Elements of Weather and Climate - PDF.pdf

Figure 7.33 Mammatus Clouds near Lone Pine, California. Image by Jeremy Patrich is used under a CC-BY 4.0 license. UNIT 7: ELEMENTS OF WEATHER & CLIMATE Goals & Objectives of this unit Describe the various aspects and elements of weather and atmospheric wat...

Figure 7.33 Mammatus Clouds near Lone Pine, California. Image by Jeremy Patrich is used under a CC-BY 4.0 license. UNIT 7: 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 mid-latitude cyclones and describe the three phases a thunderstorm goes through in its life cycle. Differentiate between weather and climate and explain their interrelationships. Characterize the five general types of climate as defined by the Köppen climate classification. 95 | P H Y S I C A L G E O G R A P H Y 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 place at a time. Weather can ch 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 of this energy is the sun. Climate The climate for a 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 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 s climate. Humidity Humidity is the amount of water vapor in the air in a 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 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. 96 | P H Y S I C A L G E O G R A P H Y  Figure 7.34 Diagram Explaining the Calculation of Relative Humidity. Image is used under a Attribution-Share Alike 3.0 Unported license. Clouds Clouds have a big influence on weather by preventing solar radiation from reaching the ground; absorbing warmth that is re-emitted 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. 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 meets a cold surface or when it rises. 97 | P H Y S I C A L G E O G R A P H Y 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. High-level clouds form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds. Cirrocumulus clouds are small, white puffs that ripple across the sky, often in rows. Cirrus clouds may indicate that a storm is coming. Middle-level clouds, including altocumulus and altostratus clouds, may be made of water droplets, ice crystals or both, depending on the air temperatures. Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation, is coming. Low-level clouds, are nearly all composed of water droplets. Stratus, stratocumulus and nimbostratus clouds are common low clouds. Nimbostratus clouds are thick and dark, vertically instead of horizontally, and will also have their bases at lower altitudes and the tops reaching middle or higher altitudes. Cumulonimbus clouds are cooler clouds, where they have ice along the top, yet are warmer near the bottom. As the warm air rises, a classic anvil head is created. 98 | P H Y S I C A L G E O G R A P H Y  Figure 7.35 Cloud Identification Diagram. Image is used under an Attribution-Share Alike 3.0 Unported 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. There are 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 if there is snow or cool moisture on the ground it will meet the warm, moist winds. This contact between the air and ground will cause the air blowing in to become cool. Then the dew point is reached and fog will form. Radiation Fog: This fog forms when most solar energy exits the earth and allows the ground temperature cools down to the dew point. The best condition to have radiation 99 | P H Y S I C A L G E O G R A P H Y  fog is 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 mph 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's 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 of the 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, hail, and graupel. Their respective intensities are classified either by rate of fall or by visibility restriction. 100 | P H Y S I C A L G E O G R A P H Y AIR MASSES Where an air mass exhibits its characteristics of temperature and humidity it is called the source region. Air masses are slowly pushed along by high-level 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 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? 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 slowly pushed along by high-level winds. Air masses are classified based on their temperature and humidity characteristics. Below are examples of how air masses are classified over North America: Maritime tropical (mT) moist, warm air mass Continental tropical (cT) dry, warm air mass Maritime polar (mP) moist, cold air mass Continental polar (cP) dry, cold air mass Figure 7.36 Air Masses. Image is in the public domain. 101 | P H Y S I C A L G E O G R A P H Y 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 mark the leading edge of moving warm air. Figure 7.5 Diagram Depicting how Weather Fronts are Drawn on Weather Maps. Image by COC OER Team is used under a CC-BY 4.0 license COLD FRONT Imagine that you are standing in one spot as a cold front of air approach. 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 cirrus, cirrostratus and altostratus 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 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 low-pressure 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: If the 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. 