Section 1: Earth’s Atmosphere

• Atmospheric Composition

  • Earth’s atmosphere is a mixture of gases, including the oxygen your body needs and water vapor.

  • Photosynthetic organisms use carbon dioxide and produce almost all the oxygen in the atmosphere.

  • In northern latitudes, carbon dioxide concentration increases during summer and decreases during winter.

  • Microorganisms in swamps, rice paddies, and soil produce nitrogen and methane. The microorganisms in the digestive tracts of animals such as termites, cows, and sheep produce methane.

  • The ozone layer shields Earth from ultraviolet rays that can harm living organisms.

• Atmospheric Structure

  • Earth’s atmosphere extends more than 1,000 km above Earth’s surface.

  • Troposphere: a layer extending about 12 km, on average, above Earth’s surface; temperature normally decreases with height.

  • Temperature Inversion: When temperature increases with height.

  • Above the troposphere is the stratosphere, which includes the ozone layer

  • Temperature decreases with height in the troposphere up to the tropopause. Above the tropopause, temperature increases with height in the stratosphere.

• Heating the Atmosphere

  • The energy that heats the atmosphere ultimately comes from the Sun.

  • About half of the solar energy that strikes the atmosphere passes right through and is absorbed by Earth’s surface.

  • Earth’s surface absorbs some of these waves and heats up

  • Some gases in the atmosphere, such as water vapor and carbon dioxide, absorb infrared waves and are heated. These gases then emit infrared waves in all directions. Some of these infrared waves are absorbed by Earth’s surface.

  • Greenhouse Effect: The emission of infrared waves by gases in Earth’s atmosphere

  • Convection transfers thermal energy by movement of air from place to place

  • Latent Heat: the thermal energy released when water changes state from a gas to a liquid, or from a liquid to a solid.

• A Varied Surface

  • As Earth’s surface warms, it also heats the air in contact with the surface.

  • Surfaces covered by snow, ice, water, vegetation, and bare soil absorb different amounts of energy.

  • This uneven heating of Earth’s surface causes differences in pressure and wind.

• Water in the Atmosphere

  • The uneven heating of Earth’s surface produces currents of air that generally rise upward over warm areas.

  • Clouds form when water vapor cools and condenses to form droplets of water.

  • For cloud droplets to form, water vapor in the rising air must cool enough for condensation to occur.

  • Three basic cloud types are stratus clouds, cumulus clouds, and cirrus clouds.

    • Stratus clouds are layered sheetlike clouds that form at heights below 2,000 m. Stratus clouds often produce long periods of rain.

    • Cumulus clouds are puffy in shape and also usually form below 2,000 m. Under certain conditions, a cumulus cloud can grow upward and become a towering cumulonimbus cloud, or thunderhead.

    • Cirrus clouds form at heights above 6,000 m and are made of ice crystals.

  • Different types of clouds form at different heights in the atmosphere. Clouds with vertical development grow upward from where they first form.

  • The formation of clouds often results in precipitation. The main types of precipitation are rain, snow, sleet, and hail.

• Global Water Cycle

  • Water is always moving between Earth’s surface and the atmosphere. This movement of water is called the water cycle.

    • Energy from the Sun evaporates water from Earth’s surface.

    • Rising water vapor cools and condenses to form clouds.

    • Condensation results in precipitation that falls over land and water.

    • Some of the water that falls on land evaporates and some runs off into bodies of water. There evaporation also occurs and the cycle continues.

Section 2: Weather

• Atmospheric Pressure

  • The atmosphere exerts a force on any surface in contact with it.

  • The pressure exerted on a surface is the force exerted on the surface divided by the surface area.

  • When air is heated, it expands and becomes less dense.

  • Wind is the movement of air from a region of high pressure to a region of low pressure.

  • Large-scale differences in atmospheric pressure cause global wind patterns.

  • In each hemisphere there are four major pressure zones—the equatorial low, the subtropical high, the subpolar low and the polar high. These pressure zones produce the trade winds, the westerlies, and the polar easterlies.

  • Westerlies: winds that blow from the west in middle latitudes

  • Two factors produce these global patterns — unequal heating between the equator and poles and the rotation of Earth.

  • Jet Streams: control many weather processes, such as storm development.

• High and Low Pressure Systems

  • A low-pressure system, or low, is a region of the lower troposphere where the air pressure is lower than the air pressure around it.

  • Air flowing into a low-pressure system rises, while air flowing out of a high-pressure system sinks. In the northern hemisphere, the Coriolis effect causes air to flow counter-clockwise around a low and clockwise around a high.

  • A high-pressure system, or high, is a region of the lower troposphere where the air pressure is higher than the air pressure around it.

  • The direction of airflow is also affected by Earth’s rotation. The effect of Earth’s rotation on the motion of objects is called the Coriolis effect.

• Air Masses and Weather Fronts

  • Weather around low-pressure systems is produced by interaction of air masses—large volumes of air with relatively uniform moisture and temperature.

  • Air masses can be polar or tropical and continental or maritime.

    • Continental air originates over land. It is relatively dry and can be extremely cold or extremely warm.

    • Maritime air masses are moist because they originate over the oceans.

  • Weather Fronts: associated with low pressure systems.

  • Weather fronts occur when air masses of different temperatures meet.

  • In a warm front, warm air rises gently above the cold air, usually forming layered, stratus-type clouds or fog—a cloud with its base on the ground. Most layered clouds produce only drizzle or steady rain. In a cold front, cold air pushes the warm air aloft in a random and chaotic fashion forming cumulus clouds.

• Severe Weather

  • Severe weather often occurs in the continental United States.

  • A thunderstorm can form when moist air rises high in the atmosphere.

