Unit 4 · Topic 4.8 Beta

Earth's Geography and Climate

Geography reshapes climate.

Practice 2: Visual RepresentationsPractice 1: Concept Explanation

Part 1 · Hook

Why this matters

Drive east from Seattle across the Cascade Mountains and in two hours you go from mossy forest on the western slopes, which gets more than 2 meters of rain and snow a year, to sagebrush desert that gets less than 25 centimeters. The two places sit at the same latitude and get the same sunlight. A wall of mountains, and the ocean behind it, make the difference.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which way do the westerlies blow?

  1. From the west toward the east
  2. From the east toward the west
  3. From the poles toward the equator
  4. Straight up from the ground
Show the answer

Winds are named for where they come from; the westerlies blow from the west, between about 30° and 60°.

  • Correct: From the west toward the east:
  • From the east toward the west:
  • From the poles toward the equator:
  • Straight up from the ground:

2. How does air temperature change with height in the troposphere?

  1. It falls, about 6.5 °C per km
  2. It rises, about 6.5 °C per km
  3. It stays the same
  4. It falls only above 50 km
Show the answer

The troposphere is heated from the ground, so it cools with height.

  • Correct: It falls, about 6.5 °C per km:
  • It rises, about 6.5 °C per km:
  • It stays the same:
  • It falls only above 50 km:

3. What does rising air do to rainfall?

  1. It cools, so clouds and rain form
  2. It warms, so clouds evaporate
  3. It has no effect on rainfall
  4. It always brings snow
Show the answer

Rising air expands and cools, and its water vapor condenses.

  • Correct: It cools, so clouds and rain form:
  • It warms, so clouds evaporate:
  • It has no effect on rainfall:
  • It always brings snow:

Part 4 · See it

See it first

Moist air from the ocean blows toward a mountain range. On the windward side the air is pushed up the slope, so it cools; its water vapor condenses into clouds and falls as heavy rain, and the windward slope is green and forested. The air that crosses the peak has lost most of its water. On the leeward side it sinks and warms, so it is dry, and the land there is a rain shadow: a dry region, often a desert, with sparse plants.
The rain shadow effect: moist air rises up the windward side, cools and drops its rain; dry air sinks and warms on the leeward side. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Prevailing winds push moist ocean air against a mountain rangethe air rises up the windward slope, expands and cools
  2. Cool air holds less water vaporclouds form and heavy rain or snow falls on the windward side
  3. The dried-out air sinks down the leeward side and warmsthe leeward side is a dry rain shadow, often a desert
  4. Winds drag the ocean's surface, and Earth's rotation turns the moving waterocean currents carry warm water toward the poles and cold water toward the equator, warming or cooling nearby coasts

Part 6 · Key ideas

Key ideas

  • Climate depends on latitude, but also on geography: mountains, nearness to the ocean, ocean currents and elevation.
  • Rain shadow: the windward side of a mountain range is wet; the leeward side is dry, because the air lost its moisture on the way up and warms as it sinks.
  • Water heats and cools more slowly than land, so coastal climates are milder (cooler summers, warmer winters) than inland climates at the same latitude.
  • Ocean currents, driven mainly by prevailing winds and turned by the Coriolis effect and the continents, form large loops called gyres. Warm currents (the Gulf Stream) warm nearby coasts; cold currents (the California Current) cool them.
  • Higher elevation means colder air, about 6.5 °C cooler per kilometer of height.

Part 7 · Misconception

A common mistake

The wrong idea: Places at the same latitude have the same climate.

What actually happens: Latitude sets the sunlight, but mountains, oceans, ocean currents and elevation change rain and temperature a lot. Seattle and eastern Washington, or London and Calgary, share a latitude but not a climate.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Why does air drop most of its moisture as it rises up the windward side of a mountain?

  1. Rising air expands and cools, so vapor condenses
  2. Mountains have colder soil, which pulls water from the air
  3. The trees on the slope absorb water vapor directly from the air
  4. Rising air is squeezed, so the water is pushed out of it
Show the answer

As air rises, the pressure on it drops, so it expands and cools. Cool air holds less water vapor, so clouds form and rain or snow falls.

  • Correct: Rising air expands and cools, so vapor condenses: Right: cooling of rising air causes condensation.
  • Mountains have colder soil, which pulls water from the air: Soil temperature is not what makes clouds form.
  • The trees on the slope absorb water vapor directly from the air: Trees release water vapor; they do not pull it from the air.
  • Rising air is squeezed, so the water is pushed out of it: Rising air expands; it is sinking air that is compressed.

2. Why do coastal towns usually have smaller differences between summer and winter temperatures than inland towns at the same latitude?

  1. Water warms and cools more slowly than land
  2. The ocean gets far more sunlight than the land nearby does
  3. Coastal towns tend to be at lower latitudes than inland ones
  4. Sea breezes carry the cold air away from the coast in winter
Show the answer

Water needs much more energy to change its temperature than rock and soil do. So the ocean stays cooler in summer and warmer in winter, and it moderates the air over nearby land.

