1. 1. What type of weather is associated with a low pressure system? 2. How does pressure change as you go towards the center of a high pressure system? 3. What direction do the winds blow in a high pressure system in the Northern hemisphere? Why does the direction change in the Southern hemisphere

2. 1. Why does it take a plane longer to travel from NC to Russia than from Russia to NC? 2. How are air masses named? 3. What type of air is associated with a mT air mass?

3.  Most common features on any weather map & weather generalizations can be made using them  - Winds are influenced by pressure (pressure gradients) centers and the Coriolis effect

4. CYCLONE TYPE CHARACTERISTICS (Northern hemisphere) EXPECTED WEATHER AIR PRESSURE LOW (Cyclones) * Winds blow counterclockwise * Net flow of air inward * Rising air * Move roughly west to east direction across the often requiring a few days to more than a week to move from the west to the east * Unpredictable – making accurate estimation of movement difficult * Cloudy conditions * Precipitation * Produces violent storms if the Low pressure system is strong enough Pressure decreases from the outside towards the center HIGH (Anticyclone s) * Winds blow clockwise * Net flow of air outward (due to friction) * Sinking air * Fair weather * Clear skies Pressure increases as you move from the outside toward the center

6.  Horizontal Convergence- inward movement of air causes the area occupied by the air mass to shrink  - must increase in height to allow for the decreased area it occupies.  - produces a taller and heavier air column

7.  a surface low can exist only as long as the column of air above it exerts less pressure than the air that surrounds it  low-pressure center causes a net accumulation of air increasing its pressure.

8.  When there is a converging air mass at the surface, it must be balanced by outflow  a surface convergence can be maintained if a divergence occurs above the low at the same rate as the inflow below and vise versa

9.  Air spreads out (diverges) above surface cyclones and comes together (converges) above surface anticyclones

10. Recall:  Warm Air RISES, and Cool Air SINKS. (ADIABATIC temp. changes)  At low altitudes, air pressure is HIGH. At high altitudes, air pressure is LOW.

11.  So….. in a giant convection cell… 1. At the equator, air becomes WARMER and RISES to an area of LOWER pressure. 2. The LOW pressure causes the air to EXPAND, moving it toward the POLES.

12. 3. Next, the air COOLS and SINKS back to the earth. 4. Finally, the air will move toward the EQUATOR, where the cycle begins again.

13.  THE EARTH ROTATES TOWARD THE EAST.  The resulting wind movement is called the CORIOLIS EFFECT.

14.  link link

16.  NOTE: ---Wind systems are measured using an ANEMOMETER (see p. 545)  ---Wind systems are named for THE DIRECTION THEY COME FROM  Ex: a wind would specifically be called a SOUTHWESTERLY wind.

17. 1. TRADE WINDS  Occurs at 30° north and south latitudes.  Air SINKS, WARMS, and moves toward the equator in an EASTERLY wind.  At the equator, it RISES again and moves back toward 30°, continuing the cycle.  Called a HADLEY CELL for George Hadley (who described it in 1735)

18.  High pressure of sinking air at 30° latitude forms very CALM winds.  Called HORSE LATITUDES because SAILORS WERE STUCK HERE, COULDN’T FEED THE HORSES, & HAD TO THROW THEM OVERBOARD.  Low pressure of converging air at the equator also forms calm winds.  Called DOLDRUMS because SAILORS WERE STUCK HERE, ALSO.

19. 1. PREVAILING WESTERLIES  Between 30° and 60° north and south latitude  Circulates OPPOSITE of the Trade Winds.  Air moves AWAY FROM the equator  in an easterly.

