Chapter 12 Meteorology
NC ESES Unit 6: The Atmosphere and Weather NC Earth Science Essential Standards EEn. 2.5 - Understand the structure of and processes within our atmosphere. EEn. 2.5.1 - Summarize the structure and composition of our atmosphere. EEn. 2.5.2 - Explain the formation of typical air masses and the weather systems that result from air mass interactions. EEn. 2.5.3 - Explain how cyclonic storms form based on interaction of air masses. EEn. 2.5.4 - Predict the weather using available weather maps and data (including surface, upper, atmospheric winds and satellite imagery). EEn. 2.5.5 - Explain how human activities affect air quality.
NC ESES Unit 6: The Atmosphere and Weather Reading Assignment: Read Chapter 12; pages: 298-321 Objective: -Compare and contrast weather and climate; -Analyze how imbalances in the heating of Earth s surface create weather; -Describe how and where air masses form; -Describe how the rotation of Earth affects the movement of air; -Compare and contrast wind systems; -Identify the various types of fronts; -Recognize the importance of accurate weather data; - Describe the technology used to collect weather data; -Analyze the strengths and weaknesses of weather observation systems; -Analyze a basis surface weather chart; - Describe problems with long-term forecasts. Vocabulary: Meteorology Weather Climate Air mass Air mass - modification Coriolis effect Trade winds Prevailing -westerlies Polar easterlies Jet stream Front Thermometer Barometer Anemometer Hygrometer Ceilometer Radiosonde Doppler effect station model Isopleth Digital forecast Analog forecast
Weather and Climate Meteorology is the study of atmospheric phenomena. The root word of meteorology meteor: means flaming rock, but the ancient Greeks meaning was high in the air, which pertains to our view of meteorology. Clouds, raindrops, snowflakes, fog, dust, and rainbows are all types of atmospheric meteors. The primary types of cloud droplets and forms of precipitation that contain water in any phase are known as hydrometeors. Smoke, haze, dust, and other condensation nuclei are called lithometeors. Thunder and lightning are examples of electrometeors. Weather: the current state of the atmosphere; short-term variations affecting our lives. Climate: the long term weather average weather usually over 30 years or more. Meteorology, weather, and climate are all related.
A Question of Balance Sun s rays strike Earth more directly at the tropics than at the poles. At the poles, the same amount of solar radiation is spread over a larger area than at equator. Therefore, polar regions are never very warm. As the Sun heats the surface of Earth, the Earth radiates back to space about as much energy as it receives over the course of a year.
Balancing the Budget The tropics and other places maintain fairly constant average temperatures because heat energy is redistributed around the world. The continual motion of air and water reallocates heat energy among Earth s surface, oceans, and atmosphere and brings it into balance. Virtually everything that we consider to be weather, meaning every atmospheric motion, from the tiniest convection current to thunderstorms to large-scale weather systems is part of this constant redistribution of Earth s heat energy.
Air Masses When air over a warm surface, such as a parking lot, becomes warmer than the surrounding air because of conduction, the warm air rises. Now, imagine this same process, of warm air rising, taking place over thousands of kilometers, for days or even weeks and you have the formation of an air mass. An air mass is a large body of air that takes on the characteristics and temperature of the area over which it forms, which meteorologists call a source region. All air masses form over land or water. Those that form over land have less exposure to large amounts of moisture, so they re drier than ones formed over water.
Classifying Air Masses Air masses are classified according to their source regions. The main types of air masses are: Continental tropical (ct): warm and dry Maritime tropical (mt): warm and humid Continental polar (cp): cold and dry Maritime polar (mp): cold and humid Arctic (A): basically the same as cp but colder
Source Regions All five main types of air masses are found in North America because of the continent s proximity to the source regions associated with each air mass. mp air masses form over the cold waters of North Atlantic and Pacific; affects west coast bringing heavy winter rains cp air masses form over interiors of Canada and Alaska; results in frigid winters and cool dry summers. tropical and subtropical oceans of the Caribbean Sea and Gulf of Mexico form the mt air masses. Deserts of southwest and Mexico form the ct air masses which results in hot and dry summers. Arctic air masses develop over latitudes above 60 N in the ice- and snow-covered regions of Siberia and the Arctic.