102 | P H Y S I C A L G E O G R A P H Y  Figure 7.37 A 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 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 cirrostratus clouds form. As the front approaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is winter, snowflakes fall. The clouds thicken and nimbostratus clouds form. Snowfall increases. Winds grow stronger as the low-pressure 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, in turn melting the ice crystals. Warm and cold air mix at the front, leading to the formation of stratus clouds and fog. 103 | P H Y S I C A L G E O G R A P H Y  Figure 38.7 Image of a Warm Front (Green Arrow) Pushing Away a Parcel of Air. Image in the public domain. OCCLUDED FRONTS An occluded front usually forms around a low-pressure 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 Coriolis 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 7.39 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 low pressure. This means that weather is typically 104 | P H Y S I C A L G E O G R A P H Y unstable 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 often, these weather fronts are not isolated events. Often, they are part of a larger rotating system called a mid-latitude cyclone. This type of cyclone will be discussed later in this chapter, but as an introduction, it is a low-pressure 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. TYPES OF EXTREME WEATHER Weather is experienced every day, but only some days experience extreme weather, such as storms. A storm s 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 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 three-stage 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 to continue 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 downdrafts 105 | P H Y S I C A L G E O G R A P H Y within the cloud. The downdrafts also begin to pull cold, dry air from outside the cloud toward 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 downdrafts exist side- by-side 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 downdrafts spread within the cloud and at first make the cloud become wider, but eventually overtaking the updrafts. Cool downdrafts 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 downdrafts 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 downdrafts 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 downdrafts 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 (mT) air mass from the Gulf of Mexico meets the continental polar (cP) 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 kph (110 mph), but some winds are much faster. A tornado travels over the ground at about 45 km per hour (28 miles per hour) and goes about 25 km (16 miles) before losing energy and disappearing. 106 | P H Y S I C A L G E O G R A P H Y 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 are killed by tornadoes each year. The most violent 2% of tornadoes account for 70% of the deaths by tornadoes. Tornadoes form at the front of severe thunderstorms. Lines of these thunderstorms form in the spring where maritime tropical (mT) and continental polar (cP) 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 mid-west 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 cP and mT, 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 Fujita Scale, which assigns a value based on wind speed and damage. Figure 7.9 The Enhanced Fujita Scale. (Image on Wikimedia Commons by Pfly, CC BY-SA 3.0) 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 low-pressure center. The swirling air rises and cools, creating clouds and precipitation. 107 | P H Y S I C A L G E O G R A P H Y There are two types of cyclones: middle latitude (mid-latitude) cyclones and tropical cyclones. Mid-latitude cyclones are the main cause of winter storms in the middle latitudes. Tropical cyclones are also known as hurricanes. Mid-Latitude Cyclones Mid-latitude 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. Coriolis 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 mid-latitude cyclones. The warm air at the cold front rises and creates a low-pressure 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. If the rising air contains enough moisture, rain, or snow may fall. Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two- to five-day storms can reach 1,000 to 2,500 km (625 to 1,600 miles) in diameter and produce winds up to 125 km (75 miles) per hour. Like tropical cyclones, they can cause extensive beach erosion and flooding. Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states where they are called , because they come from the 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 willi- storms on Earth. Hurricanes arise in the tropical latitudes (between 10° and 25°N) in summer and autumn when sea surface temperatures are 28° C (82° F) or higher. The warm seas create a large humid air mass. The warm air rises and forms a low-pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression. If the temperature reaches or exceeds 82° F 28 (28° C) 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. 