AP Environmental Science

## Learning Outcomes

- Explain how unequal solar heating and the Coriolis effect create global atmospheric circulation
- Identify the main features of the three-cell model: Hadley, Ferrel, and Polar cells
- Understand how circulation patterns shape rainfall belts, deserts, and storm paths
- Explain how climate warming changes atmospheric moisture and why heavy precipitation often intensifies
- Distinguish between more certain climate effects and more uncertain circulation shifts
- Apply circulation ideas to subtropical drying, jet stream shifts, storm tracks, and monsoon change

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand climate change and circulation patterns, with a focus on the following syllabus points:

- Know that global atmospheric circulation redistributes heat and moisture across Earth
- Identify the three-cell model in each hemisphere: Hadley, Ferrel, and Polar cells
- Explain how the Coriolis effect changes wind direction and produces trade winds, westerlies, and polar easterlies
- Understand why rising air is linked to wetter regions and sinking air to drier regions
- Know that many deserts occur near about **30° N/S** because of descending dry air
- Explain how warming can alter Hadley cells, jet streams, storm tracks, and regional precipitation
- Understand that warmer air can hold about **7% more water vapor per °C** warming, which helps intensify heavy precipitation
- Distinguish between broad climate tendencies, such as wetter high latitudes and drier subtropics, and regional uncertainty in exact circulation changes

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why are many of the world’s deserts located near **30°** latitude?
2. How does the Coriolis effect change wind direction in the Northern and Southern Hemispheres?
3. What is the difference between a Hadley cell and a jet stream?
4. Why can heavy rainfall increase even in places where average annual rainfall changes only a little?
5. How might a poleward shift in circulation patterns affect drought risk in subtropical regions?

## Introduction

Climate change is not only about warmer temperatures. It also changes the movement of heat and moisture through the atmosphere and oceans. That matters because global circulation patterns help determine where rain falls, where dry belts form, where storms travel, and which regions face drought or flood risk.

For AP Environmental Science, this topic connects Earth systems, climate, water availability, and ecological systems change. You need to know both the basic circulation model and how warming can alter it. A strong exam answer explains the physical process first, then links that process to changes in precipitation, storm intensity, and regional climate effects.

> **Key Term: global atmospheric circulation**
> The large-scale movement of air that redistributes heat and moisture around Earth.

### How circulation patterns form and why they shape climate zones

Earth receives more solar energy at the equator than at the poles. That uneven heating is the starting point for global circulation. Warm air near the equator becomes less dense and rises. As it rises, it cools, and water vapor condenses into clouds and precipitation. This is why equatorial regions are often warm and wet.

Farther from the equator, that air loses moisture and eventually sinks. Sinking air warms as it descends, which lowers relative humidity and suppresses cloud formation. That is a major reason many dry regions form in the subtropics.

> **Key Term: three-cell model**
> A model of atmospheric circulation in which each hemisphere contains Hadley, Ferrel, and Polar cells.

The **Hadley cell** is the most important part of this model for many APES questions. Air rises near the equator, moves poleward high in the atmosphere, sinks around **30° N/S**, and returns toward the equator near the surface.

> **Key Term: Hadley cell**
> A tropical circulation cell in which warm air rises near the equator, moves poleward aloft, sinks around 30° latitude, and returns equatorward at the surface.

This explains two climate patterns students must know:

- Equatorial regions are rainy because rising air cools and condenses
- Subtropical regions are dry because descending air suppresses rainfall

That cause-and-effect chain matters more than just memorizing locations. If a question asks why tropical rainforests cluster near the equator, the answer is not only “it is hot there.” The better answer is that strong solar heating causes warm, moist air to rise, leading to cooling, condensation, and frequent rainfall. If a question asks why deserts are common near **30°**, the answer is that air descending from the Hadley cell is dry and warming, so cloud formation and precipitation are limited.

The **Ferrel cell** lies between about **30° and 60°** latitude. It is associated with the **westerlies**, which help move weather systems across midlatitudes. The **Polar cell** occurs closer to the poles, where cold, dense air sinks and precipitation is usually low.

