Physical Geography - Version 1 Unit 7 Elements of Weather & Climate PDF Free Download

Physical Geography - Version 1 Unit 7 Elements of Weather & Climate PDF Download

Figure Clouds near Lone Pine , California . Image by Jeremy is used under a license . UNIT ELEMENTS OF WEATHER CLIMATE Goals Objectives of this unit Describe the various aspects and elements of weather and atmospheric water . Explain how air masses and weather fronts together form cyclones and describe the three phases a thunderstorm goes through in its life cycle . Differentiate between weather and climate and explain their . Characterize the five general types of climate as defined by the Koppen climate classification . GEOGRAPHY

WEATHER ATMOSPHERIC MOISTURE If someone across the country asks you what the weather is like today , you need to consider several factors . Air temperature , humidity , wind speed , the amount and types of clouds , and precipitation are all part of a thorough weather report . In this unit , you will learn about many of these features in more detail . Weather is what is going on in the atmosphere at a particular place at a particular time . Weather can change rapidly . A location weather depends on air temperature air pressure fog humidity cloud cover precipitation wind speed and direction . All of these are directly related to the amount of energy that is in the system and where that energy is . The ultimate source ofthis energy is the sun . Climate is the average of a weather over time . The climate for a particular place is steady and changes only very slowly . Climate is determined by many factors , including the angle of the Sun , the likelihood of cloud cover , and the air pressure . All of these factors are related to the amount of energy that is found in that location over time most meteorologists use data spanning nearly 30 years to identify a region climate . Humidity Humidity is the amount of water vapor in the air in a particular parcel of air . We usually use the term to mean relative humidity , the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain . If the humidity today is 80 , it means that the air contains 80 of the total amount of water it can hold at that temperature . What will happen if the humidity increases to more than 100 ?

The excess water condenses and forms precipitation . This is a simplistic look at this topic , because depending on the temperature of the air , the capacity of water content per kilogram of air changes . Warm air can hold more water vapor than cool air , so raising or lowering the temperature can change the air relative humidity . The temperature at which air saturated air can condense is called the dew point . This term makes sense , because the water condenses from the air as the dew . A smaller scale example of this would be a cup full of ice water . Depending on the temperature and humidity levels for the day , if the contents in the cup are cooler than the surrounding air , the glass will cause the moisture in the air around the cup to condense along the glass surface . The image below shows the relationship between relative humidity , dew point and overall air temperature . GEOGRAPHY

Amount of Water in Air at 100 Relative Humidity Across a Range of Tem Ca with tool at 100 I I I I I I Relative Humidity ( Dew Point ) 80 70 60 50 Water in Air ( grams per ) 10 20 30 40 50 Tem re ( degrees ) Figure Diagram Explaining the Calculation of Relative Humidity . Image is used under a Alike license . Clouds Clouds have a big influence on weather by preventing solar radiation from reaching the ground absorbing warmth that is from the ground and as the source of precipitation . When there are no clouds , there is less insulation . As a result , cloudless days can be extremely hot , and cloudless nights can be very cold . For this reason , cloudy days tend to have a lower range of temperatures than clear days . There are a variety of conditions needed for clouds to form . First , clouds form when air reaches its dew point . This can happen in two ways Air temperature stays the same but humidity increases . This is common in locations that are warm and humid . GEOGRAPHY