108 | P H Y S I C A L G E O G R A P H Y Hurricanes are large systems with high winds. The exception is the relatively calm eye of the storm where the air is rising upward. Rai 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 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 simple, though the processes involved are quite complex. As a low-pressure disturbance forms, the warm, moist air rushes towards the low pressure to rise upward to form towering thunderstorms. Around the low-pressure disturbance is a wall of clouds called an eyewall. Within the eyewall, 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 eyewall are organized, intense thunderstorms, called spiral rain bands s are the first hurricanes are assigned to categories based on their wind speed. The categories are listed on the Saffir- Simpson Scale. Table 7.1 Saffir Simpson Hurricane Scale Category Wind Speed (mph) Type of Damage 1 74 95 Some Damage 2 96 110 Extensive Damage 3 111 129 Devastating 4 130 156 Catastrophic Damage 5 157 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 westerlies, they switch direction and travel toward the north or northeast. Hurricanes may cover 800 km (500 mi) in one day. Damage from hurricanes comes from the high winds, rainfall, and storm surge. Storm surge occurs as the sto low-pressure center comes onto land, causing the sea level to rise seawater across the ocean onto the shoreline. Flooding can be devastating, especially along low-lying coastlines such as the Atlantic and Gulf Coasts. Hurricane Michael in 2018 had peak winds of 260 km/h (160 mph) and storm surges up to 4.3 m (14 ft). 109 | P H Y S I C A L G E O G R A P H Y source shutdowns and the storm weakens. When a hurricane disintegrates, it is often replaced with intense rains and tornadoes. 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 (as weather changes constantly). Climate is defined in terms of the average (mean) of weather elements (such as temperature and precipitation) over a specified period. (The World Meteorological Organization defines the typical period as 30 years). Climate also encompasses weather extremes for a place. Scientists have developed a variety of ways for classifying climate. In the early 20th century, a German scientist named Vladimir Köppen developed one of the most widely used classification systems. The Köppen system categorizes climate into five main types, which can be further divided into subcategories. Table 7.2 Basic Characteristics of the Köppen Climate Classification Type of Climate Characteristics Tropical (A) Average temperature of 18 °C (64.4 °F) or higher every month of the year, with significant precipitation (very humid). Dry (B) Evaporation exceeding precipitation with constant water deficiency throughout the year. Average temperature greater than 10 °C (50 °F) Temperate (C) Humid and warm or hot summers, and mild or dry winters, with average temperatures between -3°C (27°F) and 18°C (64°F) Continental (D) At least one month averaging below 0 °C (32 °F) °C (27 °F)) and at least one month averaging above 10 °C (50 °F). Polar (E) Extremely cold winters and an average temperature of the warmest summer month below 10°C (50°F) 110 | P H Y S I C A L G E O G R A P H Y over the past million years, Earth has experienced several glacial periods interspersed with interglacial (warmer) periods. The relatively constant and favorable interglacial period of possible. Figure 7.10 Köppen-Geiger 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 g concern is what is known as abrupt climate change. According to the National Oceanic and Atmospheric Administration (NOAA), 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 decades rather than centuries or millennia). Meteorologists focus primarily on real-time (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. 111 | P H Y S I C A L G E O G R A P H Y  Figure 7.11 Annual Temperatures During 1880-2018. Notice the Winter/Summer Patterns & Then the Overall Dramatic Increase of The Patterns During the Past 20 Years. Image is in the public domain. Climate scientists or climatologists, 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 ground-based 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 mid-1800s. They have grown increasingly more precise and sophisticated over time, to such an extent that meteorologists can now consistently provide reasonably accurate near-term (1 week or less) weather forecasts. Climate monitoring requires data covering all areas of the planet over a much longer time. Sophisticated Earth-observing satellites equipped with remote-sensing equipment circle the globe. With each pass, they can record sea surface and other temperatures, measure. 112 | P H Y S I C A L G E O G R A P H Y Extreme Weather All-weather events that cause loss of life, disrupt normal human activities, and result in 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 2 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 weather 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. 113 | P H Y S I C A L G E O G R A P H Y UNIT 7 SUMMARY Weather, the state of the atmosphere at a 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 of that region. There are four types of air masses. A weather front is a transition zone between two different air masses at the Earth's 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 Köppen climate classification is one of many systems that help identify a region s 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 generally 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 long- 114 | P H Y S I C A L G E O G R A P H Y