20.  POLAR EASTERLIES  Between 60° and the POLES  Air moves similar to the TRADE WINDS  Characterized by COLD AIR

22.  Wind caused by TEMPERATURE and PRESSURE differences  Jet Stream: narrow bands of FAST, high- ALTITUDE, WESTERLY winds.  Speed: up to 185 km/hr  Altitude: 10.7 km to 12.2 km

23.  Resemble JETS of WATER  2 types:  * POLAR = stronger of the two  SUBRTOPICAL = controls much of the weather in the US

25.  an immense body of air that is characterized by similar temperature and amounts of moisture at any given altitude  can be 1,600 km or more across… take several days to move over an area

26.  When an air mass moves out of the region over which it formed, it carries its temp & moisture  - The characteristics of an air mass change as it moves and so does the weather in that area  - Air masses are classified according to the surface over which they form (ex: continental form over the continent)

29.  Air masses collide forming _fronts_.  Front: narrow region separating 2 air masses of different densities____  Can cause dramatic __changes__ in weather.  4 Types: _cold, __warm__, ___stationary__, and __occluded___

30.  Cold, dense air displaces ___warm___ air, forcing the warm air _up__ along a steep front.  Warm air _rises _, cools, and _condenses_  Forms __clouds__, __showers__, and sometimes __thunderstorms_____  Twice as __steep _ as a warm front and advance more ___rapidly___ than a warm front

32.  Advancing _warm___ air displaces __cold___ air, and the warm air rises above the cold air.  The warm air moves easily, forming a rather __gradual___ frontal slope  Extensive _cloudiness__ and ___precipitation____  Gradual _warming__ in temp occurs in the area  Wind shift from the _E___ to the _SW__ is associated with a warm front

34.  Warm and Cold air meet, but neither moves into the other’s territory, which __stalls__ the front.  Wind moves __parallel _ to the lines of the front  They stall because the ___temperature__ and __pressure___ gradients are small.

36.  A cold air mass moves so rapidly that it __overtakes a warm front____.  The cold air masses collide, pushing the warm air ___upward____.  Causes precipitation on both sides of the front_______.

38.  Main weather producers in the United States  Large centers of low pressure that generally travel from west to east causing storms  Air moves in a counter clockwise direction towards the center of the low.  Most middle latitude cyclones have a cold front and a warm front extending from a central area

40.  Thunderstorms: Form when warm humid air rises in an unstable environment  generates lightning and thunder and frequently produces gusty winds, heavy rain & hail

41. Strong updrafts Supply moist air Cloud grows vertically Amount of precipitation is too great for updrafts to support Heavy precipitation Gust winds, lightning Downdrafts dominate through the cloud Cooling effect of falling rain and the flowing of colder air from high above calms the storm

42.  Violent windstorms that take the form of a rotating column of air called a vortex that extends downward from a cumulonimbus cloud.

43. 1. mesocyclone- vertical cylinder of rotating air that develops in the updraft of a thunderstorm  strong winds high in the atmosphere  cause winds lower in the atmosphere to roll Violent windstorms that take the form of a rotating column of air called a vortex that extends downward from a cumulonimbus cloud.

44. 2. updrafts cause the rolling air to tilt and become well established 3. not all mesocyclones produce tornadoes

45.  Low pressures within a tornado cause air near the ground to rush into a tornado from all directions  As the air streams inward, it spirals upward around the core  Rating of a tornado is determined by the worst damage left behind  Fujita tornado intensity scale (F0- F5) Maximum winds can reach 480 km/hr

46.  Whirling tropical cyclones that produce winds of at least 119 Km/hr  Cause high winds, huge waves, and extensive flooding hundreds of miles away  Growing threat- >50% of the US population lives within 75 km of the coast.  The north pacific has the greatest number of storms (~20 per year)

47.  Late summer when temps are warm enough to provide heat and moisture the air  Begins as a tropical disturbance- disorganized clouds & thunderstorms w/ low pressure  Inward rush of warm moist air moves towards the center

48.  Air turns upwards and rises in a cumulonimbus cloud (eye wall)  eye wall has the greatest wind speeds & heaviest rain  Rising air is carried away from the storm center- provides room for more inward flow  At the very center of storm is the eye  zone where precipitation ceases and winds subside.  air gradually descends in the eye and compressed- warmest part of the storm

50.  Storm Surge: dome of water 65- 80 km  wide that sweeps across the coast- where the eye moves onto land  Hurricanes weaken when they move over cool ocean waters- can’t supply heat and moisture necessary to maintain its strength

51.  friction with rough land surfaces causewinds to subside  Eventually, the hurricane reaches a location where the air aloft is unfavorable and it will die out.