Atmosphere and Weather AP Environmental Science Mark Ewoldsen, Ph.D. Dr. E www.ai.mit.edu/people/jimmylin/pictures/2001-12-seattle.htm
Origin of Modern Atmosphere • original atmosphere surrounded the homogenousplanet Earth and probably was composed of H and He • second atmosphere evolved from gases from molten Earth • H2O, CO2, SO2, CO, S2, Cl2, N2, H2, NH3, and CH4 • allowed formation of oceans and earliest life • modern Atmosphere • evolved after Cyanobacteria started photosynthesizing • oxygen produced did not reach modern levels until about 400 million years ago www.degginger.com/digitalpage.html
Earth’s Atmosphere • compared to the size of the Earth (104 km), the atmosphere is a thin shell (120 km). http://www.gsfc.nasa.gov/gsfc/earth/pinatuboimages.htm
AtmosphereLayers • Exosphere • Thermosphere • (Ionosphere) • Mesosphere • Stratosphere • Troposphere
Troposphere • 8 to 14.5 kilometers high (5 to 9 miles) • most dense • the temperature drops from about 17 to -52 degrees Celsius • almost all weather is in this region • Air for breathing
Stratosphere • extends to 50 kilometers (31 miles) high • dry and less dense • temperature in this region increases gradually to -3 degrees Celsius, due to the absorption of ultraviolet radiation • ozone layer absorbs and scatters the solar ultraviolet radiation • ninety-nine percent of "air" is located in first two layers • every 1000-m 11% less air pressure
Composition • Nitrogen (N2, 78%) • Oxygen (O2, 21%) • Argon (Ar, 1%) • myriad of other very influential components are also present which include the Water (H2O, 0 - 7%), "greenhouse" gases or Ozone (O3, 0 - 0.01%), Carbon Dioxide (CO2, 0.01-0.1%),
Solar output and ocean absorption also influence climate • Solar output: the sun varies in the radiation it emits • At each peak of its 11-year sunspot cycle, the sun emits flares strong enough to disrupt satellite communications. • Ocean absorption:the ocean holds 50 times more carbon than the atmosphere and absorbs it from the atmosphere • Warmer oceans absorb less CO2 because gases are less soluble in warmer water, which accelerates warming.
Seasonal Changes • Occur because the earth’s axis is tilted • Creates opposite seasons in the northern and southern hemisphere • Factor that determines global air circulation patterns
Ocean circulation also influences climate • Ocean circulation: ocean water exchanges tremendous amounts of heat with the atmosphere • Moves energy from place to place • Thermohaline circulation: a worldwide current system • Warmer, fresher water moves along the surface and carries heat to Europe • North American Deep Water (NADW): the deep portion of the thermohaline circulation, consisting of denser, saltier, cool water that sinks and moves deep beneath the surface
NADW is vulnerable • If Greenland’s ice melts, freshwater runoff would dilute surface waters, making them less dense, and stopping NADW. – This has occurred in the past. • Data suggest thermohaline circulation is slowing.
Ocean Currents Ocean Currents are influenced by two types of forces 1.Primary Forces--start the water moving 1. Solar Heating 2. Winds 3. Gravity 4. Coriolis 2.Secondary Forces--influence where the currents flow
Solar energy causes air to circulate • Convective circulation: less dense, warmer air rises and creates vertical currents MATTER AND ENERGY! • Rising air expands and cools • Cool air descends and becomes denser, replacing warm air • Influences both weather and climate
The sun and atmosphere warm the Earth • Three factors exert more influence on climate than all others: • The sun — source of most of the heat/energy on Earth • The atmosphere —the Earth’s temperature would be much colder • The oceans — store and transport heat and moisture
Rain shadow Figure 9-6 Rain shadow effect
Thermal (temperature) inversion • Usually, tropospheric air temperature decreases as altitude increases. • Warm air rises, causing vertical mixing. • Thermal inversion: a layer of cool air occurs beneath a layer of warmer air • Inversion layer: the band of air in which temperature rises with altitude • Denser, cooler air at the bottom of the layer resists mixing. • Pollutants are trapped at ground level.
Coriolis Effect • The Earth is a spinning globe where a point at the equator is traveling at around 1100 km/hour, but a point at the poles is not moved by the rotation. • This fact means that projectiles moving across the Earth's surface are subject to Coriolis forces that cause apparent deflection of the motion.
