Lesson Objectives

• Describe the effect of latitude on climate.
• Diagram the Hadley, Ferrell, and Polar atmospheric circulation cells and show how they influence the climate of various locations.
• Discuss the other important location factors that influence a location’s climate: position in the global wind belts, proximity to a large water body, position relative to a mountain range, and others.

Vocabulary

• continental climate
• Intertropical Convergence Zone (ITCZ)
• maritime climate

Introduction

Although almost anything can happen with the weather, climate is more predictable. The weather on a particular winter day in San Diego may be colder than on the same day in Lake Tahoe, but, on average, Tahoe’s winter climate is significantly colder than San Diego’s (Figure below). Climate then is the long-term average of weather. Good climate is why we choose to vacation in Hawaii in February, even though the weather is not guaranteed to be good!

What is Climate?

Climate is the average of weather in that location over a long period of time, usually for at least 30 years. A location’s climate can be described by its air temperature, humidity, wind speed and direction, and the type, quantity, and frequency of precipitation. Climate can change, but only over long periods of time.

The climate of a region depends on its position relative to many things. These factors are described in the next sections.

Latitude

The main factor influencing the climate of a region is latitude because different latitudes receive different amounts of solar radiation. To review from the Earth’s Atmosphere chapter:

• The equator receives the most solar radiation. Days are equally long year-round and the sun is just about directly overhead at midday.
• The polar regions receive the least solar radiation. The night lasts six months during the winter. Even in summer, the sun never rises very high in the sky. Sunlight filters through a thick wedge of atmosphere, making the sunlight much less intense. The high albedo, because of ice and snow, reflects a good portion of the sun’s light.

Atmospheric Circulation Cells

Recall from the Earth’s Atmosphere chapter the circulation cells and global wind belts (Figure below):

The position of a region relative to the circulation cells and wind belts has a great effect on its climate. In an area where the air is mostly rising or sinking, there is not much wind.

The ITCZ

The Intertropical Convergence Zone (ITCZ) is the low pressure area near the equator in the boundary between the two Hadley Cells. The air rises so that it cools and condenses to create clouds and rain (Figure below). Climate along the ITCZ is therefore warm and wet. Early mariners called this region the doldrums because their ships were often unable to sail because there were no steady winds.

The ITCZ migrates slightly with the season. Land areas heat more quickly than the oceans. Because there are more land areas in the Northern Hemisphere, the ITCZ is influenced by the heating effect of the land. In Northern Hemisphere summer, it is approximately 5° north of the equator while in the winter it shifts back and is approximately at the equator. As the ITCZ shifts, the major wind belts also shift slightly north in summer and south in winter, which causes the wet and dry seasons in this area (Figure below).

Hadley Cell and Ferrell Cell Boundary

At about 30°N and 30°S, the air is fairly warm and dry because much of it came from the equator where it lost most of its moisture at the ITCZ. At this location the air is descending, and sinking air warms and causes evaporation

Mariners named this region the horse latitudes. Sailing ships were sometimes delayed for so long by the lack of wind that they would run out of water and food for their livestock. Sailors tossed horses and other animals over the side after they died. Sailors sometimes didn’t make it either.

Ferrell Cell and Polar Cell Boundary

The polar front is around 50° to 60°, where cold air from the poles meets warmer air from the tropics. The meeting of the two different air masses causes the polar jet stream, which is known for its stormy weather. As the Earth orbits the Sun, the shift in the angle of incoming sunlight causes the polar jet stream to move. Cities to the south of the polar jet stream will be under warmer, moister air than cities to its north. Directly beneath the jet stream, the weather is often stormy and there may be thunderstorms and tornadoes.

Prevailing Winds

The prevailing winds are the bases of the Hadley, Ferrell, and Polar Cells. These winds greatly influence the climate of a region because they bring the weather from the locations they come from. For example, in California, the predominant winds are the westerlies blowing in from the Pacific Ocean, which bring in relatively cool air in summer and relatively warm air in winter. Local winds also influence local climate. For example, land breezes and sea breezes moderate coastal temperatures.

