Air pressure › Pressure exerted by the weight of air above › Exerted in all directions › One...
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Transcript of Air pressure › Pressure exerted by the weight of air above › Exerted in all directions › One...
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
› 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
How to measure air pressure?› Barometer
Invented by Galileo
Direct correlation between air pressure and rising mercury
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
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
3 factors control wind› Pressure differences› Coriolis effect› Friction
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
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
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
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
Local Winds?› Small scale winds› Caused by topographic effects or surface
composition(land or water)
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)
› 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
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
Measuring Wind› Direction and Speed› Wind vane - direction
Always point into the wind
› Anemometer – speed
› Prevailing winds Consistently blowing from same direction
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
El Nino Stage
Note the large pool of warm (red) water in the western Pacific where water is piled up by the trade winds
› 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