Air Mass Modification Air masses do not stay in one place indefinitely, eventually, they move, transferring heat form one area to another and thus establishing the heat balance discussed earlier. As an air mass moves, it may travel over land or water that has different characteristics from those of its source region. The air mass then starts to acquire some characteristics of the new surface beneath it, when this happens, it is said to undergo air mass modification, which is the exchange of heat or moisture with the surface over which an air mass travels.
Weather Systems If Earth had no rotation and we were all land or water, a large convection cell would have less dense air rise at the equator, cool and flow directly towards the poles and begin again. But because Earth does rotate, particles such as air are deflected to the right in the Northern hemisphere and left in the Southern hemisphere (from the equator). The Coriolis effect caused by Earth s rotation, combined with the heat imbalance, creates the trade winds, polar easterlies, and prevailing westerlies.
Trade Winds Are winds that flow from 30 N toward equator in a westerly direction, air heats and rises then cools and sinks to 30ºN; This circulation pattern are named after English scientist George Hadley (1753), called the Hadley cell. Around 30º latitude, the sinking air associated with the trade winds creates a belt of high pressure that in turn causes generally weak surface winds, so calm that a ship could be stranded in these waters due to near-calm winds; Spanish sailors called this belt the horse latitudes.
ITCZ / Doldrums Trade winds (near equator) from both hemispheres move together from two different directions and as the air converges, it is forced upward, and creates an area of low pressure; this convergence can be on a large or small scale. Near the equator, it occurs over a large area called the intertropical convergence zone (ITCZ). The ITCZ migrates south or north of the equator as the seasons change; in essence, it follows the path of the Sun s rays, which are directly over the equator in September and March. Because the ITCZ is a region of rising air, it is characterized by a band of cloudiness and occasional showers that help provide moisture for many of the world s tropical rain forests. The ITCZ is also called the doldrums.
Other Wind Zones Prevailing westerlies, surface winds move air from 30 N to 60 N in an easterly direction (SW to NE) which is opposite direction than trade winds; in this zone, surface winds move toward the poles in a generally easterly direction (west to east) (Southern hemisphere: Prevailing westerlies move east to west.) Polar easterlies, this wind zone lies between 60 latitude and the poles and is characterized by cold air flowing from the northeast to southwest in the northern hemisphere. (Southern hemisphere: Polar easterlies move SE to NW.)
Jet Streams Earth s weather is strongly influenced by atmospheric conditions and events that occur at the boundaries between wind zones. Both surface and upper-level air differs greatly in temperature and pressure and it is these differences that cause wind. The large temperature gradient in upper-level air results in strong westerly winds, called the jet streams, which are narrow bands of high-altitude, westerly winds that flow at speeds up to 185 km/h at elevations of 10.7 km to 12.2 km. Polar jet stream separates polar easterlies from prevailing westerlies. And the subtropical jet stream is located where the trade winds meet the prevailing westerlies.
Large-Scale Weather Systems Position of jet stream varies, can split into different branches and later reform into a single stream. Jet stream: strongest core of westerly winds; most weather systems follow jet stream path; Jet stream affects intensity of weather systems by moving air of different temperatures from one region to another.
Fronts In the middle latitudes, air masses with different characteristics sometimes collide, forming a front. Fronts are narrow regions separating two air masses of different densities that are caused by differences in temperature, pressure, and humidity. Fronts can stretch over thousands of kilometers across Earth s surface. The interaction between the colliding air masses can bring dramatic changes in weather. There are four main types of fronts: cold, warm, stationary, and occluded fronts.
Cold Fronts Cold, dense air displaces warm air and forces the warm air up along a steep front. As the warm air rises, it cools and condenses. Clouds, showers, and sometimes thunderstorms are associated with cold fronts. Cold front diagram: solid blue line with blue triangles pointing in direction of front s motion
Warm Fronts Warm front: advancing warm air displaces cold air, because the air ahead of a warm front moves more slowly than an advancing cold air mass, the warm air encounters less friction with the ground it develops a gradual frontal slope rather than a steep boundary. Warm front: characterized by extensive cloudiness and precipitation. Warm front diagram: solid red line with red semicircles pointing in the direction of front s motion.
