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Climate Patterns
Explained

Discover how global circulation patterns shape the climates of different regions: from scorching deserts to lush rainforests.

Earth & Space Science

What Is Climate?

Climate is the long-term average of weather conditions in a region, typically measured over \( 30 \) years or more. It includes patterns of temperature, precipitation, humidity, and wind.

Temperature
Average highs, lows, and seasonal range
Precipitation
Rain, snow, and moisture patterns year-round
Wind
Prevailing direction, speed, and seasonal shifts

Why Does Climate Vary by Region?

The Sun heats Earth's surface unevenly. Near the equator, sunlight strikes almost directly overhead, delivering intense energy. Near the poles, sunlight arrives at a low angle and spreads over a larger area, delivering less energy per unit of surface.

Equator
Direct
rays
Angled rays
spread
N Pole
S Pole
HOT
COLD
COLD

The Three Circulation Cells

Earth's atmosphere is organized into three major convection cells in each hemisphere. Warm air rises, cools aloft, sinks, and returns: creating predictable wind belts.

\( 0° \)
Equator
\( 30°\text{N} \)
\( 60°\text{N} \)
\( 90°\text{N} \)
\( 30°\text{S} \)
\( 60°\text{S} \)
\( 90°\text{S} \)
Hadley
Ferrel
Polar
Trade Winds
Westerlies
Polar Easterlies
Solid: rising air
Dashed: sinking air

From Cells to Wind Belts

Where air rises (equator, \( 60° \) latitudes), low pressure forms and precipitation is heavy. Where air sinks (\( 30° \) latitudes, poles), high pressure creates dry conditions. The Coriolis effect curves these winds.

1
Hadley Cell (\( 0° \) to \( 30° \)): Warm air rises at the equator, flows poleward aloft, sinks at \( 30° \). Surface winds curve west: these are the Trade Winds.
2
Ferrel Cell (\( 30° \) to \( 60° \)): Air sinking at \( 30° \) flows poleward at the surface, curves east. These are the Westerlies that drive most mid-latitude weather.
3
Polar Cell (\( 60° \) to \( 90° \)): Cold dense air sinks at the poles, flows equatorward, curves west. These are the Polar Easterlies.

The Coriolis Effect

Because Earth rotates, moving air does not travel in a straight north-south line. Instead, it curves: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Rotation
Intended
Actual
(curves right)
Actual
(curves left)
NH
SH
Northern Hemisphere
Winds curve RIGHT
of their path
Southern Hemisphere
Winds curve LEFT
of their path

Core Rule: Latitude Determines Climate

The single most powerful predictor of a region's climate is its latitude: how far north or south of the equator it sits. Latitude determines which circulation cell controls the region and how much solar energy it receives.

\( 0° \) to \( 30° \)
Tropical and subtropical: hot, often wet near equator, dry at \( 30° \)
\( 30° \) to \( 60° \)
Temperate: moderate temperatures, variable weather from Westerlies
\( 60° \) to \( 90° \)
Polar: very cold, low precipitation, stable high pressure

Weather vs. Climate

Weather Climate
Time Scale Minutes to days Decades to centuries
Variability Changes rapidly Relatively stable
Measurement Today's temperature, humidity \( 30 \)-year averages
Predictability Accurate up to ~\( 10 \) days Long-term patterns reliable
Example "It's raining now" "Summers here are dry"

Other Factors That Shape Climate

Latitude and circulation cells are the primary drivers, but several additional factors modify a region's climate:

Altitude
Higher elevation = cooler temperatures. Temperature drops about \( 6.5°\text{C} \) per \( 1{,}000\text{ m} \) of elevation gain.
Ocean Currents
Warm currents raise coastal temperatures; cold currents lower them and reduce rainfall.
Distance from Ocean
Coastal areas have milder, wetter climates. Inland areas are more extreme: hotter summers, colder winters.
Mountain Barriers
Mountains force air up, causing rain on the windward side and dry "rain shadow" on the leeward side.

