AP Environmental Science: Topic 7.3 Thermal Inversions Study Notes

Lesson Introduction and Objectives

• Speaker and Series: The video is presented by Mr smees and is part of the Apes video notes series.
• Topic Identifier: This session covers Topic $7.3$, which focuses on thermal inversions.
• Primary Learning Objective: The main objective is to describe thermal inversions and explain how they relate to pollution.
• Skill Development: The lesson concludes with a skill-practice exercise based on FRQ $7.3$, focusing on explaining an environmental process or concept represented visually.

The Urban Heat Island Effect

• Context for Thermal Inversion: Understanding how a thermal inversion forms requires prior knowledge of the urban heat island effect.
• Definition: The urban heat island effect refers to the concept that urban areas are often warmer than the surrounding rural or suburban areas.
• Factor 1: Lower Albedo:     * Albedo Description: Albedo refers to how reflective a surface is.     * Urban Materials: Cities have a high prevalence of black top and Asal. Because these surfaces are darker in color and there is less vegetation, they possess a lower albedo.     * Absorption and Radiation: Darker surfaces absorb more of the sun's rays. When a surface absorbs more sunlight and reflects less, it radiates that energy out as infrared radiation.     * Human Perception: Humans perceive infrared radiation as heat. This is why sun rays striking a surface make it warmer.     * Visual Graphic: In the provided diagram, the sun (previously featured in Video $7.2$) sends sun rays to both a downtown area and a rural area with more trees. Red squiggly lines represent the infrared radiation emitted by the black top and asphal in urban areas when absorbing the sunf phace.     * Rural Contrast: Rural areas are heated by the sun, but the effect is less pronounced because vegetation has a higher albedo (lighter in color) and reflects more sunlight, resulting in a lower surface temperature.
• Factor $2$: Evapo Transpiration:     * Mechanism (Unit $1$ Recurrence): Evapo transpiration is defined as the combined effect of evaporation (water from land surfaces) and transpiration (water leaving the pores of leaves).     * Cooling Effect: This process carries heat away from the surface and disperses it into the atmosphere, providing a cooling effect.     * Visual Representation: Large blue squiggly arrows are used in the rural area graphic to represent trees transpiring and moving heat away.     * Urban Deficiency: In downtown areas, fewer trees result in less transpiration. Additionally, high levels of runoff result in less evaporation. Therefore, urban areas do not benefit from the evaporative and transpirational cooling seen in rural areas.

Normal Atmospheric Temperature Gradients

• General Rule: Normally, Earth heats the atmosphere most intensely near its surface. Temperature generally gets cooler As We rise in altitude.
• Reasons for the Gradient:     1. Earth's surface absorbs sunlight and releases it as infrared radiation.     2. Atmospheric pressure is lower at higher altitudes, and lower pressure results in lower temperature.
• Pollution Dispersal:     * This normal gradient is beneficial for air quality because warm air rises, creating convection currents.     * Pollutants formed near Earth—such as tropospheric ozone, smog, or particulate matter—are carried up and away from urban areas.     * This process is likened to the Hadley cell mechanism discussed in Topic $4.5$, where warm air rises at the equator.

Formation and Mechanism of Thermal Inversions

• Inversion Definition: In a thermal inversion, the temperature gradient is altered or inverted.
• The "Sandwich" Layer: A warm air mass becomes trapped beneath a colder air mass above and a cooler air mass below at Earth's surface.
• Common Causes:     1. Warm Fronts: A warm air mass can move in and cover a colder air mass. This is common in coastal areas like California, where warm ocean air masses move inland over cooler air.     2. Nighttime Summer Cooling in Urban Areas: During the summer, urban centers absorb sunlight all day. At night, when the sun goes down, the Earth's surface cools off, but the heat absorbed during the day is still being released into the air layers above, creating a trapped warm pocket.
• Stagnant Air: The presence of this warm air mass shuts off normal convection. Instead of rising away, air plutons are trapped close to Earth.
• Geological Factors:     * Inversions are worsened in geographical basins where mountains are present on either side.     * Mountains prevent wind from dispersing smog or particulate matter.     * The physical barrier of the mountains makes it difficult for the warm air mass to move out of the area, exacerbating pollution conditions.

Environmental and Health Impacts of Thermal Inversions

• Visual Evidence: A picture from La (Los Angeles) illustrates a shocking and extreme example of air plutons trapped by an inversion.
• Respiratory Irritation: Trapped pollutants (smog, particulate matter) lead to human health issues, specifically respiratory irritation.
• Specific Health Conditions:     * Asthma: Inversions cause asthma flare-ups. Data shows a scientific link between thermal inversion events and increased emergency room visits for asthma in the following days.     * Chronic Diseases: Conditions like Chronic Obstructive Pulmonary Disorder (COPD) and emphysema are significantly worsened.
• Economic Costs:     * Healthcare Burden: Increased hospitalizations tax the healthcare system.     * Workforce Impact: Individuals cost money in lost income and jobs lose productivity due to illness.     * Tourism: Cities or countries may see decreased revenue from tourism because visitors are discouraged by poor visibility and health risks (using La as an example).
• Biological Impact on Photosynthesis:     * The thick blanket of trapped smog makes it harder for sunlight to penetrate the atmosphere.     * This results in decreased photosynthetic activity for plants.

Practice FRQ $7.3$

• Prompt: Using a diagram showing normal conditions on the left and a temperature inversion on the right, explain what the arrows indicate regarding how temperature inversions impact air plutons such as smog.
• Analysis: In the normal condition, arrows pointing upwards indicate convection and the dispersal of pollutants. In the thermal inversion, the arrows indicate that pollutants are suppressed and held near the surface because the warm inversion layer acts as a cap, preventing vertical movement.