8/18/2014. Chapter 17: Weather Patterns. Section 1 (Air Masses and Fronts) 8 th Grade

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1 Section 1 (Air Masses and Fronts) Chapter 17: Weather Patterns 8 th Grade air mass a huge body of air that has similar temperature, humidity, and pressure throughout Cold, dense air has a higher pressure because the particles are closer together and hit things more often. (Warm, less dense air has a lower pressure.) tropical air mass polar air mass maritime air mass an air mass that forms over oceans and is humid due to evaporated water continental air mass a warm air mass that forms in the tropics and has low pressure a cold air mass that forms north of 50 o north latitude and south of 50 o south latitude (have higher pressure) a dry air mass that forms over land Types of Air Masses: (classified according to temperature and humidity) 1. maritime tropical warm, humid air mass that comes from the South Pacific, South Atlantic, or Gulf of Mexico toward the U.S. bring hot, humid weather in the summer bring wet heavy snow in the winter 2. continental tropical hot, dry air mass that form mostly in summer over dry areas of the Southwest and northern Mexico bring hot, dry weather to the Midwest as they travel northeast 3. maritime polar cool, humid air mass that form over the North Pacific and North Atlantic oceans affect the West Coast more than the East Coast with fog, rain, and cool weather 4. continental polar cool, dry air mass that forms over northern Canada and Alaska bring bitterly cold weather with very low humidity in winter In summer, they develop storms when they move south and collide with a maritime tropical air mass moving north. Prevailing westerlies and jet streams usually push air masses from west to east. (This is why most of our weather comes from the west, southwest, or northwest) the boundary where unlike air masses meet, but do not mix Storms often develop along s as two air masses collide and the less dense mass is pushed upward. 1

2 Types of Fronts: Cold s where a fast- moving air mass overtakes a slow-moving warm air mass The denser cold air slides under the warm air, pushing the warm air upward. As the warm air rises, water vapor condenses, clouds form. Since cold s move quickly, rapid changes in weather can take place. Stationary s where cold and warm air masses meet, but neither one can move the other Where the air masses meet, water vapor condenses, clouds form, and may result in clouds or precipitation for many days as the is stalled. Occluded s where a warm air mass is caught between two cooler air masses The warm air is pushed upward, clouds form, and precipitation may result. Weather maps: L = H = area of relatively low air pressure that often result in precipitation area of relatively high air pressure that usually result in clear skies Cyclone a swirling center of low air pressure (called lows on a weather map) As air masses collide, mountains or strong winds (jet stream) can distort, or bend the boundary along the. The air along the begins to swirl and can cause a lowpressure center to form. As warm air at the center of a cyclone rises, the air pressure decreases even more. Cooler, higher pressure air rushes into this area, creating the swirling action. Cyclones have winds that swirl inward and upward toward the center (counterclockwise as viewed from above in the Northern Hemisphere) Cyclones bring clouds, winds, and precipitation. Anticyclones a high pressure center of dry air (called highs on a weather map) Winds spiral outward and downward from the center, moving toward areas of lower pressure. Spin clockwise, as viewed from above in the Northern Hemisphere. Cool air moves down from above to take the place of the air that spiraled out of the area. The descending air usually causes dry, clear weather. fulgurite Section 2 (Storms) a tubelike figure left after lightning strikes sandy ground and melts the sand grains and fuses them together storm a violent disturbance in the atmosphere Storms may or may not involve precipitation. Storms involve sudden changes in air pressure that cause sudden changes in air pressure. 2

3 Thunderstorms: form in large cumulonimbus clouds, also known as thunderheads form when warm, humid air rises rapidly on hot, humid afternoons, or when warm air is forced upward along a cold Within the thunderhead are strong upward and downward winds (updrafts and downdrafts). Cold cold air mass moving into a warm air mass cold air moves downward warm, humid air moves upward lightning a sudden spark, or energy discharge, caused when electrical charges jump between parts of a cloud, from one cloud to a nearby cloud, or between a cloud and the ground. How Lightning Forms: 1. As a warm air mass collides with a cold air mass there is friction between water particles (in the warm air mass) and ice particles (hail) in the cold air mass. 2. The heavier hail particles gain negative charges from the water particles, leaving the water particles with a positive charge. 3. The heavier negative particles (hail) fall to the bottom of the cloud, making the bottom of the cloud very negative Negative charges are repelled into the ground by the negatives in the cloud, leaving the surface of the ground very positive Electrons from the cloud are rapidly attracted to the positive ground surface. (Taking the easiest path.) ground surface

4 6. The friction between the speeding electrons and the air creates extreme heat and air expansion along the line of travel, leading to a visible flash of light (heat) and a loud clap of thunder (air expanding) Because light travels faster than sound, you see the lightning before hearing the thunder. Calculating Thunderstorm Distance: Count seconds between seeing the lightning and hearing the thunder, then divide by 5 for miles. (Divide by 3 for kilometers.) How Tornadoes Form: 1. Tornadoes form in the same clouds that bring thunderstorms (cumulonimbus). 2. Warm, moist air flows in at the bottom of a cumulonimbus cloud and moves upward. 3. This forms a low-pressure area in the cloud. 4. The rising warm air begins to rotate within the cloud as it meets winds blowing at different directions in the cloud. 5. Downdrafts bring the rotating air to the ground to form a funnel. 6. Updrafts can tilt the rotating funnel and cause it to spin even faster. (up to 500 Km/hr or 300 miles/hour) 7. The extreme low pressure inside a tornado sucks dust and other objects into the funnel. Fujita scale tornado watch conditions are right to form tornadoes tornado warning waterspout rates tornadoes by the amount of damage they cause, from light damage (F0) to extreme damage (F5) a tornado has been actually spotted or detected on radar a tornado that touches down over a lake or ocean tropical disturbance tropical storm a low pressure area over warm ocean water When a tropical disturbance intensifies, or becomes more severe, it becomes a tropical storm. a mild cyclone that forms in a low pressure area over warm ocean water When a tropical storm intensifies, it becomes a hurricane hurricane a tropical cyclone that has winds of about 119 Km/hr to 320 Km/hr. ( mph) As warm, humid air rises and forms thick clouds, cooler air rushes in to take its place. Winds spiral inward at very high speeds to the area of lowest pressure at the center. The lower the air pressure at the center of the storm, the faster the winds move. Hurricanes last longer than most storms (usually a week or more) but lose strength as they hit land due to the lack of warm, moist air that fuels them. 4

