Transcript air mass
Chapter 17-21 • Atmosphere • Weather • Climate 17.1 Atmosphere Characteristics Composition of the Atmosphere Weather is constantly changing, and it refers to the state of the atmosphere at any given time and place. Climate, however, is based on observations of weather that have been collected over many years. Climate helps describe a place or region. 17.1 Atmosphere Characteristics Composition of the Atmosphere Major Components • Air is a mixture of different gases and particles, each with its own physical properties. 17.1 Atmosphere Characteristics Composition of the Atmosphere Variable Components • Water vapor is the source of all clouds and precipitation. Like carbon dioxide, water vapor absorbs heat given off by Earth. It also absorbs some solar energy. • Ozone is a form of oxygen that combines three oxygen atoms into each molecule (O3). • If ozone did not filter most UV radiation and all of the sun’s UV rays reached the surface of Earth, our planet would be uninhabitable for many living organisms. 17.1 Atmosphere Characteristics Composition of the Atmosphere Human Influence • Emissions from transportation vehicles account for nearly half the primary pollutants by weight. 17.1 Atmosphere Characteristics Height and Structure of the Atmosphere The atmosphere rapidly thins as you travel away from Earth until there are too few gas molecules to detect. Pressure Changes • Atmospheric pressure is simply the weight of the air above. 17.1 Atmosphere Characteristics Height and Structure of the Atmosphere Temperature Changes • The atmosphere can be divided vertically into four layers based on temperature. • The troposphere is the bottom layer of the atmosphere where temperature decreases with an increase in altitude. • The stratosphere is the layer of the atmosphere where temperature remains constant to a height of about 20 kilometers. It then begins a gradual increase until the stratopause. 17.1 Atmosphere Characteristics Height and Structure of the Atmosphere Temperature Changes • The mesosphere is the layer of the atmosphere immediately above the stratosphere and is characterized by decreasing temperatures with height. • The thermosphere is the region of the atmosphere immediately above the mesosphere and is characterized by increasing temperatures due to the absorption of very short-wave solar energy by oxygen. 17.1 Atmosphere Characteristics Earth-Sun Relationships Earth’s Motions • Earth has two principal motions—rotation and revolution. Earth’s Orientation • Seasonal changes occur because Earth’s position relative to the sun continually changes as it travels along its orbit. Tilt of Earth’s Axis 17.1 Atmosphere Characteristics Earth-Sun Relationships Solstices and Equinoxes • The summer solstice is the solstice that occurs on June 21 or 22 in the Northern Hemisphere and is the “official” first day of summer. • The winter solstice is the solstice that occurs on December 21 or 22 in the Northern Hemisphere and is the “official” first day of winter. • The autumnal equinox is the equinox that occurs on September 22 or 23 in the Northern hemisphere. • The spring equinox is the equinox that occurs on March 21 or 22 in the Northern Hemisphere. 17.1 Atmosphere Characteristics Length of Daylight The length of daylight compared to the length of darkness also is determined by Earth’s position in orbit. Solstices and Equinoxes 17.2 Heating the Atmosphere Energy Transfer as Heat Heat is the energy transferred from one object to another because of a difference in the objects’ temperature. Temperature is a measure of the average kinetic energy of the individual atoms or molecules in a substance. 17.2 Heating the Atmosphere Energy Transfer as Heat Three mechanisms of energy transfer as heat are conduction, convection, and radiation. Conduction • Conduction is the transfer of heat through matter by molecular activity. Convection • Convection is the transfer of heat by mass movement or circulation within a substance. Energy Transfer as Heat 17.2 Heating the Atmosphere Energy Transfer as Heat Electromagnetic Waves • The sun emits light and heat as well as the ultraviolet rays that cause a suntan. These forms of energy are only part of a large array of energy emitted by the sun, called the electromagnetic spectrum. Visible Light Consists of an Array of Colors 17.2 Heating the Atmosphere Energy Transfer as Heat Radiation • Radiation is the transfer of energy (heat) through space by electromagnetic waves that travel out in all directions. • Unlike conduction and convection, which need material to travel through, radiant energy can travel through the vacuum of space. 