Climate and global circulation

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Unequal Heating and Latitude

The Sun's rays strike Earth's surface most directly at the Equator, making it warmer.
Near the poles, the Sun's rays are less direct and spread over a wider area, so temperatures are lower.
Latitude (distance north or south of the equator, measured in degrees) causes unequal heating of Earth's surface.
Places receiving more solar energy are warmer; places receiving less are cooler.
Warm air rises and cool air sinks, driving atmospheric circulation that redistributes heat.

The Sun's rays and latitude

Atmospheric Circulation

The atmosphere forms convection cells that move air from the equator toward the poles and back.
There are three main air cells in each hemisphere, averaging out the movement of air.
Atmospheric circulation brings warm air toward the poles and cold polar air toward the equator.
This movement moderates Earth's temperatures, balancing heat between warm and cold regions.
Global winds blow consistently in the same direction, influenced by unequal heating and Earth's rotation.

Global wind patterns showing westerlies and trade winds

Oceanic Circulation

Wind patterns push ocean water, creating surface currents that can flow for thousands of kilometers.
Surface currents distribute heat around the planet and are a major factor influencing climate.
The global conveyor belt describes the movement of warm water from the equator to the poles, where it cools, sinks, and returns.
The Gulf Stream is a warm ocean current that carries heat from the equator to Europe, raising temperatures and precipitation.
Ocean currents transfer heat from equatorial regions to higher latitudes, preventing extreme climate differences.

The global conveyor belt showing warm shallow currents and cold salty deep currents.

Temperature Inversions

A temperature inversion occurs when air temperature in the troposphere increases with altitude, with warm air sitting over cold air.
Inversions are stable and can last for days or weeks, trapping pollutants and causing unhealthy air in cities.
They form over cold ground at night or in winter, or near coasts where cold seawater cools the air above it.
The cooler, denser air is trapped beneath warmer, less dense air, preventing mixing with the stratosphere.
Salt Lake City experiences winter inversions, where the valley is covered in fog-like clouds while the mountains are clear.

A temperature inversion trapping pollutants over a city.

Altitude and Temperature

Air temperature falls at higher altitudes because air is less dense, so its molecules collide less often and produce less heat.
Latitude is not the only factor in climate — altitude matters too.
Mount Kilimanjaro sits only about 3° south of the equator, yet its 6-kilometer peak is capped with snow year-round.
A single mountain can have very different climates at its base and at its summit.

Air temperature vs. height

Mountains and Precipitation

Mountains can cast a rain shadow: as prevailing winds push warm, moist air up the windward side, the air cools and drops its moisture as precipitation.
By the time the air crosses the peak, it has lost most of its moisture.
On the leeward (far) side, the now-dry air sinks and warms, so very little precipitation falls there.
Rain shadows are a geographic factor, alongside latitude and ocean currents, that shape a region's climate.

The rain shadow effect

Climate Definition

Climate is the 30-year average of weather conditions in an area.
Climate is determined by atmospheric and oceanic circulation, along with geographical factors.
The transfer of energy from the equator to the poles, combined with other factors, creates areas with common weather patterns.
Understanding climate requires considering both the atmosphere and the ocean as interconnected systems.