The Coriolis effect changes the direction of moving air because Earth rotates. In the Northern Hemisphere, motion is deflected to the right. In the Southern Hemisphere, it is deflected to the left.

> **Key Term: Coriolis effect**
> The apparent deflection of moving air and water caused by Earth’s rotation.

Without the Coriolis effect, air would move much more directly north-south. With it, the surface winds become the **trade winds**, **westerlies**, and **polar easterlies**. These wind belts matter because they transport moisture, steer storms, and influence ocean currents.

> **Test Tip:** For FRQs, do not stop at naming a cell or wind belt. Add the process: unequal heating -> rising or sinking air -> humidity change -> rainfall or drought pattern.

### Jet streams, storm tracks, and why warming changes rainfall patterns

Students often confuse circulation cells with jet streams. They are related, but not the same thing. A Hadley cell is a broad circulation loop. A **jet stream** is a narrow, fast-moving band of upper-atmosphere winds.

> **Key Term: jet stream**
> A fast-moving upper-atmosphere air current that forms along strong temperature gradients and helps steer weather systems.

Jet streams are especially important in the **tropopause**, where they guide storms and influence where precipitation falls. The common path followed by many midlatitude storms is called a **storm track**.

> **Key Term: storm track**
> The typical path followed by midlatitude storm systems.

Why does climate warming affect these? First, warming changes temperature differences between regions. Since jet streams are linked to temperature gradients, their position and behavior can shift. Second, warmer air holds more water vapor. This means storms can carry and release more moisture.

A key climate relationship here is the **Clausius-Clapeyron relationship**.

> **Key Term: Clausius-Clapeyron relationship**
> The rule that the atmosphere’s moisture-holding capacity increases by about 7% for each 1°C of warming.

This is one of the most testable ideas in this subtopic. It explains why heavy precipitation often increases in a warmer climate. If a storm forms in warmer air, that storm can draw from a more moisture-rich atmosphere. As a result, rainfall extremes often intensify.

But there is an important distinction. Atmospheric moisture can rise by about **7% per °C**, yet average global precipitation increases more slowly. That is because the water cycle is also limited by the atmosphere’s energy budget. So students should not assume “7% more water vapor” means “7% more rain everywhere.”

This leads to a common exam distinction:

- **Heavy precipitation** often increases quite clearly
- **Mean annual precipitation** may change less, or may vary by region

That is why a place can experience stronger downpours without a large increase in average yearly rainfall. In some places, average rainfall may even decrease while intense rain events become more severe when they do occur.

Warming is also linked to broad circulation shifts. Evidence supports a **poleward expansion** of the tropical circulation, including the edge of the Hadley cell, though the exact size of the shift varies by region, season, and hemisphere.

If the dry subtropical belt shifts poleward, areas on its edge may face greater drying risk. This helps explain why some subtropical regions are expected to become drier, while many higher-latitude regions are expected to become wetter.

The Southern Hemisphere shows a clearer average poleward shift in jets and storm tracks than the Northern Hemisphere. That means some regions on the equatorward side of storm tracks may receive less frontal rainfall.

A simple chain to remember is:

- warming changes atmospheric temperature structure
- warmer air holds more moisture
- circulation belts and storm paths can shift
- where storms travel and where air rises or sinks changes
- rainfall patterns and drought risk change by region
  > **Test Tip:** If asked to predict precipitation change, separate the answer into two parts: thermodynamic effect and circulation effect. This shows higher-level reasoning and avoids overgeneralising.

### Regional effects: subtropical drying, monsoons, and tropical Pacific circulation

Once you know the physical pattern, you can apply it to regions. A common broad result of climate warming is:

- increased precipitation at many high latitudes
- increased drying risk in many subtropical regions

This does not mean every high-latitude place gets wetter or every subtropical place gets drier. Oceans, mountains, local wind patterns, and seasonal shifts all matter. But it is a useful large-scale pattern.

Subtropical drying links directly to the Hadley cell. If descending dry air shifts poleward, regions near the subtropical margins can spend more time under drying conditions. That increases drought risk, water stress, wildfire risk, and pressure on agriculture.