Humidity can remain the same , but temperature decreases . When the air cools enough to reach 100 humidity , water droplets form . The air cools when it comes into contact with a cold surface or when it rises . Rising air creates clouds when it has been warmed at or near the ground level and then is pushed up over a mountain or mountain range or is thrust over a mass of cold , dense air . Water vapor is not visible unless it condenses to become a cloud . Water vapor condenses around a nucleus , such as dust , smoke , or a salt crystal . This forms a tiny liquid droplet . Billions of these water droplets together make a cloud . Clouds are classified in several ways . The most common classification used today divides clouds into three separate cloud groups which are determined by their altitude and if precipitation is occurring or not . clouds form from ice crystals where the air is extremely cold and can hold little water vapor . Cirrus , and are all names of high clouds . clouds are small , white puffs that ripple across the sky , often in rows . Cirrus clouds may indicate that a storm is coming . clouds , including and clouds , may be made of water droplets , ice crystals or both , depending on the air temperatures . Thick and broad clouds are gray or . They often cover the entire sky and usually mean a large storm , bearing a lot of precipitation , is coming . clouds are nearly all water droplets . Stratus , and clouds are common low clouds . clouds are thick and dark that produce precipitation . Clouds with the prefix grow vertically instead of horizontally and have their bases at low altitudes and their tops at high or middle altitudes . Clouds grow vertically when strong unstable air currents are rising upward . GEOGRAPHY

Ci Cirrus High Cumulonimbus AC AS Cu . Cumulus Low Stratus Figure 735 Cloud Identification Diagram . Image is used under an Alike license . FOG Fog is a cloud located at or near the ground . When humid air near the ground cools below its dew point , fog is formed . The several types of fog that each form in a different way . Radiation fog forms at night when skies are clear and the relative humidity is high . As the ground cools the bottom layer of air will cool below its dew point . Tule fog is an extreme form of radiation fog found in some regions . San Francisco , California , is famous for its summertime advection fog . Warm , moist Pacific Ocean air blows over the cold California current and cools below its dew point . Sea breezes bring the fog onshore . Steam fog appears in autumn when cool air moves over a warm lake . Water evaporates from the lake surface and condenses as it cools , appearing like steam . Warm humid air travels up a hillside and cools below its dew point to create upslope fog . Advection fog This type of fog forms from surface contact of horizontal winds . This fog can occur in windy conditions . Warm air , moist air blows in from the south and ifthere is snow or cool moisture on the ground it will come in contact with the warm , moist winds . This contact between the air and ground will cause the air blowing in to become cool . Then the dew point rises and creates high humidity and forms fog . Radiation fog This fog forms when all solar energy exits the earth and allows the temperature to meet up with the dew point . The best condition to have radiation fog is GEOGRAPHY

when it had rained the previous night . This helps to moisten up the soil and create higher dew points . This makes it easier for the air to become saturated and form fog . However , the winds must be less than 15 to prevent moisture and dry air from mixing . Valley Fog Valley fog forms in the Valley when the soil is moist from previous rainfall . As the skies clear solar energy exits earth and allows the temperature to cool near or at the dew point . This form deep fog , so dense it sometimes called tule fog Precipitation Precipitation is an extremely important part of the weather . Some precipitation forms in place . The most common precipitation comes from clouds . Rain or snow droplets grow as they ride air currents in a cloud and collect other droplets . They fall when they become heavy enough to escape from the rising air currents that hold them up in the cloud . Millions of cloud droplets will combine to make only one raindrop . If temperatures are cold , the droplet will hit the ground as a snowflake . In meteorology , the various types of precipitation often include the character or phase ofthe precipitation which is falling to ground level . There are three distinct ways that precipitation can occur . Convective precipitation is generally more intense , and of shorter duration , than stratiform precipitation ( arranged in layers ) Orographic precipitation occurs when moist air is forced upwards over rising terrain , such as a mountain . Precipitation can fall in either liquid or solid phases , or transition between them at the freezing level . Liquid forms of precipitation include rain and drizzle and dew . Rain or drizzle which freezes on contact within a subfreezing air mass gains the preceding adjective freezing , becoming known as freezing rain or freezing drizzle . Frozen forms of precipitation include snow , ice crystals , ice pellets ( sleet ) hail , and graupel . Their respective are classified either by rate of fall or by visibility restriction . GEOGRAPHY

AIR MASSES Where an air mass receives its characteristics of temperature and humidity is called the source region . Air masses are slowly pushed along by winds when an air mass moves over a new region , it shares its temperature and humidity with that region . The temperature and humidity of a particular location depend partly on the characteristics of the air mass that sits over it . Storms arise if the air mass and the region it moves over have different characteristics . For example , when a colder air mass moves over the warmer ground , the bottom layer of air is heated . That air rises , forming clouds , rain , and thunderstorms . How would a moving air mass form an inversion ?