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練習題

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1.What is the 30-year average of weather in an area called?
Easy
- AClimate
- BWeather
- CTemperature
- DPrecipitation
2.Which of the following best describes latitude?
Easy
- ADistance north or south of the equator, measured in degrees
- BDistance east or west of the prime meridian, measured in degrees
- CHeight above sea level
- DThe amount of solar energy received at a location
3.Surface currents are created by global wind patterns and the rotation of the Earth.
Easy
True or false?
4.Why do places near the equator receive more solar energy than places near the poles?
Easy
- AThe Sun's rays strike the equator more directly, focusing energy over a smaller area.
- BThe equator is closer to the Sun.
- CThe equator has more hours of daylight each day.
- DThe atmosphere is thinner at the equator.
5.Match each term with its correct description.
Easy
- Climate
- Weather
- Latitude
- 30-year average of weather
- Current atmospheric conditions
- Distance from the equator
6.What is the primary reason that warm air rises and cool air sinks?
Medium
- AWarm air is less dense than cool air.
- BWarm air is denser than cool air.
- CWarm air contains more water vapor.
- DCool air has more pressure.
7.The Gulf Stream is a cold ocean current that flows from the poles toward the equator.
Easy
True or false?
8.Which of the following best explains why London has a milder winter than Quebec, despite being at a similar latitude?
Medium
- ALondon is warmed by the Gulf Stream.
- BLondon is closer to the equator.
- CQuebec is at a higher altitude.
- DLondon receives more direct sunlight.

複習筆記

## Climate and Global Circulation - **Unequal heating** by the Sun and Earth's rotation drive atmospheric and ocean circulation, redistributing heat from the tropics to the poles. - **Three circulation cells per hemisphere:** Hadley, Ferrel, and Polar cells. ### Hadley Cell - **Location:** 0°–30° latitude. - **Process:** Warm, moist air rises at the equator (low pressure), moves poleward aloft, cools, and descends at ~30° (high pressure). Surface air flows back toward the equator, deflected west by Coriolis → **trade winds** (easterlies). - Shifts north/south with the thermal equator. ### Polar Cell - **Location:** 60°–90° latitude. - **Process:** Air rises at ~60° (low pressure), moves poleward aloft, cools, and descends at the poles (high pressure). Surface air flows toward 60°, deflected west → **polar easterlies**. - Creates cold, dry conditions; record low −89.2°C in Antarctica. ### Ferrel Cell - **Location:** 30°–60° latitude. - **Process:** Surface air flows poleward from the horse latitudes (30°), deflected east → **prevailing westerlies**. Air rises at ~60° and returns aloft toward 30°. ### Key Concepts - **Coriolis effect:** Deflects moving air to the right in the Northern Hemisphere, left in the Southern. - **Horse latitudes:** High-pressure zones at ~30° with light winds and little rain. - **Jet streams:** Fast, narrow air currents at the boundary between cells (e.g., polar jet stream). - **Thermal equator:** Zone of maximum heating; shifts seasonally. - **Rossby waves:** Large-scale waves in the atmosphere that influence jet stream paths. ### Summary Table | Cell | Latitude | Surface Winds | Pressure | |---------|----------|---------------------|----------| | Hadley | 0°–30° | Trade winds (E) | Low at 0°, high at 30° | | Ferrel | 30°–60° | Westerlies (W) | High at 30°, low at 60° | | Polar | 60°–90° | Polar easterlies (E)| Low at 60°, high at 90° | *Note: Winds named from where they come (e.g., trade winds blow from east to west).*

Hadley cell: A large-scale atmospheric circulation cell that extends from the equator to about 30° latitude, driven by rising warm air at the equator and descending cool air at the subtropics.
Ferrel cell: A mid-latitude atmospheric circulation cell between about 30° and 60° latitude, characterized by surface westerlies and rising air at the polar front.
Polar cell: A high-latitude atmospheric circulation cell from about 60° to 90° latitude, with rising air at 60° and descending cold air at the poles, producing polar easterlies.
Coriolis effect: The deflection of moving air (or water) due to Earth's rotation; to the right in the Northern Hemisphere, to the left in the Southern Hemisphere.
Trade winds: Steady easterly winds that blow from the subtropical high-pressure belts toward the equator, part of the Hadley cell.
Polar easterlies: Cold, dry winds that blow from the polar high-pressure areas toward lower latitudes, deflected west by the Coriolis effect.
Prevailing westerlies: Winds that blow from the west in the mid-latitudes (30°–60°), part of the Ferrel cell.
Horse latitudes: Subtropical high-pressure belts at about 30°–35° latitude, characterized by light winds and little precipitation.
Jet stream: A narrow, fast-moving air current in the upper troposphere, often found at the boundary between circulation cells (e.g., polar jet stream).
Thermal equator: The zone of maximum solar heating on Earth, which shifts seasonally north and south of the geographic equator.
Rossby waves: Large-scale, meandering waves in the atmosphere that influence the path of jet streams and weather systems.