17.2 Heating the Atmosphere Energy Transfer as Heat Radiation • All objects, at any temperature, emit radiant energy. • Hotter objects radiate more total energy per unit area than colder objects do. • The hottest radiating bodies produce the shortest wavelengths of maximum radiation. • Objects that are good absorbers of radiation are good emitters as well. 17.2 Heating the Atmosphere What Happens to Solar Radiation? When radiation strikes an object, there usually are three different results. 1. Some energy is absorbed by the object. 2. Substances such as water and air are transparent to certain wavelengths of radiation. 3. Some radiation may bounce off the object without being absorbed or transmitted. Solar Radiation 17.2 Heating the Atmosphere What Happens to Solar Radiation? Reflection and Scattering • Reflection occurs when light bounces off an object. Reflection radiation has the same intensity as incident radiation. • Scattering produces a larger number of weaker rays that travel in different directions. 17.2 Heating the Atmosphere What Happens to Solar Radiation? Absorption • About 50 percent of the solar energy that strikes the top of the atmosphere reaches Earth’s surface and is absorbed. • The greenhouse effect is the heating of Earth’s surface and atmosphere from solar radiation being absorbed and emitted by the atmosphere, mainly by water vapor and carbon dioxide. 17.3 Temperature Controls Why Temperatures Vary Factors other than latitude that exert a strong influence on temperature include heating of land and water, altitude, geographic position, cloud cover, and ocean currents. 17.3 Temperature Controls Why Temperatures Vary • • • • Land and Water --Land heats more rapidly and to higher temperatures than water. Land also cools more rapidly and to lower temperatures than water. Geographic Position--The geographic setting can greatly influence temperatures experienced at a specific location. Altitude--The altitude can greatly influence temperatures experienced at a specific location. Cloud Cover and Albedo Albedo is the fraction of total radiation that is reflected by any surface. Many clouds have a high albedo and therefore reflect back to space a significant portion of the sunlight that strikes them. Clouds Reflect and Absorb Radiation 17.3 Temperature Controls World Distribution of Temperature Isotherms are lines on a weather map that connect points where the temperature is the same. • Isotherms generally trend east and west and show a decrease in temperatures from the tropics toward the poles. 18.1 Water in the Atmosphere Water’s Changes of State Precipitation is any form of water that falls from a cloud. When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere. 18.1 Water in the Atmosphere Water’s Changes of State Solid to Liquid • The process of changing state, such as melting ice, requires that energy be transferred in the form of heat. • Latent heat is the energy absorbed or released during a change in state. Liquid to Gas • Evaporation is the process of changing a liquid to a gas. • Condensation is the process where a gas, like water vapor, changes to a liquid, like water. 18.1 Water in the Atmosphere Water’s Changes of State Solid to Gas • Sublimation is the conversion of a solid directly to a gas without passing through the liquid state. • Deposition is the conversion of a vapor directly to a solid. 18.1 Water in the Atmosphere Humidity Humidity is a general term for the amount of water vapor in air. Saturation • Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure. • When saturated, warm air contains more water vapor than cold saturated air. 18.1 Water in the Atmosphere Humidity Relative Humidity • Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure. • To summarize, when the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity. 18.1 Water in the Atmosphere Humidity Dew Point • Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation. Measuring Humidity • A hygrometer is an instrument to measure relative humidity. • A psychrometer is a hygrometer with dry- and wetbulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity. 18.2 Cloud Formation Air Compression and Expansion Adiabatic Temperature Changes • When air is allowed to expand, it cools, and when it is compressed, it warms. Expansion and Cooling • Dry adiabatic rate is the rate of cooling or heating that applies only to unsaturated air. • Wet adiabatic rate is the rate of adiabatic temperature change in saturated air. Cloud Formation by Adiabatic Cooling 18.2 Cloud Formation Processes That Lift Air Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting. Orographic Lifting • Orographic lifting occurs when mountains act as barriers to the flow of air, forcing the air to ascend. • The air cools adiabatically; clouds and precipitation may result. 18.2 Cloud Formation Processes That Lift Air Frontal Wedging • A front is the boundary between two adjoining air masses having contrasting characteristics. 