Storms arise if the air mass and the region it moves over have different characteristics . For example , when a colder air mass moves over the warmer ground , the bottom layer of air is heated . That air rises , forming clouds , rain , and sometimes thunderstorms . How would a moving air mass form an inversion ?

When a warmer air mass travels over colder ground , the bottom layer of air cools and , because of its high density , is trapped near the ground . In general , cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles . This brings heat to cold areas and cools down warm areas . It is one of the many processes that act towards balancing out the planet temperatures . Weather Front A front is identified as the zone between two masses of air , and these zones respond differently based on the temperature of the air merging . There are four types of fronts , Cold , Warm , Occluded and Stationary . With cold fronts and warm fronts , the air mass at the leading edge of the front gives the front its name . In other words , a cold front is right at the leading edge of moving cold air and a warm front marks the leading edge of moving warm air . Warm Front Stationary Front Figure Diagram Depicting how Weather Fronts are Drawn on Weather Maps . Image by OER Team is used under a license COLD FRONT Imagine that you are standing in one spot as a cold front of air approaches . Along the cold front , the denser , cold air pushes up the warm air , causing the air pressure to decrease . If the humidity is high enough , some types of cumulus clouds will develop . High in the atmosphere , winds blow ice crystals from the tops of these clouds to create and cirrus clouds . At the front , there will be a line of rain showers , snow showers , or thunderstorms with blustery winds . A squall line is a line of severe thunderstorms that forms along a cold front . Behind the GEOGRAPHY

front is the cold air mass . This mass is drier , so precipitation stops . The weather may be cold and clear or only partly cloudy . Winds may continue to blow into the zone at the front . The weather at a cold front varies with the season . Spring Summer The air is unstable so thunderstorms or tornadoes may form . Spring Ifthe temperature gradient is high , strong winds blow . Autumn Strong rains fall over a large area . Winter The cold air mass is likely to have formed in the frigid arctic so there are frigid temperatures and heavy snows . Cold Front Figure Cold Front ( Blue Arrow ) Moving in and Forcing an Air Parcel ( Green Arrow ) Upward . Image is in the public domain . WARM FRONT Along a warm front , a warm air mass slides over a cold air mass . When warm , less dense air moves over the colder , denser air , the atmosphere is relatively stable . Imagine that you are on the ground in the wintertime under a cold winter air mass with a warm front approaching . The transition from cold air to warm air takes place over a long distance so the first signs of changing weather appear long before the front is actually over you . Initially , the air is cold the cold air mass is above you and the warm air mass is above it . High cirrus clouds mark the transition from one air mass to the other . Over time , cirrus clouds become thicker and clouds form . As the front approaches , and clouds appear and the sky turns gray . Since it is winter , snowflakes fall . The clouds thicken and clouds form . Snowfall increases . Winds grow stronger as the approaches . As the front gets closer , the cold air mass is just above you , but the warm air mass is not too far above that . The weather worsens . As the warm air mass approaches , temperatures will rise , and snow GEOGRAPHY

turns to sleet and freezing rain . Warm and cold air mix at the front , leading to the formation of stratus clouds and fog . Warm Front Figure Image ofa Warm Front ( Green Arrow ) Pushing Away a Parcel . Image in the public domain . OCCLUDED FRONTS An occluded front usually forms around a system . The occlusion starts when a cold front catches up to a warm front . The air masses , in order from front to back , are cold , warm , and then cold again . The Effect curves the boundary where the two fronts meet towards the pole . If the air mass that arrives third is colder than either of the first two air masses , that air mass slips beneath them both . This is called a cold occlusion . If the air mass that arrives third is warm , that air mass rides over the other air mass . This is called a warm occlusion . The weather at an occluded front is especially fierce right at the occlusion . Precipitation and shifting winds are typical . The Pacific Coast has frequently occluded fronts . Figure Image of an Occluded Front . Image is in the public domain . Remember , a weather front is the boundary between two air masses of different densities . At the center of each air , mass is typically a high pressure . This means that weather is typically GEOGRAPHY