Global wind patterns • The convective cells also interact with Earth’s rotation to produce global wind patterns. • As Earth rotates, equatorial regions spin faster. Coriolis effect:the north-south air currents appear to be deflected from a straight path • Results in curving global wind patterns that helped ocean travel • Coriolis Effect Animations
Circulation systems produce climate • Hadley cells: near the equator,surface air warms, rises, and expands • Releases moisture and heavy rainfall near the equator • Ferrel cells and polar cells: lift air • Creates precipitation at 60 degrees latitude north and south • Causes air to descend at 30 degrees latitude and in the polar regions • Hadley Cell Animation
Which shows air pressure that is less dense? • A • B • Pressure Link B A
Air Pressure • air pressure is caused by the weight of the air pressing down on the Earth, the ocean and on the air below • the pressure depends on the amount of air above the measuring point and falls as you go higher • air pressure changes with weather
… and Weather • air in a high pressure area compresses and warms as it descends • the warming inhibits the formation of clouds, meaning the sky is normally sunny in high-pressure areas • haze and fog might form • the opposite occurs in an area of low pressure
Humidity • relative humidity is the amount of water vapor in the air compared with the potential amount at the air's current temperature • expressed as a percentage • depends on air temperature, air pressure, and water availability • the Earth has about 326 million cubic miles of water • only about 3,100 cubic miles of this water is in the air as water vapor > clouds > precipitation www.rowcamp.com/photos.htm
Cloud cover • moisture in the atmosphere forms clouds which cover an average of 40% of the Earth at any given time • a cloudless Earth would absorb nearly 20 percent more heat from the sun • clouds cool the planet by reflecting sunlight back into space. This is known as Albedo However • clouds reduce the amount of heat that radiates into space by absorbing the heat radiating from the surface and reradiating some of it back down • the process traps heat like a blanket • “Cloud www.nasm.si.edu/earthtoday/ cloudlg.htm • -Dec-2002 15:52:11 EST
Precipitation • Air containing water vapor cools in atmosphere and therefore condenses to form droplets of liquid water • Rain: liquid, falls, d >0.5 mm (sphere) • Freezing Rain: occurs when drop touches frozen surface • Sleet: ice pellets, d < 0,5 mm, begins as rain but enters air below freezing • Snow: water deposits in hexagonal nuclei below freezing • Snow Pellets: grains of ice, d = 2-5 mm • Hail: 5-190 mm in diameter, concentric rings of ice
Winds • horizontal wind moves from areas of high to low pressure • vertical wind moves from low to high pressure • speed is determined by differences in pressure • Coriolis effect causes winds to spiral from high pressure zones and into low pressure zones www.iiasa.ac.at/Admin/INF/OPT/ Spring98/feature_story.htm
Winds • wind speed is detected by a anemometer and direction by a weather vane • wind direction is based on where the wind is coming from: an wind from the east is an easterly • Beaufort Wind Speed Scale is has a range from 0 for calm to 12 for a hurricane with waves greater than 37 feet www.mountwashington.org/notebook/ transcripts/1999/07/07.htm
Fronts • Warm Front • separates warm air from the cooler air it moves into (6 mph, NE) • rises over cool air masses • develops clouds and light precipitation • Cold Front • cold air advancing into warm (9 to 30 mph, SE) • pushes under warm air – rising air just ahead of front • vertical movement strong and thus thunderstorms
Fronts • Stationary Front • boundary of fronts does not move • generally due to winds running parallel to each other in two areas • Occluded • where cold front overtakes warm front
Severe Weather - Thunderstorms • occur from equator to Alaska • may have hail, strong winds, lightning, thunder, rain &Z tornadoes • moist air rises due to frontal zone lifting causing loss of heat leading to cumulus clouds with updrafts • at 42,000 feet downdrafts and precipitation start • may last an hour • severe thunderstorms occur when cold front approaches warm front (which supplies moisture and energy) • winds over 60 mph • hail > 3/4 inch
Severe Weather - Tornadoes • swirling masses of air with speeds of 300 mph+ • waterspouts occur over water • center of tornado is extreme low pressure which causes buildings to implode • destruction is usually less than 0.5 miles wide and 15 miles long • tornado alley is from Texas to Indiana (usually trailer homes) • Fujita Scale
Severe Weather - Hurricanes • cause most property damage and loss of life • winds speeds greater that 74 mph at the center • begin over warm oceans of the tropics • solar insolation (water >80oF) provides energy for huge evaporation, cloud formation, and atmospheric lifting Thorpe, Gary S., M.S., (2002). Barron’s How to prepare for the AP Environmental Science Advanced Placement Exam
Severe Weather - Hurricanes • stages • separate thunderstorms over tropical ocean • winds speeds of 23 to 40 mph lead to Tropical Depression • Tropical Storms have lower pressure and higher wind speeds (40-75) • Category 1 - 5 • center is the eye • Saffir-Simpson Scale
ENSO also influences climate • El Niño-southern oscillation (ENSO): a systematic shift in atmospheric pressure, sea surface temperature, and ocean circulation in the tropical Pacific Ocean • Normally, winds blow from east to west along the equator, from high to low pressure. • Water “piles up” in the western Pacific. • Westward-moving surface waters allow nutrient-rich upwelling along the coast of Peru.
El Niño • El Niño: occurs when air pressure increases in the western Pacific and decreases in the eastern Pacific, weakening the equatorial winds • Water flows eastward, suppressing an upwelling, shutting down delivery of nutrients to aquatic life • Coastal industries are devastated; global weather is changed
La Niña events • The opposite of El Niño events • Cold surface waters extend far westward in the equatorial Pacific and weather patterns are affected in opposite ways. • ENSO cycles are periodic, occurring every 2-8 years. • Globally warming air and sea temperatures may be increasing their frequency and strength.
Climate • Average long term weather of an area • Seasonal variations and weather extremes averaged over a long period (at least 30 years) • 2 Main factors • Temperature • Precipitation • amount • distribution
Milankovitch cycles also influence climate • Milankovitch cycles: periodic changes in Earth’s rotation and orbit around the Sun • Alter the way solar radiation is distributed over Earth’s surface • By modifying patterns of atmospheric heating, these cycles trigger long-term climate variation such as periodic glaciations. • Animations
Our dynamic climate • Climate: an area’s long-term atmospheric conditions • Temperature, moisture content, wind, precipitation, etc. • Weather: conditions at localized sites over hours or days • Global climate change: describes trends and variations in Earth’s climate • Temperature, precipitation, storm frequency • Global warming: an increase in Earth’s average temperature • Earth’s climate has varied naturally through time. • WRAL links