  • (c) $DALR$ is $1.5^{\circ}C\,per\,100\,m$ and $ELR$ is $1.6^{\circ}C\,per\,100\,m$.
• Question 7: Identify atmospheric layers (A-F) based on standard structural models and temperature/pressure/density relationships.