sunny within air masses , but their temperatures could vary with the season and humidity could vary based on the source region of the air mass . Now more often than not , these weather fronts are not isolated events . Often , they are part of a larger rotating system called a cyclone . This type of cyclone will be discussed later in this chapter , but as an introduction , it is a system that is usually mixing warmer air from the south ( in the Northern Hemisphere ) and colder air from the north STATIONARY FRONT At a stationary front , the air masses do not move . A front may become stationary if an air mass is stopped by a barrier , such as a mountain range . A stationary front may bring days of rain , drizzle , and fog . Winds usually blow parallel to the front , but in opposite directions . After several days , the front will likely break apart . When a cold air mass takes the place of a warm air mass , there is a cold front . TYPES OF EXTREME WEATHER Weather is experienced every day , but only some days experience extreme weather , such as storms . A storm magnitude can vary immensely depending on whether they are composed of warm or cold air , originating off the ocean or off a continent , occurring in summer or winter , and many other factors . The effects of storms also vary depending on whether they strike a populated area or a natural landscape . Thunderstorms Thunderstorms are extremely common Worldwide there are 14 million per year that per day . Most precipitate a lot of rain in a small area quickly , but some storms can be severe and highly damaging . Thunderstorms form when ground temperatures are high , ordinarily in the late afternoon or early evening in spring and summer . All thunderstorms go through a life cycle . The first stage is called the cumulus stage , where an air parcel is forced to rise , cool , and condense , called the lower condensation level , to develop into a cumulus cloud . The process of water vapor condensing into liquid water releases large quantities of latent heat , which makes the air within the cloud warmer , and unstable causing the cloud continues to grow upward like a hot air balloon . These rising air parcels , called updrafts , prevent precipitation from falling from the cloud . But once the precipitation becomes too heavy for the updrafts to hold up , the moisture begins to fall creating within the cloud . The also begin to pull cold , dry air from outside the cloud toward GEOGRAPHY

the ground in a process called entrainment . Once the precipitation begins to fall from the cloud , the storm has reached the mature stage . During this stage , updrafts and exist and the cumulonimbus is called a cell . If the updrafts reach the top of the troposphere , the cumulus cloud will begin to spread outward creating a defined anvil . At the same time , the spread within the cloud and at first make the cloud become wider , but eventually overtaking the updrafts . Cool form when precipitation and the cool air from entrainment are dragged down to the lower regions of a thunderstorm . It is also during the mature stage when the storm is most intense producing strong , gusting winds , heavy precipitation , lightning , and possibly hail . Once the overtake the updrafts , which also prevents the release of latent heat energy , the thunderstorm will begin to weaken into the third and final stage , called the dissipating stage . During this stage , light precipitation and become the dominant feature within the cloud as it weakens . In all , only 20 of the moisture within the cloud fell as precipitation whereas the other 80 evaporates back into the atmosphere . With severe thunderstorms , the are so intense that when they hit the ground it sends warm air from the ground upward into the storm . The warm air gives the convection cells more energy . Rain and hail grow before gravity pulls them to Earth . Severe thunderstorms can last for hours and can cause a lot of damage because of high winds , flooding , intense hail , and tornadoes . Thunderstorms can form individually or in squall lines along a cold front . In the United States , squall lines form in spring and early summer in the Midwest where the maritime tropical ( air mass from the Gulf of Mexico meets the continental polar ( air mass from Canada . So much energy collects in cumulonimbus clouds that a huge release of electricity , called lightning , may result . The electrical discharge may be between one part of the cloud and another , two clouds , or a cloud and the ground Tornadoes Tornadoes , also called twisters , are fierce products of severe thunderstorms . As the air in a thunderstorm rises , the surrounding air races in to fill the gap , forming a funnel . A tornado lasts from a few seconds to several hours . The average wind speed is about 177 ( 110 ) but some winds are much faster . A tornado travels over the ground at about 45 per hour ( 28 miles per hour ) and goes about 25 ( 16 miles ) before losing energy and disappearing . An individual tornado strikes a small area , but it can destroy everything in its path . Most injuries and deaths from tornadoes are caused by flying debris . In the United States , an average of 90 people is killed by tornadoes each year . The most violent of tornadoes account for 70 of GEOGRAPHY

