CHAPTER 19Air Pressure

Astronomy/Meteorology

UNDERSTANDING AIR PRESSURE 19.1• Air Pressure Defined

– Pressure exerted by the weight of the air above– Exerted in all direction– http://www.youtube.com/watch?v=vGulvutZJpg - 6 minutes

• Measuring Air Pressure– Barometer – device used to measure air pressure

• Bar= pressure• Metron = measuring instrument• Pressure increases pushes mercury up

– unit = millibar• Air pressure at sea level is 1013.2 mbars

Factors Affecting Wind 19.1• Air moves from high pressure to low pressure– Wind result of differences in air pressure• Differences in air pressure b/c of unequal heating (temp)

– Solar radiation source of wind

• Air doesn’t move in straight lines b/c– Pressure differences– Coriolis Effect– Friction

Pressure Difference 19.1• Great difference = greater wind• Isobars – lines on a map that connect places of

equal air pressure– Pressure gradient – spacing between lines• Close: steep pressure gradient = high winds• Widely spaced: weak pressure gradient = light wind

• http://www.youtube.com/watch?v=hKGQJEp-Pys– 2 minutes

• Driving force of wind• High to low

Coriolis Effect 19.1• Describes how Earth’s rotation affects moving

objects– Northern Hemisphere = deflected to the right – Southern Hemisphere = deflected to the left

• Only Affects wind direction– Strong winds equals strong deflection– Strongest at poles– Basically nonexistent at equatorhttp://www.youtube.com/watch?v=mcPs_OdQOYU

2 minutes

Friction 19.1• Affects wind speed and direction• Only affects a few km from Earth’s surface• Jet streams = fast moving rivers of air that travel

between 120 – 240 km/hr from east to west

Pressure and Centers and Winds 19.2• Highs and Lows– Cyclones – centers of Low Pressure• Pressure decreases from the outer isobars towards center• Northern Hemi: counterclockwise

– Low = Left

• Southern Hemi: clockwise

– Anticyclones – centers of High Pressure• Pressure increases from outer to inner• Northern Hemi: clockwise• Southern Hemi: counterclockwise

Pressure and Centers and Winds 19.2• Weather and Air Pressure– Rising air = clouds and precipitation• Surface Convergence = Cyclonic (low)

– Sinking air = clear skies• Surface Divergence = Anticyclone (high)

Pressure and Centers and Winds 19.2

• Weather Forecasting– Low-pressure centers – can produce bad weather– Lows move W to E• Predicting is difficult

Global Winds = Non-Rotating Earth Model 19.2

Global Winds = Rotating Earth Model 19.2

Regional Wind Systems 19.3• Local Winds – small-scale winds– Caused by either topographic effects or variation in

surface (land and water)– Land and Sea Breezes• Land heats and cools faster than water

• Valley and Mountain Breezes– Air on slopes of mountains heated more during the

day than the valley floor• Slope air = less dense glides up slope to generate a valley

breeze

– Air cools quickly at night = more dense = sinks into valley

How Wind is Measured 19.3• Wind Direction– Prevailing Wind – wind consistently blows more often

from one direction• US = westerlies move from west to east

• Wind Speed– Anemometer – measure wind speed

El Nino and La Nina• El Nino = irregular intervals of 3 to 7 years of

warm countercurrents that replace normal cold offshore waters– Affects Ecuador and Peru

• La Nina= temperature in eastern Pacific are colder than average

Normal Conditions

El Nino Conditions

• http://www.youtube.com/watch?v=SR5VPAqVQBw– 3 minutes

• http://www.youtube.com/watch?v=fN_NmCpry38– 4 min 30 sec

• http://www.youtube.com/watch?v=mHpZhKUldBg– 3 min 15 sec