Stationary Fronts If two air masses meet and neither is advancing into the other s territory, and the boundary between them stalls they create a stationary front; this frequently occurs when two air masses have very small differences in their temperature and pressure gradients. Stationary fronts have similar precipitation to warm fronts. Stationary front diagram is represented with a combination of short segments of cold- and warm-front symbols.
Occluded Fronts Sometimes a cold air mass moves so rapidly that it overtakes a warm front. The advancing cold air wedges the warm air upward; when the warm air is lifted, the cold air mass collides with the advancing cold front and the warm air is then squeezed upward between the two cold air masses, we call this an Occluded front; precipitation is common on both sides of these fronts. Occluded fronts are represented by alternating purple triangles and semicircles pointing in front s motion.
Pressure Systems Rising air is associated with low pressure and sinking air is associated with high pressure. Rising or sinking air, combined with the Coriolis Effect, results in the formation of rotating low- and high-pressure systems in the atmosphere. Air in these systems moves in a general circular motion around either a high- or low-pressure center.
High-Pressure System In a high-pressure systems, air sinks, so that when it reaches Earth s surface, it spreads away from the center. This sinking air makes cloud formation difficult, and as a result, high-pressure systems are usually associated with fair weather, clear skies, few clouds. The deflection of air to the right caused by the Coriolis Effect makes the overall circulation around a high-pressure center move in a clockwise direction in the northern hemisphere. Some high-pressure systems are associated with cold air masses that move and modify; others may be subtropical high-pressure systems that are more stationary. [The direction of movement for the high-pressure system in the southern hemisphere is reversed due to the Coriolis Effect.]
Low-Pressure System In surface low-pressure systems, air rises, causing an inward net flow toward the center and then upward; in contrast to a high-pressure system, air in a low-pressure system moves in a counterclockwise direction in the northern hemisphere. Remember, as warm air rises, it cools, condenses, and forms some type of precipitation; this is why low-pressure systems are usually associated with clouds, inclement weather and precipitation. One of the main producers of inclement weather in the middle latitudes is a specific type of low-pressure system called a wave cyclone. [The direction of movement for the low-pressure system in the southern hemisphere is reversed due to the Coriolis Effect.]
Wave Cyclone Wave cyclone usually begins along a stationary front. Some imbalance in temperature, pressure, or density causes part of the front to move south as a cold front and another part of the front to move north as a warm front. This sets up a counterclockwise or cyclonic circulation and if upper-level conditions are favorable, a fully developed lowpressure system forms. Pushed by the prevailing westerlies, this system may travel thousands of kilometers affecting large areas in the middle latitudes.
Gathering Weather Data Meteorologists measure atmospheric variables of temperature, air pressure, wind, and relative humidity to make accurate weather forecasts. The most important factors in weather forecasting is accuracy and density, which means the amount of data available. Some instruments include: Thermometer for measuring temperature, barometers for measuring air pressure, anemometers for measuring wind speed and hygrometers for measuring relative humidity.
Automated Surface Observing Systems (ASOS) ASOS collect and send data about current weather conditions to the National Weather Service to make weather forecasts. Ceilometer measures the height of cloud layers and estimates the amount of sky covered by clouds. To make accurate forecasts, meteorologist must gather atmospheric data at heights up to 30km, through the use of radiosonde are balloon-borne package of sensors, which can measure upper level temperature, air pressure, humidity and can also track wind speed and direction.