the deaths by tornadoes . Tornadoes form at the front of severe thunderstorms . Lines ofthese thunderstorms form in the spring where maritime tropical ( and continental polar ( air masses meet . Although there is an average of 770 tornadoes annually , the number of tornadoes each year varies greatly . In late April 2011 , the region of the United States experienced a tornado Super Outbreak , totaling over 300 tornadoes , traveling through 15 states , in only three days . In addition to the meeting of and , the jet stream was blowing strongly in from the west . The entire region was alerted to the possibility of tornadoes in those late April days . But meteorologists can only predict tornado danger over a very wide region . No one can tell exactly where and when a tornado will touch down . Once a tornado is sighted on the radar , its path is predicted , and a warning is issued to people in that area . The exact path is unknown because the tornado movement is not very predictable . The intensity of tornadoes is measured on the Scale , which assigns a value based on wind speed and damage . Enhanced Scale Light damage Moderate damage Considerable damage Severe damage Devastating damage 200 Incredible damage Figure The Enhanced Scale . Image on Commons by , Cyclones Cyclones can be the most intense storms on Earth . A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a center . The swirling air rises and cools , creating clouds and precipitation . There are two types of cyclones middle latitude ( cyclones and tropical cyclones . cyclones are the main cause of winter storms in the middle latitudes . Tropical cyclones are also known as hurricanes . GEOGRAPHY

Cyclones cyclones , sometimes called extratropical cyclones , form at the polar front when the temperature difference between two air masses is large . These air masses blow past each other in opposite directions . Effect deflects winds to the right in the Northern Hemisphere , causing the winds to strike the polar front at an angle . Warm and cold fronts form next to each other . Most winter storms in the middle latitudes , including most of the United States and Europe , are caused by cyclones . The warm air at the cold front rises and creates a cell . Winds rush into the low pressure and create a rising column of air . The air twists , rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere . Ifthe rising air contains enough moisture , rain , or snow may fall . cyclones form in winter in the and move eastward with the westerly winds . These to storms can reach to ( 625 to miles ) in diameter and produce winds up to 125 ( 75 miles ) per hour . Like tropical cyclones , they can cause extensive beach erosion and flooding . cyclones are especially fierce in the and New England states where they are called nor , because they come from the northeast . About 30 nor strike the region each year . Hurricanes Tropical cyclones have many names . They are called hurricanes in the North Atlantic and Eastern Pacific oceans , typhoons in the western Pacific Ocean , tropical cyclones in the Indian Ocean , and in the waters near Australia . By any name , they are the most damaging storms on Earth . Hurricanes arise in the tropical latitudes ( between and 25 ) in summer and autumn when sea surface temperatures are ( or higher . The warm seas create a large humid air mass . The warm air rises and forms a cell , known as a tropical depression . Thunderstorms materialize around the tropical depression . If the temperature reaches or exceeds 28 ( the air begins to rotate around the low pressure ( counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere ) As the air rises , water vapor condenses , releasing energy from latent heat . If wind shear is low , the storm builds into a hurricane within two to three days . Hurricanes are large systems with high winds . The exception is the relatively calm eye of the storm where the air is rising upward . Rainfall can be as high as ( per hour , resulting in about 20 billion metric tons of water released daily in a hurricane . The release of latent heat generates enormous amounts of energy , nearly the total annual electrical power consumption GEOGRAPHY

