1.  Air pressure › Pressure exerted by the weight of air above › Exerted in all directions › One of the most basic weather elements › Important factor in weather forecasting › Closely tied to cause and effect relationships

2. › Low pressure = cloudy conditions and precipitation  Cyclones  pressure decreases from the outside in. › High pressure = clear skies and fair weather  Anticyclones  Pressure increases from outside in

4.  How to measure air pressure? › Barometer  Invented by Galileo  Direct correlation between air pressure and rising mercury

5.  4 global pressure zones › Subtropics and polar highs – sinking air/prevailing winds  Outward air flow › Equator and subpolar regions – low pressure  Inward and upward  Clouds and precipitation

6.  Wind – horizontal movement of air › result of differences in air pressure  Unequal heating of surface › Flows from high to low › Nature’s way of balancing differences in air pressure › Solar radiation is ultimate source of most wind

7.  3 factors control wind › Pressure differences › Coriolis effect › Friction

8.  Pressure differences › Creation of wind › Greater the difference – greater the wind › Isobars  Lines connecting areas of equal air pressure  Close – steep pressure and high winds  Wide – weak pressure and light winds › Pressure gradient  Pressure changes

10.  Coriolis effect – change in air movement due to Earth’s rotation › All free moving objects, fluids and wind are deflected right in N. hemisphere › Deflected left in S. hemisphere › Due to 15 degree rotation each hour

12.  Friction › Slows air movement › Can change wind direction › Over ocean – low friction › Over rugged terrain – high friction › Jet stream – prominent air flow feature  Fast moving rivers of air in west to east direction

13.  What about global winds? › Ultimate source is unequal heating › Tropical regions – more radiation is received than radiated back › Polar regions – less solar energy is received than lost  The atmosphere balances wind differences out by moving warm air towards high latitudes and cool air toward the equator. (giant heat-transfer system). › Ocean currents also contribute

14.  Local Winds? › Small scale winds › Caused by topographic effects or surface composition(land or water)

15.  Coastal areas: › Land heated intensely during the day more so than the adjacent water › Air above land expands, heats, and rises creating low pressure › Sea breeze develops – cool air over water(high pressure) moves toward warm land (low pressure)

17. › At night – land cools more rapidly than sea  Land breeze develops  Cooler air at higher pressure over land moves toward the sea where air is warmer and lower pressures

19.  Valley and Mountain Breeze › Function very similarly to coastal breezes › Valley breeze –  Heating during the day generates warm air that rises from the valley floor › Mountain Breeze-  After sunset, cooling air near slopes results in cool air moving into valley

21.  Measuring Wind › Direction and Speed › Wind vane - direction  Always point into the wind › Anemometer – speed › Prevailing winds  Consistently blowing from same direction

22.  El Nino and La Nina › El Nino – “the child”  Warm countercurrents replacing normally cold offshore waters with warm equatorial waters  Irregular intervals (3-7 years)  Marked by amoral weather patterns

23. El Nino Stage

24. Note the large pool of warm (red) water in the western Pacific where water is piled up by the trade winds

25. › La Nina  Opposite of El Nino  Distinctive set of weather conditions  Colder than normal air (northwest and plain) and warms other parts of US –cold event  High precipitation in NW  Increase hurricane activity  Cost is 20x greater