ATMOSPHERE, WINDS, WEATHER 7(1) I. WEATHER AND CLIMATE A. Atmospheric properties - Air always contains some water vapor - Air is more dense at sea level than higher up - -Temperature and humidity control density - -Warm air less dense than cold (less mass, less molecules) -Humid air less dense than dry... Water molecules weigh less than Nitrogen and Oxygen molecules that they displace. -Temp changes; look at PV=nRT -Air cools when it expands (P drops, so T drops) - -Air heats up when it compresses (P up, so T goes up) • So air heats up as it descends from upper to lower atmosphere and compresses -Warm air holds more water vapor than cold air • As result, when warm air rises and gets colder it can no longer hold water vapor--the water vapor condenses into clouds (aggregates of tiny water droplets). If cooling continues, droplets coalesce and grow into rain drops or snowflakes. • These tall to earth as precipitation B. Definitions of some terms -Weather- "state of atmosphere at specific place and time" -Climate-"lonq term average of weather in an area" I 7(2) Influences on weather and climateinclude • Solar radiation • Terrain • Nearby large lakes or ocean Weather can be predict ...currently can't be too long term) climate tougher to predicted (too large an area and II. ATMOSPHERIC CIRCULATION (controls weather and climate) A. Wind -mass movement of air over Earth (Air doesn't remain stationary) B. Uneven solar heating and latitude -more sunlight in equatorial regions than at polesdue to angle of Incidence Light filtered through more atmosphere at polesless energy gets through to Earth -This change in incoming energy is in mid- latitudes, 3 x solar energy per day in June than December -Due to 23 1/2 degrees of the Earth's axis C. Uneven heating and seasons This tilt causes a chance of seasons -North America leans toward sun in summer, leans away from sun in winter -Causes Southern Hemisphere summer to be opposite Northern Hemisphere -Causes Alaska to light 24/hrs/day in summer, dark 24 hrs/day in winter 7(3) Sun looks high in sky during summer, low in sky during winter Sun never appears overhead North of 23 1/2 degrees North lot. or south of 23 1/2 degrees South latitude Most incoming solar energy is concentrated between these two bands of latitude Atmospheric Circulation due to uneven solar heating process -Convection process transfers heat due to temperature differences (just like in Earth's mantle) -Heat energy moves from high to low (hot to cold) This sets up a circulation pattern. Hot air moves toward cold, and cold air moves back toward heat source. Hot air rises at tropics, moves toward poles, cools and sinks near poles moves back toward heat source in tropics along earth surface - does this in reality using 3 convection cells per hemisphere If not for earth's rotation, this simple picture would suffice. But, because of rotation, we must consider a force that complicates the picture. The result of this force is the Coriolis Effect. 7(4) 1. II. Coriolis Effect Wind Patterns I. Coriolis Effect From earthbound perspective, any object moving freely above earth appears to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is due to the fact that movement is the sum of vector quantities. A. Eastward velocity--look at Eastward velocity vectors due to Earth rotation Buffalo, NY vs Quito, Equador Quito travels further/day, has higher Eastward velocity than Buffalo B. Now, combine Eastward velocity with North-South motion Fire cannonball from: Quito north toward Buffalo Conclusion: Viewed from the point of origin ( or, differently put, from the tail of the arrow) both of the cannonball paths have deflected to the right relative to the Earth's surface. C. Coriolis Effect and Atmospheric Circulation Now modify the original model to account for right and left deflections due to Coriolis Effect 7(5) Air does not move from tropics straight up to poles-Instead, there are 3 atmospheric circulator cells per hemisphere 0-30 degrees -- air makes it up about 1/3 of the way from the equator to North and South poles, where it cools, becomes more dense, falls back toward Earth. Air then starts flowing back down toward equator (toward the tropical heat source, like the radiator in the house - But rather than flowing straight toward equator, air is deflected by Coriolis Effect into the "Tradewinds". This air warms and becomes more humid as it approaches the equator. It rises and once again moves poleward to complete the cycle. -The cells on each side of the equator are known as Hadlev cells (Hadley, 1735). a 30-60 degrees-Another pair of cells is at mid-latitudes -Some air flows Poleward at the outer edges of Hadley cells. This air is deflected to form the "westerlies". These cells are known as Ferrel(Ferrel, mid 1800s). a 60-90 degrees-Polar easterlies move equator-ward from poles along surface of earth, air warms as it moves away from poles, rises at about 60 degrees. -This is a polar call. Each of these Cells IS: I Driven by uneven solar heating 2. Influenced by Coriolis effect -An aside on remembering these cells and their circulation: a 7(6) Both the polar cells and the equatorial Hadley cells flow from colder to warmer along the Earth's surface, rise when the air warms. This air is deflected to the right in the Northern Hemisphere and the left in the Southern Hemisphere. 