Weather Radar Weather radar systems transmit electromagnetic waves that bounce, or scatter, off large raindrops. Receiving antennae receive the scattered waves, or echoes, which are then amplified. Computers process the signals, provides a display of the storm system. The Doppler effect is the change in wave frequency that occurs in energy, such as sound or light, as that energy moves toward or away from an observer. Doppler radar, plots the speed at which raindrops move toward or away from a radar station; it provides a good estimation of wind speeds associated with precipitation, including those experiencing severe weather: thunderstorms and tornados. Low pitch: away; high pitch: toward
Weather Satellites Cameras mounted aboard a weather satellite take photos of Earth at regular intervals. Satellites track clouds but not necessarily precipitation, by combining data from the two types of technology, meteorologists can determine where the clouds and precipitation are occurring. Infrared imagery detects differences in thermal energy; used to map cloud cover or surface temperatures and allows meteorologists to determine the type, height, and temperature of clouds which can be used to predict a storm s potential to produce severe weather.
Weather Analysis A station model is a record of weather data for a particular site at a particular time. Meteorological symbols are used to represent weather data in a station model (Appendix E). Isopleths are lines that connect points of equal or constant values of pressure or temperature on maps. Lines of equal pressure are called isobars; lines of equal temperature are called isotherms. Isobars that are close together indicate a large pressure difference over a small area and thus, strong winds. Isobars that are spread far apart indicate a small difference in pressure which equates to light winds. Isobars also indicate locations of high- and low-pressure systems.
Short-Term Forecasts Weather systems change direction, speed, and intensity with time, in response to changes in the upper atmosphere. A reliable forecast must analyze data from different levels in the atmosphere. Digital forecasts are generated from highspeed computers analyzing large amounts of data. Whereas analog forecast involves comparing current weather patterns to past patterns, with the assumption, weather systems behave in a similar fashion.
Short-Term Forecasts Analog forecasting: useful for conducting monthly or seasonal forecasts, based on the past behavior of cyclic weather patterns. Due to number of variables involved, forecasts become less reliable when they attempt to predict long-term changes in the weather. Hourly forecasts are the most reliable forecasting method. Forecasts in the one- to three-day range are dependent on the behavior of larger surface and upper-level features, such as low-pressure systems.
Long-Term Forecasts Four- to seven-day range, forecasts must attempt to predict changes in surface weather systems based on circulation patterns in the troposphere and stratosphere. One- to two-week range, forecasts based on changes in largescale circulation patterns. Long-term forecasts, involving months and seasons are based largely on patterns or cycles involving changes in the atmosphere, ocean currents, and solar activity.
Unit 6: The Atmosphere and Weather END
The Causes of Weather Section Assessment 1. Match the following terms with their definitions. B meteorology A weather C climate E air mass D air mass modification A. the current state of the atmosphere B. the study of atmospheric phenomena C. the average weather over a long period of time D. the exchange of heat or moisture with the surface over which an air mass travels E. a large body of air that takes on the characteristics of an area over which it forms
The Causes of Weather Section Assessment 2. Identify the winter characteristics of each air mass. B A F cp E ct D mp (Pacific) C mp (Atlantic) A mt (Pacific) A mt (Atlantic) A. warm, humid B. bitter cold, dry C. cold, humid D. mild, humid E. warm, dry F. very cold, dry
The Causes of Weather Section Assessment 3. Why does the angle that the sun s rays strike Earth s surface affect regional temperatures? When the sun s rays strike Earth at a lower angle, the same energy is spread out over a much larger area than when the rays strike Earth more directly.
Weather Systems Section Assessment 4. Match the following terms with their definitions. D Coriolis effect C trade winds A jet streams B front A. narrow bands of high-altitude, westerly winds that flow at high speeds B. the narrow region separating two air masses of different densities C. the major wind zones that occur at 30 north and south latitude D. a result of Earth s rotation that causes moving particles such as air to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere
Weather Systems Section Assessment 5. Identify whether the following statements are true or false. false Low pressure systems are usually associated with fair weather. true The ITCZ generally follows the path of the sun s rays. true The subtropical jet stream is located where the trade winds meet the prevailing westerlies. false The overall circulation in a high-pressure system always rotates in a clockwise direction.
Weather Systems Section Assessment 6. Why are there generally weak winds in the horse latitudes? Around 30º latitude, the sinking air associated with the trade winds creates a belt of high pressure that in turn causes generally weak surface winds.