18.2 Cloud Formation Processes That Lift Air Convergence • Convergence is when air flows together and rises. Localized Convective Lifting • Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy. 18.2 Cloud Formation Stability Density Differences • Stable air tends to remain in its original position, while unstable air tends to rise. Stability Measurements • Air stability is determined by measuring the temperature of the atmosphere at various heights. • The rate of change of air temperature with height is called the environmental lapse rate. 18.2 Cloud Formation Stability Degrees of Stability • A temperature inversion occurs in a layer of limited depth in the atmosphere where the temperature increases rather than decreases with height. Stability and Daily Weather • When stable air is forced above the Earth’s surface, the clouds that form are widespread and have little vertical thickness compared to their horizontal dimension. 18.2 Cloud Formation Condensation For any form of condensation to occur, the air must be saturated. Types of Surfaces • Generally, there must be a surface for water vapor to condense on. • Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air. 18.3 Cloud Types and Precipitation Types of Clouds Clouds are classified on the basis of their form and height. • Cirrus (cirrus = curl of hair) are clouds that are high, white, and thin. • Cumulus (cumulus = a pile) are clouds that consist of rounded individual cloud masses. • Stratus (stratus = a layer) are clouds best described as sheets or layers that cover much or all of the sky. 18.3 Cloud Types and Precipitation Types of Clouds High Clouds • Cirrus clouds are high, white, and thin. • Cirrostratus clouds are flat layers of clouds. • Cirrocumulus clouds consist of fluffy masses. Middle Clouds • Altocumulus clouds are composed of rounded masses that differ from cirrocumulus clouds in that altocumulus clouds are larger and denser. • Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot. 18.3 Cloud Types and Precipitation Types of Clouds Low Clouds • Stratus clouds are best described as sheets or layers that cover much or all of the sky. • Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches. • Nimbostratus clouds are the main precipitation makers. Cloud Classification 18.3 Cloud Types and Precipitation Types of Clouds Clouds of Vertical Development • Some clouds do not fit into any one of the three height categories mentioned. Such clouds have their bases in the low height range but often extend upward into the middle or high altitudes. 18.3 Cloud Types and Precipitation Fog Fog is defined as a cloud with its base at or very near the ground. Fog Caused by Cooling • As the air cools, it becomes denser and drains into low areas such as river valleys, where thick fog accumulations may occur. Fog Caused by Evaporation • When cool air moves over warm water, enough moisture may evaporate from the water surface to produce saturation. 18.3 Cloud Types and Precipitation How Precipitation Forms For precipitation to form, cloud droplets must grow in volume by roughly one million times. Cold Cloud Precipitation • The Bergeron process is a theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water. 18.3 Cloud Types and Precipitation How Precipitation Forms Cold Cloud Precipitation Supercooled water is the condition of water droplets that remain in the liquid state at temperatures well below 0oC. Supersaturated air is the condition of air that is more concentrated than is normally possible under given temperature and pressure conditions. Warm Cloud Precipitation The collision-coalescence process is a theory of raindrop formation in warm clouds (above 0oC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop 18.3 Cloud Types and Precipitation Forms of Precipitation The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lower few kilometers of the atmosphere. Rain and Snow • In meteorology, the term rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm. • At very low temperatures (when the moisture content of air is low) light fluffy snow made up of individual six-sided ice crystals forms. 18.3 Cloud Types and Precipitation Forms of Precipitation Rain and Snow • Sleet is the fall of clear-to-translucent ice. • Hail is produced in cumulonimbus clouds. • Hailstones begin as small ice pellets that grow by collecting supercooled water droplets as they fall through a cloud. Largest hailstone recorded 19.1 Understanding Air Pressure Air Pressure Defined Air pressure is the pressure exerted by the weight of air. Air pressure is exerted in all directions— down, up, and sideways. The air pressure pushing down on an object exactly balances the air pressure pushing up on the object. 