of the United States from one storm . Hurricanes can also generate tornadoes . Hurricanes are strange phenomena because they are deadly monsters , yet have a gentle , but cold heart . The anatomy of a hurricane is fairly simple , though the processes involved are quite complex . As a disturbance forms , the warm , moist air rushes towards the low pressure to rise upward to form towering thunderstorms . Around the disturbance is a wall of clouds called an . Within the , the wind speeds are the greatest , the clouds are the tallest , the atmospheric pressure is at its lowest , and precipitation is most intense . Moving away from the are organized , intense thunderstorms , called spiral rain bands that rotate around and toward the storm . These are the first hurricanes are assigned to categories based on their wind speed . The categories are listed on the Simpson Scale . Table Simpson Hurricane Scale Category Wind Speed ( Type of Damage 74 95 Some Damage 96 110 Extensive Damage 111 129 Devastating 130 156 Catastrophic Damage 157 and above Catastrophic Damage Hurricanes move with the prevailing winds . In the Northern Hemisphere , they originate in the trade winds and move to the west . When they reach the latitude of the , they switch direction and travel toward the north or northeast . Hurricanes may cover 800 ( 500 mi ) in one day . Damage from hurricanes comes from the high winds , rainfall , and storm surge . Storm surge occurs as the storm center comes onto land , causing the sea level to rise unusually high . A storm surge is often made worse by the hurricane high winds blowing seawater across the ocean onto the shoreline . Flooding can be devastating , especially along coastlines such as the Atlantic and Gulf Coasts . Hurricane Michael in 2018 had peak winds of 260 ( 160 ) and storm surges up to ( 14 ) Hurricanes typically last for to 10 days . Over cooler water or land , the hurricane latent heat source shutdowns and the storm weakens . When a hurricane disintegrates , it is often replaced with intense rains and tornadoes . GEOGRAPHY

WEATHER VERSUS CLIMATE People often confuse weather and climate they are not identical . According to the American Meteorological Society ( AMS ) the weather is defined as the state of the atmosphere at some place and time , usually expressed in terms of temperature , air pressure , humidity , wind speed and direction , precipitation , and cloudiness . Meteorologists study the atmosphere , processes that cause weather , and the life cycle of weather systems . Climate is defined in terms of the average ( mean ) of weather elements ( such as temperature and precipitation ) over a specified period of time . The World Meteorological Organization defines the typical time period of time as 30 years ) Climate also encompasses weather extremes for a particular place . Scientists have developed a variety of ways for classifying climate . In the early century , a German scientist named Vladimir Koppen developed one of the most widely used classification systems . The Koppen system categorizes climate into five main types , which can be further divided into . Table Basic Characteristics of the Climate Classification Type of Climate Characteristics Tropical ( A ) Humid average temperate above ( Dry ( Evaporation exceeding precipitation with constant water deficiency throughout the year Temperate ( Humid and warm or hot summers and mild winters with average temperatures between ( and ( Continental ( Humid and warm summers with the average temperature of warmest Polar ( Extremely cold winters and an average temperature of the warmest summer month below ( The planet climate has changed many times over Earth long geologic history . For example , over the past million years , Earth has experienced several glacial periods interspersed with interglacial ( warmer ) periods . The relatively constant and favorable interglacial period of climate experienced over the past years has made human advancement possible . GEOGRAPHY