Monsoon systems are another good application. A **monsoon** is a seasonal wind system linked to land-sea heating differences and seasonal shifts in rising and sinking air. In many monsoon regions, warming can intensify rainfall because the air contains more water vapor. But the timing, strength, and location of monsoon rainfall remain harder to predict because circulation dynamics add uncertainty.

This is a classic APES evaluation point: some parts of the climate response are more certain than others.

More certain:

- warmer atmosphere
- more atmospheric moisture
- stronger heavy rainfall events in many regions

Less certain:

- exact jet stream changes in every region
- exact storm track position year to year
- exact monsoon onset and regional distribution
- exact regional shifts in tropical Pacific circulation

A tropical example is the **Walker circulation**, the east-west circulation across the tropical Pacific linked to trade winds, rising air, and sinking air.

> **Key Term: Walker circulation**
> An east-west tropical Pacific circulation pattern linked to trade winds, upwelling, and rainfall distribution.

Under normal conditions, trade winds push warm surface water westward across the tropical Pacific. Warm water piles up in the western Pacific, helping create rising air and rainfall there. In the eastern Pacific, colder nutrient-rich water upwells near South America.

When this pattern changes during **ENSO** events, rainfall and storm patterns shift over large areas.

> **Key Term: ENSO**
> El Niño–Southern Oscillation, a natural cycle involving changes in tropical Pacific ocean temperatures, trade winds, and atmospheric circulation.

During **El Niño**:

- trade winds weaken
- warm water shifts eastward
- upwelling decreases
- rising air and rainfall shift eastward

During **La Niña**:

- trade winds strengthen
- warm water is pushed farther west
- upwelling increases
- western Pacific rainfall strengthens while some other regions become drier

Climate change may alter the background conditions in which ENSO operates, but the exact effect on ENSO behavior remains uncertain. A safer exam answer is that warming can intensify ENSO-related consequences, such as heavier rain in wet phases and worse drought in dry phases, rather than claiming a simple permanent shift to El Niño or La Niña conditions.

Another important regional effect involves the poles. Polar regions warm faster than the global average because of feedback loops involving ice and albedo. Snow and ice reflect solar energy well. When they melt, darker land or ocean absorbs more solar energy, causing more warming and more melting.

This matters for circulation because warming at the poles can alter temperature gradients and affect atmospheric and oceanic transport of heat. It also matters for polar lifes, since species adapted to extreme cold may have nowhere colder to move.

Finally, ocean circulation matters too. Surface currents are driven partly by wind, and deeper circulation depends on temperature and salinity. Since oceans store huge amounts of heat, even relatively small circulation changes can have large climate effects. Warmer surface water also holds less dissolved carbon dioxide than cooler water, which can weaken ocean carbon uptake.

> **Test Tip:** In regional examples, always connect circulation to a concrete outcome such as drought, flooding, storm path change, upwelling reduction, or marine life stress. Avoid vague phrases like “weather becomes more extreme” without saying how.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Global atmospheric circulation redistributes heat and moisture because the equator receives more solar energy than the poles
- The three-cell model includes Hadley, Ferrel, and Polar cells in each hemisphere
- Warm air rises near the equator, cools, condenses, and helps produce frequent rainfall
- Air sinking near about **30° N/S** creates dry subtropical conditions and helps explain why many deserts occur there
- The Coriolis effect deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
- Jet streams are narrow upper-atmosphere wind bands, while circulation cells are much broader movement systems
- Storm tracks are the common paths of midlatitude storms and can shift with climate warming
- The Clausius-Clapeyron relationship explains why air can hold about **7%** more water vapor per **1°C** warming
- Heavy precipitation often increases more reliably than average annual precipitation
- Warming can shift Hadley cell edges, jets, and storm tracks poleward, raising drying risk in some subtropical regions
- Monsoon rainfall may intensify because warmer air carries more moisture, but regional circulation changes remain less certain
- ENSO and Walker circulation show how ocean-atmosphere interactions can shift rainfall, drought, and storm patterns

## Key Terms and Concepts

- global atmospheric circulation
- three-cell model
- Hadley cell
- Coriolis effect
- jet stream
- storm track
- Clausius-Clapeyron relationship
- Walker circulation
- ENSO


Global Climate Change - Climate change and circulation patterns