19.1 Understanding Air Pressure Measuring Air Pressure A barometer is a device used for measuring air pressure. When air pressure increases, the mercury in the tube rises. When air pressure decreases, so does the height of the mercury column. 19.1 Understanding Air Pressure Factors Affecting Wind Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. The unequal heating of Earth’s surface generates pressure differences. Solar radiation is the ultimate energy source for most wind. Three factors combine to control wind: pressure differences, the Coriolis effect, and friction. 19.1 Understanding Air Pressure Factors Affecting Wind Pressure Differences • A pressure gradient is the amount of pressure change occurring over a given distance. • Closely spaced isobars—lines on a map that connect places of equal air pressure—indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds. Isobars 19.1 Understanding Air Pressure Factors Affecting Wind Coriolis Effect • The Coriolis effect describes how Earth’s rotation affects moving objects. In the Northern Hemisphere, all free-moving objects or fluids, including the wind, are deflected to the right of their path of motion. In the Southern Hemisphere, they are deflected to the left. 19.1 Understanding Air Pressure Factors Affecting Wind Friction • Friction acts to slow air movement, which changes wind direction. • Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour in a west-to-east direction. 19.2 Pressure Centers and Winds Highs and Lows Cyclones are centers of low pressure. Anticyclones are centers of high pressure. In cyclones, the pressure decreases from the outer isobars toward the center. In anticyclones, just the opposite is the case— the values of the isobars increase from the outside toward the center. 19.2 Pressure Centers and Winds Highs and Lows Cyclonic and Anticyclonic Winds • When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counterclockwise around a low. Around a high, they blow clockwise. • In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow of air outward around an anticyclone. Cyclonic and Anticyclonic Winds 19.2 Pressure Centers and Winds Highs and Lows Weather and Air Pressure • Rising air is associated with cloud formation and precipitation, whereas sinking air produces clear skies. Weather Forecasting • Weather reports emphasize the locations and possible paths of cyclones and anticyclones. • Low-pressure centers can produce bad weather in any season. Airflow Patterns, Surface and Aloft 19.2 Pressure Centers and Winds Global Winds The atmosphere balances these differences by acting as a giant heat-transfer system. This system moves warm air toward high latitudes and cool air toward the equator. Non-Rotating Earth Model On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form. 19.2 Pressure Centers and Winds Global Winds Rotating Earth Model • If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells. • Trade winds are two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of the subtropical highs. • Westerlies are the dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs. 19.2 Pressure Centers and Winds Global Winds Rotating Earth Model • Polar easterlies are winds that blow from the polar high toward the subpolar low. These winds are not constant like the trade winds. • A polar front is a stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin. 19.2 Pressure Centers and Winds Global Winds Influence of Continents • The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. In the Northern Hemisphere, where land masses break up the ocean surface, large seasonal temperature differences disrupt the pressure pattern. • Monsoons are the seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea. In summer, the wind blows from sea to land. 19.3 Regional Wind Systems Local Winds The local winds are caused either by topographic effects or by variations in surface composition—land and water—in the immediate area. Land and Sea Breezes • In coastal areas during the warm summer months, the land surface is heated more intensely during the daylight hours than an adjacent body of water is heated. As a result, the air above the land surface heats, expands, and rises, creating an area of lower pressure. At night the reverse takes place. 19.3 Regional Wind Systems Local Winds Valley and Mountain Breezes • In mountainous regions during daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. Because this warmer air on the mountain slopes is less dense, it glides up along the slope and generates a valley breeze. After sunset the pattern may reverse. 19.3 Regional Wind Systems How Wind Is Measured Wind Direction The prevailing wind is the wind that blows more often from one direction than from any other. In the United States, the westerlies consistently move weather from west to east across the continent. Wind Speed An anemometer is an instrument that resembles a cup and is commonly used to measure wind speed. 