字卡

What drives atmospheric circulation?

Unequal heating of Earth's surface by the Sun and the rotation of Earth drive atmospheric circulation.

What are the three major atmospheric circulation cells in each hemisphere?

The Hadley cell, Ferrel cell, and polar cell.

Where is the Hadley cell located?

The Hadley cell is located near the equator, extending from about 0° to 30° latitude in each hemisphere.

Describe the movement of air in the Hadley cell.

Warm, moist air rises at the equator, moves poleward in the upper troposphere, cools and descends at about 30° latitude, then flows back toward the equator along the surface.

What causes the trade winds?

The trade winds are the surface winds in the Hadley cell that blow from the east toward the equator due to the Coriolis effect.

What are the horse latitudes?

The horse latitudes are high-pressure zones at about 30° to 35° latitude where winds diverge, typically with light winds and little precipitation.

Describe the polar cell.

At about 60° latitude, air rises, moves poleward in the upper atmosphere, cools and descends at the poles, then flows back toward 60° latitude along the surface.

What are polar easterlies?

Polar easterlies are surface winds in the polar cell that blow from the east (from the poles toward 60° latitude) due to the Coriolis effect.

What is the Coriolis effect?

The Coriolis effect is the deflection of moving air (or water) due to Earth's rotation, causing winds to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

How does the Coriolis effect influence global wind patterns?

It causes surface winds in the Hadley cell to blow from the east (trade winds), in the Ferrel cell from the west (westerlies), and in the polar cell from the east (polar easterlies).

What is the role of atmospheric circulation in redistributing heat?

Atmospheric circulation moves thermal energy from the equator toward the poles, balancing Earth's temperature distribution.

What is the thermal equator?

The thermal equator is the zone of greatest heating on Earth, which shifts north and south with the seasons.

How does the Hadley cell shift seasonally?

The Hadley cell shifts northward in June/July and southward in December/January, following the Sun's heating.

What are Rossby waves?

Rossby waves are large-scale waves in the atmosphere that influence the path of the polar jet stream.

What is the polar jet stream?

The polar jet stream is a fast-moving air current in the upper atmosphere, located between the Ferrel and polar cells, that influences weather patterns.

How does ocean circulation relate to atmospheric circulation?

Ocean circulation works together with atmospheric circulation to redistribute thermal energy around Earth.

What causes the rising of air at the equator?

Intense solar heating at the equator warms the air, causing it to expand, decrease in density, and rise.

Why does descending air create high pressure?

Descending air becomes denser and compresses, increasing surface pressure, which typically leads to clear skies and dry conditions.

What is a heat engine in the context of atmospheric circulation?

Earth's atmosphere acts as a heat engine driven by solar energy, with the Sun as the heat source and space as the heat sink, producing motion that redistributes heat.

How do tectonic uplift and plate tectonics affect atmospheric circulation over long timescales?

Tectonic uplift can alter jet streams, and plate tectonics can shift ocean currents, potentially changing major circulation patterns over hundreds of millions of years.

What is the approximate power of the Hadley cell as a heat engine?

The Hadley system is estimated to have a power of about 200 terawatts.

Why can't long-range weather predictions be made beyond ten days?

Small-scale weather systems are chaotic, and due to chaos theory (butterfly effect), predictions become unreliable beyond about ten days.

What is the lowest temperature recorded on Earth, and where was it measured?

−89.2 °C at Vostok Station in Antarctica, measured in 1983.

How does the polar cell contribute to cold surface temperatures in Antarctica?

The polar cell, combined with terrain and katabatic winds, creates very cold conditions at the surface.

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