Gathering Weather Data Section Assessment 7. Match the following terms with their definitions. D thermometer A barometer C anemometer E hygrometer B ceilometer A. an instrument that measures air pressure B. an instrument that measures the height of cloud layers and estimates the amount of sky covered by clouds C. an instrument that measures wind speed D. an instrument that measures temperature E. an instrument that measures relative humidity
Gathering Weather Data Section Assessment 8. Why is it important to have data gathered at the same time from many locations? It would do no good to analyze how temperature and air pressure are interacting in the atmosphere if the two variables were measured at different times. Meteorologists need an accurate snapshot of the atmosphere from as many locations as possible, at a particular moment in time, to develop reliable forecasts.
Gathering Weather Data Section Assessment 9. Why does Doppler radar have a distinct advantage over traditional radar systems? Doppler radar can provide a good estimation of wind speeds associated with precipitation areas, including areas experiencing severe weather.
Weather Analysis Section Assessment 10. Match the following terms with their definitions. C station model A isopleths B digital forecast D analog forecast A. lines that connect points of equal or constant values B. a forecast that relies on numerical data C. a record of weather data for a particular site at a particular time D. a forecast that involves comparing current weather patterns to patterns that took place in the past
Weather Analysis Section Assessment 11. Based on the map below, isobars that are close together indicate what type of winds? Isobars that are close together indicate strong winds, where as, isobars that are far apart indicate light winds.
Chapter Assessment Multiple Choice 12. What would be the most likely classification of an air mass originating over Mexico and Arizona? a. ct c. A b. cp d. mp Mexico and Arizona occupy a warm, continental regions; therefore, the air mass would be classified as a continental tropical air mass (ct).
Chapter Assessment Multiple Choice 13. The horse latitudes represent a general area of air. a. rising c. sinking b. low-pressure d. convergent Rising, low-pressure, and convergent all describe areas where air rises from Earth s surface. At the horse latitudes, the northeast trade winds and the prevailing westerlies diverge, causing air to sink to the surface and then spread apart which creates an area of high-pressure.
Chapter Assessment Multiple Choice 14. What type of front is the result of a cold air mass moving rapidly and overtaking a warm front? a. warm front c. stationary front b. cold front d. occluded front In an occluded front, the cold air squeezes the warm air upward between the two cold air masses.
Chapter Assessment Multiple Choice 15. Which of the following instruments is used to determine the height of cloud layers? a. anemometer c. hygrometer b. ceilometer d. barometer An anemometer measures wind speed. A hygrometer measures relative humidity. A barometer measures air pressure. A ceilometer also estimates the amount of sky covered by clouds.
Section 12.1 Study Guide Section 12.1 Main Ideas Meteorology is the study of the atmosphere. Weather is the current state of the atmosphere, and climate is the average weather over a long period of time. An air mass is a large body of air that takes on the characteristics of the area over which it forms.
Section 12.2 Study Guide Section 12.2 Main Ideas The Coriolis effect deflects air to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis effect combines with the heat imbalance found on Earth to form the trade winds, prevailing westerlies, and polar easterlies. Weather in the middle latitudes is strongly influenced by fast-moving, high-altitude jet streams. A front is the boundary between two air masses of different densities. The four types of fronts are cold fronts, warm fronts, occluded fronts, and stationary fronts.
Section 12.3 Study Guide Section 12.3 Main Ideas Two of the most important factors in weather forecasting are the accuracy and the density of the data. Surface data are easier to gather than upper-level data. The most common instrument for collecting upper-level data is a balloon-borne radiosonde. Radiosondes measure temperature, pressure, humidity, wind speed, and wind direction. Weather radar pinpoints exactly where precipitation occurs. Weather satellites use both visible-light imagery and infrared imagery to observe weather conditions on Earth.
Section 12.4 Study Guide Section 12.4 Main Ideas A station model is a record of weather data for a particular site at a particular time. On a weather map, lines of equal pressure are called isobars and lines of equal temperature are called isotherms. Digital forecasting uses numerical data. Analog forecasting compares current weather patterns to patterns that took place in the past. All forecasts become less reliable when they attempt to predict long-term changes in the weather.