5 Eye the name given for the center of a hurricane Winds are strongest near the eye, but are calm in the eye. storm surge a dome of water that sweeps across the coast where a hurricane lands The area covered by a hurricane has a raised water level (up to 6 meters higher) due to the low pressure and high winds. typhoon a hurricane in the Western Pacific Lake- effect snow : an event that explains why cities in upstate New York get immense amounts of snow. In the fall, the land cools more rapidly than the water in the Great Lakes. As cold, dry air moves from the NW across the lakes, it is warmed by the water and picks up a great amount of water vapor. As the warming air reaches the other side of the lake, it rises and cools again. As warm air cools, condensation, cloud formation, and precipitation (snow) falls. This ongoing process continues throughout the winter. Section 3 (Predicting the Weather) Reading the Weather: If a barometer reading shows falling air pressure rain or snow is possible. On a warm afternoon, cumulus clouds growing taller and larger may become cumulonimbus clouds. ( may produce thunderstorms) weather balloons weather satellites carry instruments high into the troposphere and stratosphere to measure temperature, air pressure, and humidity at upper levels. orbit Earth in the exosphere with cameras to make images of temperature and humidity, as well as Earth s surface features, clouds, storms, snow cover, solar radiation, wind speed, and wind direction. National Weather Service map symbols: amount of cloud cover (about 70%) wind direction (from the northeast) isobar a curved line on a weather map that joins places that have the same air pressure Numbers on the isobar represent pressure readings. temperature wind speed (The more flags, the higher the wind.) atmospheric pressure (in millibars or inches of mercury) isotherm a line on a weather map that joins places that have the same temperature. 5

6 cold warm stationary occluded Color bands on a weather map represent different temperature ranges and usually include a key. Predicted temperatures are often plotted as high and low temperature (example: 78/51 ) light rain heavy rain fog light snow heavy snow Triangles point in the direction the is moving. Half circles show the direction the is moving. H high pressure area L low pressure area Doppler Radar a device that rotates, sending out radio waves that bounce off particles in the air When waves bounce off particles and return to the device s transmitter, a computer is used to process data and create a colored image. Different colors represent amounts of precipitation, speed of winds, and directions of winds (important in detecting possible tornadoes). El Nino Chapter 18, Section 4 (Global Changes in the Atmosphere) a climate event that occurs every 2-7 years in the Pacific Ocean Global winds shift and push warm water east toward the coast of South America. The arrival of warmer surface water disrupts the cold ocean currents along the western coast of S. America. This changes weather patterns around the world, often bringing heavy rains or severe droughts in many places. La Nina a climate event in the eastern Pacific Ocean in which surface waters are colder than normal. Usually brings colder than normal winters and greater precipitation to the Pacific Northwest and north central U.S. Also results in greater hurricane activity in the western Atlantic Ocean. greenhouse effect the process by which gases in the atmosphere hold heat in the air greenhouse gases gases in the atmosphere that trap solar energy (CO 2, water vapor, methane, etc.) global warming a gradual increase in the temperature of Earth s atmosphere Over the last 120 years, the average temperature of the troposphere has increased by about 0.5 Celsius degree. 6

7 Possible causes of global warming: 1. The rise in global temperatures may be caused by natural variations in climate. Satellite measurements have shown the amount of energy the sun produces varies from year to year. 2. Human activities are adding CO 2 to the atmosphere ( making it more dense so less radiation can escape ). Burning of fossil fuels in homes, industry and autos adds CO 2 (wood, coal, oil, natural gas). Destruction of rainforests also adds CO 2 (Trees remove CO 2 during photosynthesis. Removing trees means less CO 2 is taken out of the air. Much of the removed material is then burned, adding to the CO 2 problem.) 3. CFC s are depleting the ozone layer, leaving holes in it over Antarctica. ( This lets too much radiation in. ) CFCs chlorofluorocarbons Ozone depletion ( holes in the ozone layer ): 1. CFC s are chemicals released by air conditioners, aerosol sprays, refrigerators, & Styrofoam products. 2. CFC s travel to the ozone layer and are broken apart by UV rays from the sun. 3. Chlorine atoms are released from the CFC s and destroy ozone molecules. 4. Destruction of ozone layer lets in more radiation ( UV rays ) from the sun. 5. As more radiation enters each year, and more heat is trapped because of increased CO 2, the temperature of Earth increases. Effects of global warming: 1. Places too cold for farming today may become farmland. 2. Current farmland may become too dry to farm. 3. Warming of the oceans could increase the strength of hurricanes. 4. Polar ice caps ( glaciers ) are slowly melting, resulting in rising sea levels. 5. Many species (plants and animals) are becoming extinct, maybe due to changes in climate. 7