2) The mid-latitude Ferrel cell is different. I think of it as "caught" between the polar and equatorial cells, and it's flow direction is different. If flows from warmer to colder along the surface, opposite from the other cells. II. Wind Patterns Direct result of atmospheric circulation cells and their edges ,Doldrums at equator--calm area--Low pressure due to ascending air-calm area between Hadley cells; aka Intertropical Convergence Zone or ITCZ •Tradewinds from 0 degrees to 30 °-steady east to west winds from Hadley cell circulation. Horse latitudes at 30 n- calm area -High Pressure due to descending air-name due to Spanish practice of throwing their dead horses over board when ships were becalmed and ran out of food and water for horses. • Westerlies from 30 °, to 60 u,-steady west -to-east wind • Polar from 60 °,- calm- Low pressure due to ascending air • Polar easterlies from 60-90 ° steady east to west wind ,Polar high-Hi pressure due to descending air 7(7) STORMS Definition: STORM ='regional atmospheric disturbance characterized by strong winds and often accompanied by precipitation" CYCLONE- huge rotating mass of low pressure air in which winds converge and ascend. (tropical and extratropical) TORNADO- smaller funnel of fast-spinning wind associated with severe thunderstorm. A. Air Masses Air Mass: "a large body of air with nearly uniform temperature, humidity, and density throughout" Air pausing over water or land will tend to take on the characteristics of the surface below • Land: Cold dry air; dense; high pressure zones • Water: Warm humid air; less dense; low pressure zones. Air masses can move within or between circulation cells. FRONT: boundary between air masses of different density (Vilhelm Bjerknes, after WWI battlefronts) -Cyclones can be tropical (form within one tropical air mass) or extratropical (form at front between air masses) B. Extratropical Cyclones -form at the boundary between each hemisphere's polar cell and Ferrel cell- the polar front. Occur mainly in the winter hemisphere -Development of "e.t." cyclone1) cold wind poleward of front moves from east; warmer air equatorward of front moves from west. 7(8) 2) " twist "forms along front due to difference in the wind direction 3) Cold dense air mass slides beneath warmer lighter one. 4)Twist circulates counterclockwise in the Northern Hemisphere - Extratropical cyclones driven eastward by westerlies. Typically 1000 to 2500 km ( 600-1600 mi) in diameter lasting 2-5 days - -Lifting , expansion , cooling of air mass causes precipitation [Fig 8.15, 8.17). - - When cold air advances and lifts warm air, "cold front" occurs. - "Warm front" occurs when warm air is blown on top of retreating edge of cold air. - Wind and precipitation from these fronts are called Frontal Storms. - Extratropical cyclones are principal cause of weather in mid-latitude regions. - -Nor'easters are most violent extratropical cyclones. 30 times/yr, nor'easters on coasts generate erosion of beaches and offshore barrier islands. Every 100 yrs, will devastate coastal settlement. C. Tropical Cyclones -Warm, humid rotating air mass, originates in tropics -Types: "hurricanes" -in Atlantic and E. Pacific "typhoons" - in W Pacific "tropical cyclones"- in Indian Ocean "willy-willies" - in Australia 7(9) -Criteria for classification as hurricane -Winds of greater than 74 mph; can go as high as 250 mph (book-good image-clinging to airplane wing) -Lesser storms known as tropical depressions and tropical storms. -Structure of tropical cyclone -counterclockwise rotation - 'eye' of storm calm -Genesis-not well understood -forms from tropical depression -need warm air (water temp greater than or equal to 79° F), which rises and precipitation develops -can form in pairs or trios -Source of power-energy released during condensation that was stored as latent heat of evaporation is released this energy drives strong winds -Some statistics: -20 billion tons of water can tall from a tropical cyclone in 1 day -I day of tropical cyclone generates 2,400 billion KW of power, equivalent to US electrical energy usage for I year! 7(10) -Destructive elements of cyclones 1) Wind (up to 250 mph) 2) Rain (cause flooding) 3) 3) Storm surge-most dangerous element "dome" of seawater which rises due to low atmospheric pressure. Can be as high as 40 ft. Final analogy " Nature's safety valves, flinging solar energy poleward from the tropics" D. Coriolis effect--still at work? -At first glance, no Air moves clockwise (when viewed from tail) in N. Hemisphere -Cyclones move counterclockwise What gives? Coriolis effect does cause the counter clockwise spin on Low Pressure cell. Key is to realize that airflow is moving toward the low pressure cell from all directions, since wind energy moves from high to low energy. Air flow deflects to the right, sets up counterclockwise spin.