19.3 Regional Wind Systems El Niño and La Niña El Niño • El Niño is the name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. • At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters. • A major El Niño episode can cause extreme weather in many parts of the world. Normal Conditions El Niño Conditions 19.3 Regional Wind Systems El Niño and La Niña La Niña • Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns. 20.1 Air Masses Air Masses and Weather Air Masses • An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude. Movement of Air Masses • As it moves, the characteristics of an air mass change and so does the weather in the area over which the air mass moves. 20.1 Air Masses Classifying Air Masses In addition to their overall temperature, air masses are classified according to the surface over which they form. Air Masses Are Classified by Region 20.1 Air Masses Weather in North America Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses. 20.1 Air Masses Weather in North America Continental Polar Air Masses • Continental polar air masses are uniformly cold and dry in winter and cool and dry in summer. Maritime Tropical Air Masses • Maritime tropical air masses are warm, loaded with moisture, and usually unstable. • Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States. 20.1 Air Masses Weather in North America Maritime Polar Air Masses • Maritime polar air masses begin as cP air masses in Siberia. The cold, dry continental polar air changes into relatively mild, humid, unstable maritime polar air during its long journey across the North Pacific. • Maritime polar air masses also originate in the North Atlantic off the coast of eastern Canada. Maritime Polar Air Masses 20.1 Air Masses Weather in North America Continental Tropical Air Masses • Only occasionally do cT air masses affect the weather outside their source regions. However, when a cT air mass moves from its source region in the summer, it can cause extremely hot, droughtlike conditions in the Great Plains. • Movements of cT air masses in the fall result in mild weather in the Great Lakes region, often called Indian summer. 20.2 Fronts Formation of Fronts When two air masses meet, they form a front, which is a boundary that separates two air masses. Types of Fronts • Warm Front-forms when warm air moves into an area formerly covered by cooler air. • Cold Front-forms when cold, dense air moves into a region occupied by warmer air. Formation of a Warm Front Formation of a Cold Front 20.2 Fronts Stationary Fronts • Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to the line of the front. In Such cases, the surface position of the front does not move, and a stationary front forms. Occluded Fronts • When an active cold front overtakes a warm front, an occluded front forms. 20.2 Fronts Middle-Latitude Cyclones Middle-latitude cyclones are large centers of low pressure that generally travel from west to east and cause stormy weather. 20.2 Fronts The Role of Airflow Aloft More often than not, air high up in the atmosphere fuels a middle-latitude cyclone. 20.3 Severe Storms Thunderstorms A thunderstorm is a storm that generates lightning and thunder. Thunderstorms frequently produce gusty winds, heavy rain, and hail. Occurrence of Thunderstorms-- At any given time, there are an estimated 2000 thunderstorms in progress on Earth. The greatest number occur in the tropics where warmth, plentiful moisture, and instability are common atmospheric conditions. Development of Thunderstorms-- Thunderstorms form when warm, humid air rises in an unstable environment Stages in the Development of a Thunderstorm 20.3 Severe Storms Tornadoes Tornadoes are violent windstorms that take the form of a rotation column of air called a vortex. The vortex extends downward from a cumulonimbus cloud. Occurrence and Development of Tornadoes • Most tornadoes form in association with severe thunderstorms. • A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm. Fujita Tornado Intensity Scale 20.3 Severe Storms Hurricanes Whirling tropical cyclones that produce winds of at least 119 kilometers per hour are known in the United States as hurricanes. Occurrence of Hurricanes • Most hurricanes form between about 5 and 20 degrees north and south latitude. The North Pacific has the greatest number of storms, averaging 20 per year. Satellite View of Hurricane Floyd 20.3 Severe Storms Hurricanes Development of Hurricanes • Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air. • The eye is a zone of scattered clouds and calm averaging about 20 kilometers in diameter at the center of a hurricane. • The eye wall is a doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane. 20.3 Severe Storms Hurricanes