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Earth’s Weather Chapter 17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Atmosphere A relatively thin shell of gases surrounding the solid Earth Density decreases with increasing altitude The Atmosphere Cont. 50% within 5.6 km of Earth’s surface 99% within 32 km of the surface Composition of the Atmosphere Mostly nitrogen (78%), oxygen (21%) and argon (1%) Nitrogen and oxygen cycle in and out of atmosphere Argon: inert; of radioactive origins Trace components Water, carbon dioxide, neon, helium, krypton, xenon, hydrogen, methane, nitrous oxide, … Aerosols: dust, smoke, salt and other tiny solid or liquid particles Measurement of Atmospheric Pressure Barometer: pressure measuring device Mercury barometer Depends on atmospheric pressure supporting a column of mercury Standard atmospheric pressure (1 atmosphere) corresponds to 76.00 cm of Hg Pressure variation with altitude 10.0 N/cm2 at sea level 5.0 N/cm2 at 5.6 km Closer to earth, the higher the pressure Warming the Atmosphere Heated by incoming solar radiation Direct heating 16% absorbed by air 3% absorbed by clouds Warming the Atmosphere Cont. Indirect heating 51% absorbed by surface Infrared reemitted by surface and absorbed by atmosphere Involves water and CO2 Greenhouse effect Losses 20% reflected by clouds 6% scattered by air 4% reflected by surface Layered Structure Troposphere Surface to where temperature stops decreasing with height Most weather here Stratosphere Temperature increases with height Less turbulent layer Ionosphere Merges with vacuum of space Free electrons and ions The Wind Three general motions: 1. Upward movement over a region of greater heating 2. Sinking of air over a cooler region 3. Horizontal movement between warmer and cooler regions Wind is considered this horizontal movement of air Local Wind Patterns Two important relationships 1. Air temperature and air density Density decreases with temperature Hot air rises; colder dense air sinks 2. Air pressure and air movement Upward movement leaves low pressure behind “lifting effect” Downward movement builds higher pressures “piling up effect” Air generally moves from higher pressure areas to lower pressure regions Global Wind Patterns Remember: Earth receives more direct solar radiation in the equatorial region Global Wind Patterns Cont. -Intertropical convergence zone -Hot air rises here -Rising air cools, causing precipitation -Cooler air descends at higher latitudes -High pressure belt 30º N and S of equator -Jet stream -Meandering loops of wind near the top of the troposphere -Speeds of 100 mi/h or more Wind ChillAir Temperature and Wind Speed Water and the Atmosphere Three phases of water (only compound) 1. Liquid generally above 0ºC (98% of Earth’s water) 2. Solid below 0ºC in the form of ice, snow and hail 3. Water vapor is the invisible, molecular form of water this is what will be found in the atmosphere at any given time (inch thick cloud over entire earth) Contributes to: greenhouse effect principal weathering and erosion agents Evaporation and Condensation Liquid-to-gas and gas-to-liquid phase changes occur at any temperature Liquid molecules with higher than average kinetic energy can evaporate Gas molecules with lower than average kinetic energy can condense Saturation Equilibrium between evaporation and condensation Influenced by temperature Warm air can hold more water vapor than cold air Humidity Absolute humidity The amount of water vapor in the air at a particular time Ranges from near zero to the temperature dependent saturation limit Relative Humidity Relative Humidity-The relationship between the actual absolute humidity and the maximum absolute humidity Capacity of air to hold water changes with temperature -Capacity increases, relative humidity decreases -Warm air can hold more water vapor than cold air The Condensation Process Condensation factors 1. Relative humidity 2. Temperature of the air Water molecules in air join together to: produce a liquid On a surface as dew In the air in droplets or to produce a solid As frost on a surface As snow in the air Condensation, cont. Dew and frost tend to form on cool, clear and calm nights (the 3 C’s) The air becomes cooler near surfaces Forms more in open areas Forms more on grass or other objects rather than flat, bare ground Occurs in low-lying areas near the surface before slopes or hillsides Dew Frost Fog and Clouds Fog - near the ground; clouds - higher up Comprised of small, suspended water droplets Form around condensation nuclei (dust, smoke, soot, salt) Fog Clouds Form when a mass of air above the surface is cooled to its dew point temperature Usually because of air pushed into higher levels in atmosphere Three major causes of upward air movement Convection Differential heating Mountain Ranges Barriers to moving masses Colliding air masses with different densities Cloud formation depends on atmospheric stability Atmospheric Stability Stable atmosphere Lifted parcel of air is cooler (and denser) than surrounding air Lifted parcel returns to its original level Unstable atmosphere Lifted parcel of air is warmer (and less dense) than surrounding air Moved to a higher level, it will continue to rise “thermals” Upward Mobility and Moist Air Rising moist air cools and eventually reaches the dew point Droplets condense around condensation nuclei in saturated air (No condensation nuclei: supersaturated air) Cooling of rising air slowed by release of latent heat of vaporization Huge numbers of droplets appear as clouds Clouds Cont. Cumulus clouds Stratus and stratocumulus Cirrostratus cumulus Rain shower at base of cumulonimbus Altocumulus Stratocumulus Origin of Precipitation Precipitation Water returning to Earth’s surface Dew and frost are surface processes, not precipitation Precipitation forms in two ways 1. Coalescence of cloud droplets 2. Growth of ice crystals Origin of Precipitation Cont. Coalescence process Takes place in warm cumulus clouds near the tropic