. lam mi um my in an Ill so Am As Aw awn ask ash on em Cu cab ( Own on mu Dad Dom Dan Dan ! EF Figure Climate Subdivisions . Image is in the public domain . Climate Change Climate change refers to a significant and sustained ( over decades or longer ) change from one climatic condition to another . The term global warming refers to a specific kind of climate change in which Earth average temperature is increasing . Of growing concern is what is known as abrupt climate change . According to the National Oceanic and Atmospheric Administration ( abrupt climate change is a relatively new area of scientific research whose formal definition is still being developed , but it refers to a sudden , rapid change from one climate state to another ( over a period of years rather than centuries or millennia ) Meteorologists focus primarily on ( current ) data to predict local or regional atmospheric conditions for the hours , days , or weeks ahead . Thus , weather prediction tends to be more local and relates to conditions in the immediate future from days to weeks . GEOGRAPHY

' i , i , ii ! i . I . a . i ' I I I I 880 lI ' 102 1061 I . 217 ) Year Figure Annual Temperatures During . Notice the Then the Overall Dramatic Increase of The Patterns During the Past 20 Years . Image is in the public domain . Climate scientists or , on the other hand , look at atmospheric conditions in terms of averages and trends ( patterns ) that have occurred over many decades , centuries , and millennia . Weather is variable but can be averaged over time to indicate climate trends . Therefore , climate scientists can use weather data plus proxy data to help them identify previous trends and improve their predictions of future trends . Meteorologists and climate scientists use similar tools . Weather balloons , satellites , specially designed airplanes , and radar and other data collection instruments ( to measure wind speed , precipitation , air temperature , humidity levels , etc . are all good examples . These methods and tools have enabled humans to collect reliable atmospheric data on a consistent basis since the . They have grown increasingly more precise and sophisticated over time , to such an extent that meteorologists can now consistently provide reasonably accurate ( week or less ) weather forecasts . Climate monitoring requires data covering all areas of the planet over a much longer time period . Sophisticated satellites equipped with equipment circle the globe . With each pass , they can record sea surface and other temperatures , measure atmospheric gases and rainfall amounts , take visible and infrared photos of Earth surface and calculate Earth outgoing infrared and reflected solar radiation . GEOGRAPHY

Extreme Weather events that cause loss of life , disrupt normal human activities , and result in property damage appear extreme . It is a question of perspective How do today severe weather events compare to severe weather events in the recent and distant past ?

The resolution of Global Climate Models can complicate making direct comparisons between past and present events . For example , since 1986 the global human population has grown by approximately billion . Simply said , there are more people than ever living in formerly unpopulated or sparsely populated areas . Comparing death tolls , between recent and past events may not be the most meaningful indicator of a particular weather event intensity . Nonetheless , the growing body of meteorological data indicates an increase in the number of extreme weather events occurring here in the United States since 1980 , and the number of extreme events also appears to be rising worldwide . GEOGRAPHY

SUMMARY Weather , the state of the atmosphere at a particular place during a short period of time . It involves atmospheric phenomena such as temperature , humidity , precipitation ( type and amount ) air pressure , wind , and cloud cover . An air mass is a large mass of air that has similar characteristics of temperature and humidity within it . An air mass acquires these characteristics above an area of land or water known as its source region . When the air mass sits over a region for several days or longer , it picks up the distinct temperature and humidity characteristics ofthat region . There are four types of air masses . A weather front is a transition zone between two different air masses at the Earth surface . Each air mass has a unique temperature and humidity characteristics . Often there is turbulence at a front , which is the borderline where two different air masses come together . The turbulence can cause clouds and storms . The Koppen climate classification is one of many systems that help identify a region climate . There are five main climate groups , with each group being divided based on seasonal precipitation and temperature patterns . The weather differs from the climate in that the latter includes the synthesis of weather conditions that have prevailed over a given area during a long time 30 years . Climate , by contrast , refers to weather trends and patterns occurring globally or regionally over decades , centuries , and even millennia . Extreme weather events , by definition , are rare and intense . You have learned that although scientists are still unable to conclusively link specific extreme weather events to global climate change , these events are predicted consequences of changes in Earth climate . GEOGRAPHY

Physical Geography - Version 1 Unit 7 Elements of Weather & Climate

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