  • Correct: Water warms and cools more slowly than land: Right: water changes temperature slowly, which moderates nearby climates.
  • The ocean gets far more sunlight than the land nearby does: Both get about the same sunlight at the same latitude.
  • Coastal towns tend to be at lower latitudes than inland ones: The question compares towns at the same latitude.
  • Sea breezes carry the cold air away from the coast in winter: Ocean air is milder in winter, but the cause is water's slow temperature change.

Model or diagram

Moist air crossing a mountain range

Moist air from the ocean blows toward a mountain range. On the windward side the air is pushed up the slope, so it cools; its water vapor condenses into clouds and falls as heavy rain, and the windward slope is green and forested. The air that crosses the peak has lost most of its water. On the leeward side it sinks and warms, so it is dry, and the land there is a rain shadow: a dry region, often a desert, with sparse plants.
LevlPrep original diagram.

3. In the diagram, which side of the mountain range is the windward side?

  1. The side facing the ocean, where the air rises
  2. The side away from the ocean, where the air sinks
  3. The peak, where the cloud forms
  4. Both sides equally
Show the answer

The windward side faces the oncoming wind. Here the wind blows from the ocean, so the ocean-facing slope is windward.

  • Correct: The side facing the ocean, where the air rises: Right: the windward side faces the wind coming off the ocean.
  • The side away from the ocean, where the air sinks: The side where air sinks is the leeward side.
  • The peak, where the cloud forms: The peak divides the two sides; it is not a side.
  • Both sides equally: Only one side faces the wind.

4. According to the diagram, why is the land on the far side of the mountains dry?

  1. The air lost its water rising and warms as it sinks
  2. The mountains block the sunlight, so clouds rarely form there
  3. The wind blows the clouds back toward the ocean side
  4. Sinking air cools, so it can hold very little water vapor
Show the answer

The air dropped most of its water on the windward side. Sinking down the leeward side, it is compressed and warms, so it can hold more water vapor and clouds rarely form.

  • Correct: The air lost its water rising and warms as it sinks: Right: dry, warming air brings little rain to the leeward side.
  • The mountains block the sunlight, so clouds rarely form there: Clouds form from moisture and rising air, not from sunlight blocked by mountains.
  • The wind blows the clouds back toward the ocean side: The wind keeps blowing from the ocean across the mountains.
  • Sinking air cools, so it can hold very little water vapor: Sinking air warms; it does not cool.

5. The Sierra Nevada in California has wet, forested western slopes and the dry Great Basin to its east. Which feature of the diagram does this match?

  1. Moist Pacific air rising and raining on the west side
  2. Air sinking on the west side and rising on the east side
  3. A warm ocean current flowing between the two sides of it
  4. Plate boundaries that make the whole east side volcanic
Show the answer

Westerly winds bring moist air from the Pacific. It rises over the Sierra Nevada, drops rain and snow on the western slopes, and leaves the eastern side in a rain shadow.

  • Correct: Moist Pacific air rising and raining on the west side: Right: this is the rain shadow effect, with the wet side facing the Pacific.
  • Air sinking on the west side and rising on the east side: Air rises on the windward (west) side and sinks on the east.
  • A warm ocean current flowing between the two sides of it: No ocean current flows between the slopes of a mountain range.
  • Plate boundaries that make the whole east side volcanic: The dryness comes from the rain shadow, not from volcanoes.

Data

Two towns on opposite sides of a mountain range

Two towns at about the same latitude and elevation, 120 km apart, with a mountain range between them; winds blow from the ocean toward the west slope (invented data for practice)
TownSide of the rangeYearly precipitation (mm)Coldest month average (°C)Warmest month average (°C)
FernvaleWest (facing the ocean)2,200517
Dry GulchEast250-624

6. How does yearly precipitation in Fernvale compare with Dry Gulch?

  1. Fernvale gets nearly nine times as much
  2. Fernvale gets about twice as much
  3. They get about the same amount
  4. Dry Gulch gets nearly nine times as much
Show the answer

2,200mm250mm\frac{2{,}200\,\text{mm}}{250\,\text{mm}}=8.8= 8.8, nearly nine times as much.

  • Correct: Fernvale gets nearly nine times as much: Right: 2,200 ÷ 250 = 8.8.
  • Fernvale gets about twice as much: 2,200 is much more than twice 250 (500).
  • They get about the same amount: The difference is 1,950 mm, which is large.
  • Dry Gulch gets nearly nine times as much: Dry Gulch gets far less, not more.

Part 9 · Summary

Summary

Geography reshapes climate. Mountains force moist air up, so the windward side is wet and the leeward side is a dry rain shadow. Oceans heat and cool slowly, giving coasts milder climates than inland areas. Wind-driven ocean currents carry heat: warm currents warm nearby coasts, cold currents cool them. Higher elevations are colder.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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