  • A tall cumulonimbus cloud produces a thunderstorm. Rising air causes an updraft that helps pull moist air into the cloud. Condensing water vapor releases latent heat, which warms the surrounding air and helps maintain the updraft.

  • The falling precipitation in a thunderstorm cools the air around it
    and causes cold, dense air to sink to the ground. This sinking current of cold air is called a downdraft

  • Downdrafts can result in a form of severe weather called downbursts.

  • Two types of violent wind storms that differ greatly in their origins and effects are tornadoes and hurricanes.

    • Tornadoes are intense, short-lived, localized storms in the mid-latitudes.

    • Hurricanes are tropical storms that cover vast areas and last for days.

Section 3: Climate

• Climate and Weather

  • Traditionally, climate means the long-term average of weather conditions—wind, temperature, precipitation, moisture, and other aspects of weather. Climate also describes the annual variations of these conditions and their extremes.

  • Climate is best considered as part of the whole Earth system.

  • The atmosphere includes the air around us.

  • Biosphere: all living organisms and the environments in which they live.

  • The hydrosphere is liquid water in oceans, lakes, rivers, soil, and underground. The cryosphere is frozen water in snow, ice, and glaciers.

  • The lithosphere is the solid Earth, including its soil, rocks, and mantle.

  • Water is exchanged between the atmosphere and cryosphere when frozen precipitation falls, melts, and evaporates.

  • Gases, such as nitrogen, oxygen, and carbon dioxide, are exchanged between the atmosphere and the biosphere.

• What Causes Climate?

  • The primary factor that determines the climate at a given location is the location’s latitude.

  • The long-term average temperature at a given location depends mainly on the location’s latitude.

  • The difference between average temperatures in high latitudes and low latitudes is not as large in summer as in winter.

  • Maritime Climate: A climate that is strongly affected by an ocean

  • Continental Climate: A climate that is not directly affected by an ocean

  • Wind and pressure patterns determine precipitation.

  • Climates tend to be even drier in the subtropics on the eastern sides of the subtropical highs.

  • Average precipitation over North America tends to be greater over coastal regions. Lines connecting points of equal precipitation are called isohyets.

  • Another factor affecting precipitation is the prevailing winds

  • Mountains can affect precipitation patterns

  • Air on the windward side of a mountain rises and cools, resulting in precipitation. The climate on the windward side of a mountain tends to be cool and wet. The air on the leeward side of the mountain is drier and warms as it sinks. The climate on the leeward side is often hot and dry.

  • Coasts and lake shores can affect regional climates

  • Lake-effect snow occurs in areas east and south of the Great Lakes.

  • Sea Breeze: blows from the water toward the land in the afternoon, when the land is warmer than the water.

  • A land breeze occurs when cool, dense air over land creates high pressure causing the air to blow from the land toward the sea.

  • Many small-scale variations also occur within the large-scale climate patterns.

• Types of Climates

  • Climate zone influences the types of vegetation that will grow there.

  • Geographer Glenn Trewartha and meteorologist Lyle Horn designed a system to classify climates.

    • It has three major divisions—cold or boreal, arid and semi-arid, and climates with adequate heat and precipitation. The last includes temperate, subtropical, and tropical climates. These divisions are closely correlated with vegetation.

Section 4: Earth’s Changing Climates

• Seasonal Changes

  • Seasonal changes occur as Earth orbits the Sun.

  • Seasonal changes are smallest near the equator and become greater with increasing latitude.

• Long-Term Changes

  • The period when large ice sheets cover much of Earth’s surface is called an ice age.

  • At the peak of the last ice age, about 18,000 years ago, thick ice sheets covered much of North America.

  • A warm period from the eleventh to the fourteenth centuries, called the medieval climatic optimum, enabled the Vikings to colonize Greenland. This was followed by a cooler period called the Little Ice Age that lasted from about the seventeenth century to the middle of the nineteenth century.

  • Periodic changes in Earth’s orbit around the Sun change the amount and distribution of solar radiation that reaches Earth.

  • The amount of energy emitted by the Sun also changes.

  • Volcanoes can affect climate over a period of a few years.

    • Large eruptions can produce droplets of sulfuric acid in the stratosphere that reflect incoming solar radiation. This reduces the amount of solar energy reaching Earth’s surface.

• The Human Factor

  • Human activities, such as the burning of fossil fuels, manufacturing processes, deforestation, draining of wetlands, and intensive agriculture, have affected Earth’s atmosphere.

  • When organisms die and decay, some carbon is stored as humus in the soil and some is released back to the atmosphere as carbon dioxide.

  • The concentration of atmospheric carbon dioxide can be affected by changes in the carbon cycle.

  • Water vapor and carbon dioxide are examples of greenhouse gases. These gases produce the greenhouse effect by absorbing energy emitted by Earth and emitting energy back to Earth’s surface.

    • Human activities have also increased the atmospheric concentration of other greenhouse gases, including methane and nitrous oxide.

  • About one-third of the increase in atmospheric carbon dioxide is due to land-use changes.

  • An increase in the concentration of atmospheric greenhouse gases could increase the greenhouse effect and raise surface temperatures.

  • 9Global Warming: An increase in the average global temperature

  • Over Antarctica an annual seasonal reduction occurs in atmospheric ozone.

    • The concentration of springtime ozone over Antarctica has decreased over the past several decades.

• El Niño and La Niña

  • El Niño: the warming of the Pacific Ocean off the coast of western South America that occurs every 3 to 10 years.

    • El Niño can dramatically alter global weather patterns.

    • Parts of the Pacific Coast Highway in California have been disrupted by erosion and mudslides caused by El Niño.

  • La Niña: occurs when trade winds in the Pacific are unusually strong and equatorial oceanic surface temperatures are colder than normal.

    • La Niña can cause drought in the southern United States and excess rainfall in the northwest.