Hurricane Intensity • The intensity of a hurricane is described using the Saffir-Simpson scale. • A storm surge is the abnormal rise of the sea along a shore as a result of strong winds. Saffir-Simpson Hurricane Scale 21.1 Factors That Affect Climate Factors That Affect Climate Latitude • Polar zones are between 66.5o north and south latitudes and the poles. The sun’s rays strike Earth at a very small angle in the polar zones. Elevation • The higher the elevation is, the colder the climate. Topography • Topographic features such as mountains play an important role in the amount of precipitation that falls over an area. 21.1 Factors That Affect Climate Factors That Affect Climate Water Bodies • Large bodies of water such as lakes and oceans have an important effect on the temperature of an area because the temperature of the water body influences the temperature of the air above it. Atmospheric Circulation • Global winds are another factor that influences climate because they distribute heat and moisture around Earth. 21.1 Factors That Affect Climate Factors That Affect Climate Vegetation • Vegetation can affect both temperature and the precipitation patterns in an area. 21.2 World Climates The Köppen Climate Classification System The Köppen climate classification system uses mean monthly and annual values of temperature and precipitation to classify climates. 21.2 World Climates Humid Tropical Climates Humid tropical climates are without winters. Every month in such a climate has a mean temperature above 18oC. The amount of precipitation can exceed 200 cm per year. Wet Tropical • Wet tropical climates have high temperatures and much annual precipitation. 21.2 World Climates Humid Tropical Climates Tropical Wet and Dry • Tropical wet and dry climates are climates that transition between the wet tropics and the subtropical steppes. 21.2 World Climates Humid Mid-Latitude Climates Climates with mild winters have an average temperature in the coldest month that is below 18oC but above -3oC. Climates with severe winters have an average temperature in the coldest month that is below -3oC. 21.2 World Climates Humid Mid-Latitude Climates Humid Mid-Latitude with Mild Winters • A humid subtropical climate is generally located on the eastern side of a continent and is characterized by hot, sultry summers and cool winters. • A marine west coast climate is found on o o windward coasts from latitudes 40 to 65 and is dominated by maritime air masses. Winters are mild, and summers are cool. 21.2 World Climates Humid Mid-Latitude Climates Humid Mid-Latitude With Mild Winters • A dry-summer subtropical climate is a climate located on the west sides of continents between 30o and 45o latitude. It is the only humid climate with a strong winter precipitation maximum. 21.2 World Climates Humid Mid-Latitude Climates Humid Mid-Latitude With Severe Winters • A subarctic climate is found north of the humid continental climate and south of the polar climate; it is characterized by bitterly cold winters and short, cool summers. Places within this climate realm experience the highest annual temperature ranges on Earth. 21.2 World Climates Dry Climates A dry climate is one in which the yearly precipitation is not as great as the potential loss of water by evaporation. 21.2 World Climates Polar Climates Polar climates are those in which the mean temperature of the warmest month is below 10oC. 21.2 World Climates Highland Climates In general, highland climates are cooler and wetter than nearby areas at lower elevations. 21.3 Climate Changes Natural Processes That Change Climates Volcanic Eruptions • The presence of volcanic aerosols (ash, dust, and sulfur-based aerosols) in the air increases the amount of solar radiation that is reflected back into space. This causes Earth’s lower atmosphere to cool. Ocean Circulation • Changes in ocean circulation also can result in short-term climate fluctuations. 21.3 Climate Changes Natural Processes That Change Climates Solar Activity • When the sun is most active, it contains dark blemishes called sunspots. The formation of sunspots appears to correspond with warm periods in Europe and North America. Earth Motions • Geographic changes in Earth’s land and water bodies cause changes in climate. • Changes in the shape of Earth’s orbit and the tilt of Earth on its axis are other Earth motions that affect global climates. 21.3 Climate Changes Human Impact on Climate Changes The Greenhouse Effect • The greenhouse effect is a natural warming of both Earth’s lower atmosphere and Earth’s surface from solar radiation being absorbed and emitted by the atmosphere. Global Warming • As a result of increased levels of carbon dioxide and other greenhouse gases, global temperatures have increased. This increase is called global warming.