How to Connect Circulation to Climate

Use this step-by-step approach when asked to explain why a region has a particular climate:

1
Identify the latitude. Is the location near \( 0° \), \( 30° \), \( 60° \), or \( 90° \)? This tells you the dominant circulation cell.
2
Determine rising or sinking air. Air rising = low pressure = wet. Air sinking = high pressure = dry.
3
Name the wind belt. Trade Winds, Westerlies, or Polar Easterlies: this tells you the prevailing wind direction.
4
Check modifying factors. Altitude, ocean currents, distance from the coast, mountain barriers: these refine your prediction.
5
State the connection. Clearly link the circulation pattern to the climate outcome using evidence.

Example 1: The Sahara Desert

The Sahara Desert in northern Africa sits near \( 30°\text{N} \) latitude. Why is it one of the driest places on Earth?

\( 0° \)
\( 30°\text{N} \)
Sahara
Air rises
Flows poleward aloft
Air sinks
= high pressure
= very dry
Result:
Hot, dry desert

Example 2: The Amazon Rainforest

The Amazon Basin sits near the equator (\( 0° \) to \( 10°\text{S} \)). It receives some of the highest rainfall on Earth. Why?

\( 0° \)
Amazon
Air rises
= low pressure
NE Trades
SE Trades
Result:
Warm, very wet

Example 3: London, UK at \( 51°\text{N} \)

London sits at about \( 51°\text{N} \), well within the Ferrel Cell. It has mild, wet winters and cool summers. How do global circulation and other factors explain this?

1
Latitude: \( 51°\text{N} \): within the Ferrel Cell (\( 30° \) to \( 60° \)), dominated by the Westerlies.
2
Prevailing winds blow from the southwest, carrying warm, moist air off the North Atlantic.
3
Gulf Stream: A warm ocean current raises water temperatures off western Europe, keeping winters mild (rarely below freezing).
4
Result: Maritime temperate climate: frequent clouds, moderate rain spread year-round, rarely extreme heat or cold.

The Rain Shadow Effect

Mountains interact with wind belts to create dramatically different climates on each side. This is the rain shadow effect.

Windward
(wet side)
Moist wind
Air rises,
cools, rains
Leeward
(dry side)
Air sinks,
warms, dries
Mountain

Common Mistakes to Avoid

✕ Confusing Weather and Climate
A cold day in summer does not mean the climate is changing. Climate is a \( 30 \)-year average, not a single event.
✕ Ignoring the Coriolis Effect
Forgetting that winds curve: Trade Winds blow from the northeast (NH), not straight south. Wind belt names depend on the curve.
✕ "Closer to the Sun = Hotter"
The tropics are warmer because of the angle of sunlight, NOT because they are physically closer to the Sun. Earth's tilt and shape matter far more than distance.

Climate Zones Summary

Zone Latitude Cell / Wind Belt Climate Type
Tropical \( 0° \) to \( 15° \) Hadley / ITCZ Hot, wet
Subtropical \( 15° \) to \( 30° \) Hadley / Trade Winds Hot, dry (deserts)
Temperate \( 30° \) to \( 60° \) Ferrel / Westerlies Mild, variable
Subpolar \( 60° \) to \( 75° \) Polar front (rising) Cool, wet
Polar \( 75° \) to \( 90° \) Polar / Polar Easterlies Very cold, dry

Question 1 of 5

The Sahara Desert is located near \( 30°\text{N} \) latitude. Which of the following best explains why it is so dry?

Question 2 of 5

The Coriolis effect causes winds in the Northern Hemisphere to:

Question 3 of 5

A city at \( 50°\text{N} \) on the western coast of a continent has mild, rainy winters. Which combination of factors best explains this?

Question 4 of 5

Two cities are at the same latitude (\( 35°\text{N} \)). City A is on the coast; City B is \( 1{,}000\text{ km} \) inland. Compared to City A, City B most likely has:

Question 5 of 5

A mountain range runs north-south. Prevailing Westerlies carry moist air from the ocean toward the mountains. What climate pattern is most likely on the eastern (leeward) side?