oceans Clouds contain giant salt condensation nuclei Ice-crystal process Takes place in clouds at middle latitudes Ice crystals capture nearby water molecules and grow Fall as snow in the winter; melt and turn to rain in summer Weather Producers Region 10ºN and 10ºS of equator receives more direct solar energy Air heats up, rises and spreads toward poles Air cools and becomes more dense as it rises, sinking back to the surface at latitudes 30ºN and 30ºS End result Band of low pressure near the equator, bands of higher pressure 30ºN and 30ºS of the equator Large convective cells form to equalize pressure Large, horizontally uniform bodies of air Moisture and temperature conditions nearly the same Four main types 1. Continental polar 2. Maritime polar 3. Continental tropical 4. Maritime tropical Dictate air mass weather Weather conditions remain the same over several days Weather changes when a new mass moves in or when the air mass acquires local conditions Air Masses Weather Fronts Boundaries between air masses at different temperatures Stationary front Forces influencing warm and cold air masses become balanced Weather Fronts Cont. Cold front Cold air mass moves into and displaces warmer air upward Moist rising air cooled, leading to large cumulus and thunder clouds Weather Fronts Cont. Warm front -Warm air mass advances over a cooler air mass -Long, gently sloping front -Clouds and rain may form in advance of the front Thunderstorms Usually develop in warm, very moist, and unstable air Three stages 1. Cumulus Associated with convection, mountain barriers or a cold front 2. Mature Updraft can no longer support growing ice crystals and snow flakes Falling frozen water melts and becomes rain Hail formed through ice accumulation cycles 3. Final All updrafts are exhausted Lightning and Thunder Updrafts, downdrafts and circulating precipitation separate electrical charges Charges accumulate in different parts of the thunderhead Lightning Discharge between charge centers Discharge can be cloud to ground, ground to cloud or cloud to cloud Expanding pressure wave from heated air produces crack of thunder Hail Frozen precipitation Irregular, spherical or flattened forms of ice Alternating layers of clear and opaque ice Believed to form as hailstone cycles through falling and returning to upper parts of thundercloud X-Section of Hail Stone Largest Hail Stone (Where?) Tornadoes Smallest, most violent weather disturbance Rapidly whirling column of air Diameter of 100-400 meters 330-1300 ft Wind speeds up to 480 km/ 300 h Damage produced by high winds, drop in pressure at the center, and flying debris Associated with intense thunderstorms Ranked by F0- F5 Tornado Alley Some Tornado Facts Deadliest U.S. tornado? March 18, 1925 killed 695 people Largest outbreak? April 25-28, 2011 (355 tornadoes and 324 deaths) (1974) Largest path? May 3, 1999 El Reno, OK 2.6 miles Most tornadoes in one month? April 2011: 765 confirmed tornadoes Strongest? (?) 302 mph clocked on May 3, 1999, Bridge Creek, OK Costliest? May 22, 2011 Joplin, MO $2.8 billion Highest Winds? 296 mph El Reno, MO 1999 Where not to live… Codell, KS; Tanner, AL; Moore, OK; Tuscaloosa, AL; Birmingham, AL; St. Louis, MO; Haysville, KS April 3 and 4, 1974 Xienna, Ohio Xennia, Ohio 1974 Hallam, Nebraska 2004 Widest path on record– 2 1/2 miles F5 Tornadoes (1950-today) F5 Damage One reason not to live in a mobile home! Iowa Scout Camp, 2008 Hurricanes Tropical depression Area of low pressure Winds generally moving 55 km/35 h or less Tropical storm More intense low pressure area Winds between 56 and 120 km/35 to 75 h Hurricane Very intense low pressure Winds in excess of 120 km/ 75 h Fully developed hurricane has a calm eye surrounded by intense rain and thunderstorms Classified between 1-5 Hurricanes Cont. Cross section of a hurricane The eye will average 6 to 9 miles across Most damage from floods, winds and tornadoes Katrina August 29, 2005 hits gulf coast Damage from: 25 foot storm surge Wind damage Levee system failure in New Orleans Results: 81 Billion dollars in damages 1,836 fatalities Damage Bay Saint Louis, MS Biloxi, MS Two Weeks Before Katrina Biloxi New Orleans A Typhoon will be in the Pacific- west of international date line Weather Forecasting Based upon mathematical models of the atmosphere Billions of calculations necessitate use of supercomputers Fairly accurate forecasts up to three days possible Major uncertainty: insufficient technology to connect small and large scale events Ultimately oceanic influences need to be better understood Climate Changes of atmospheric conditions over a brief period of time is considered to be the “weather” Composite, larger weather patterns occurring over a number of years will be the “climate” Climate will determine Types of plants and animals at a given location Types of houses built Lifestyles Influences Shape of the landscape Types of soil Agricultural type and productivity Climate and latitude 1. Low latitudes • High solar radiation • Yearly variation small • Temperatures uniformly high 2. Middle latitudes • Higher solar radiation during one part of the year; lower during the other • Overall temperatures lower with greater variation than low latitudes Climate zones Defined in terms of yearly temperature averages 1. Tropical climate zone • • Near equator Receives most solar radiation; hot 2. Polar climate zone • • Least solar radiation; cold Constant daylight part of summer; constant darkness part of winter 3. Temperate climate zone • Intermediate between others Tropical Climate Temperate Climate Polar Climate Regional Climatic Influences Four major factors 1. Altitude • 2. 3. Mountains • Cooler air at higher altitudes • Upwind slopes receive more precipitation; downwind slopes less Large bodies of water • 4. Higher altitude air radiates more energy into space High specific heat of water moderates temperature changes Ocean currents • Can bring water nearby that has a different temperature than the land Regional Climatic Influences