Continental Position

When a particular location is near an ocean or large lake, the body of water plays an extremely important role in affecting the region’s climate.

• A maritime climate is strongly influenced by the nearby sea. Temperatures vary a relatively small amount seasonally and daily. For a location to have a true maritime climate, the winds must most frequently come off the sea.
• A continental climate is more extreme, with greater temperature differences between day and night and between summer and winter.

The ocean’s influence in moderating climate can be seen in the following temperature comparisons. Each of these cities is located at 37°N latitude, within the westerly winds (Figure below).

Ocean Currents

The temperature of the water offshore influences the temperature of a coastal location, particularly if the winds come off the sea. The cool waters of the California Current bring cooler temperatures to the California coastal region. Coastal upwelling also brings cold, deep water up to the ocean surface off of California, which contributes to the cool coastal temperatures. Further north, in southern Alaska, the upwelling actually raises the temperature of the surrounding land because the ocean water is much warmer than the land. The important effect of the Gulf Stream on the climate of northern Europe is described in the chapter, Earth’s Oceans.

Altitude and Mountain Ranges

Air pressure — and air temperature — decrease with altitude. The closer molecules are packed together, the more likely they are to collide. Collisions between molecules give off heat, which warms the air. At higher altitudes, the air is less dense and air molecules are more spread out and less likely to collide. A location in the mountains has lower average temperatures than one at the base of the mountains. In Colorado, for example, Lakewood (5,640 feet) average annual temperature is 62°F (17°C), while Climax Lake (11,300 feet) is 42°F (5.4°C).

Mountain ranges have two effects on the climate of the surrounding region:

• rainshadow effect, which brings warm dry climate to the leeward size of a mountain range, was described in the Earth’s Atmosphere chapter (Figure below).
• separation in the coastal region from the rest of the continent. Since a maritime air mass may have trouble rising over a mountain range, the coastal area will have a maritime climate but the inland area on the leeward side will have a continental climate.

Lesson Summary

• A region’s position on the globe and on a continent determines its fundamental climate.
• Latitude determines a location’s solar radiation and location within the wind belts.
• If a region is near a large water body, its climate will be influenced by that water body.
• Mountain ranges separate land areas from the oceans and create rainshadow effect, which influences climate.

Review Questions

1. Describe the weather of the location where you are right now. How is the weather today typical or atypical of your usual climate for today’s date?

2. In what two ways could a desert be found at 30°N?

3. Could a desert form at 45°N latitude? Explain how.

4. Why is there so little wind in the locations where the atmospheric circulation cells meet?

5. If it is windy at 30°N where there is normally little wind, does that mean the model of the atmospheric circulation cells is wrong?

6. What is the ITCZ? What winds do you expect to find there?

7. How does the polar jet stream move from summer to winter? How does this affect the climate of the locations where it moves?

8. Imagine two cities in North America. How does the climate of a city at 45°N near the Pacific Ocean differ from one at the same latitude near the Atlantic Coast?

9. Why does the ocean water off California cool the western portion of the state, while the water off the southeastern United States warms that region?

10. Think about what you know about surface ocean currents. How would you expect the climate of western South America to be influenced by the Pacific Ocean? Could this same effect happen in the Northern Hemisphere?

11. The Andes Mountains line western South America. How do you think they influence the climate of that region and the lands to the east of them?

Points to Consider

• Describe how two cities at the same latitude can have very different climates. For example, Tucson, Arizona, has a hot, dry desert climate and New Orleans, Louisiana, has a warm, muggy climate even though both cities are at approximately the same latitude.
• How does climate influence the plants and animals that live in a particular place?
• Would you expect climate at similar latitudes to be the same or different on the opposite side of the equator. For example, how would the climate of a city at 45°N be similar or different to one at 45°S latitude?