Chapter 28Chapter 28Atmospheric Atmospheric

Pressure and Winds Pressure and Winds

BFRB Pages 111-BFRB Pages 111-116 116

Air PressureAir Pressure

• the weight of the air in a certain areathe weight of the air in a certain area• Air pressure is measured using an Air pressure is measured using an

instrument called a instrument called a barometerbarometer (aneroid or mercury)(aneroid or mercury)

• It is measured in It is measured in inchesinches (height of (height of the mercury column in the the mercury column in the barometer), or with the metric units barometer), or with the metric units called called millibarsmillibars

• Use your reference tables for Use your reference tables for “converting” inches to millibars “converting” inches to millibars

• (ESRT’s Page 13 Right Top)(ESRT’s Page 13 Right Top)

• Right here tells Right here tells you which side you which side is millibars and is millibars and which side is which side is inchesinches

• FYI – the increments FYI – the increments are not the same on are not the same on both sides. Figure out both sides. Figure out each sides’ each sides’ increments and write increments and write them in right now…them in right now…

Inches:Inches:Increment = Increment =

____________________

Millibars:Millibars:Increment = Increment =

____________________

• Air pressure decreases with increasing Air pressure decreases with increasing altitudealtitude (less air above you means less (less air above you means less pressure pushing down on you) pressure pushing down on you)

• This is an This is an inverse (indirect) relationshipinverse (indirect) relationship• Weather maps show Weather maps show sea levelsea level air pressure. air pressure.

It ranges from 960mb – 1050mbIt ranges from 960mb – 1050mb• IsoIsobarsbars are lines on the weather maps are lines on the weather maps

which connect areas of equal air pressurewhich connect areas of equal air pressure• VIF!!!!VIF!!!! - The same rules apply for isobars as - The same rules apply for isobars as

isotherms and contour lines. If the isotherms and contour lines. If the isobars isobars are spaced close together, the pressure are spaced close together, the pressure gradient is steep and winds are fastgradient is steep and winds are fast..

Changes in Air PressureChanges in Air Pressure • How does temperature affect air How does temperature affect air

pressure?pressure?• Warm air’s molecules are further Warm air’s molecules are further

apart than cold air’s molecules! apart than cold air’s molecules! • It is less dense and not as many It is less dense and not as many

molecules are on top of an area to molecules are on top of an area to add pressure.add pressure.

• Therefore, as temperature Therefore, as temperature ________________ , ________________ , air pressure _________________. air pressure _________________.

Changes in Air PressureChanges in Air Pressure

• How does humidity (the amount of How does humidity (the amount of water vapor held in a parcel of air) water vapor held in a parcel of air) affect air pressure?affect air pressure?

• Humid (moist) air contains more water Humid (moist) air contains more water vapor molecules (Hvapor molecules (H22O) O)

• These HThese H22O molecules have less atomic O molecules have less atomic mass than Nmass than N22 and O and O22 molecules that molecules that they take the place of!!!they take the place of!!!

• Let’s PROVE it!!!!!Let’s PROVE it!!!!!

Winds and Air PressureWinds and Air Pressure

• Winds are caused by the unequal Winds are caused by the unequal heating of the atmosphere heating of the atmosphere (remember that temperature (remember that temperature causes changes in air pressure)causes changes in air pressure)

• Warm air is lighter (lower Warm air is lighter (lower pressure) and colder air is pressure) and colder air is heavier (higher pressure)heavier (higher pressure)– Ex: Sea breezes and land Ex: Sea breezes and land

breezesbreezes

Sea Breeze – the surface wind is blowing in FROM the sea.

Land Breeze – the surface wind is blowing out FROM the land.

• Winds blow Winds blow out of highout of high pressure pressure and and into lowinto low pressure pressure

• The greater the difference The greater the difference between the high pressure and between the high pressure and low pressure, the faster the wind low pressure, the faster the wind speed (this is called the speed (this is called the pressure pressure gradientgradient))

• This is shown on a weather map This is shown on a weather map by by closely spaced isobarsclosely spaced isobars (think of (think of closely space contour lines on a closely space contour lines on a topographic map – steeper slope)topographic map – steeper slope)

Air Pressures Effect on Winds Air Pressures Effect on Winds on a Larger Scaleon a Larger Scale

What parts of the US and Canada have the fastest winds?

the slowest?

Coriolis Revisited!Coriolis Revisited!

• The The Coriolis ForceCoriolis Force (caused by the spin of (caused by the spin of the Earth) the Earth) causes winds to bend to the causes winds to bend to the rightright in the northern hemisphere in the northern hemisphere

• Winds blow out of the High and in to the Winds blow out of the High and in to the Low and bend to the rightLow and bend to the right, causing a High , causing a High Pressure System to spin Pressure System to spin clockwiseclockwise

• Winds blow into the Low from the High Winds blow into the Low from the High and bend to the right, causing the Low and bend to the right, causing the Low Pressure System to spin Pressure System to spin counter-clockwisecounter-clockwise

• FYI - FYI - HurricanesHurricanes are areas of extreme low are areas of extreme low pressure and always pressure and always flow counter-flow counter-clockwise in the Northern Hemisphereclockwise in the Northern Hemisphere

In a High, the air wants to move directly out from High to Low Pressure but is deflected to its right by the Coriolis Force, creating a Clockwise flow out from the High.

VIF!!!!!!

H

In a Low, the air wants to move directly in from High to Low Pressure but is deflected to its right by the Coriolis Force creating a Counterclockwise flow into the Low.

VIF!!!!!!

L

MEASURING WINDMEASURING WIND

• Wind directionWind direction is is determined by using determined by using a a wind vane wind vane or aor a wind sockwind sock (like you (like you see at the airport)see at the airport)

• Wind speedWind speed is is measured using an measured using an anemometeranemometer (units (units are are knotsknots; 1 knot = ; 1 knot = 1.15 mph) 1.15 mph) 

Pressure Belts and WindsPressure Belts and Winds • The The jet streamjet stream is a narrow zone of high is a narrow zone of high

speed air found in the upper troposphere speed air found in the upper troposphere (about 30,000 feet)(about 30,000 feet)– Wind speeds here may be over 300 Wind speeds here may be over 300

knots!knots!• Large convection currents of air occur on Large convection currents of air occur on

the Earth’s surface due to the unequal the Earth’s surface due to the unequal heating of the atmosphereheating of the atmosphere

ESRT’s Page 14 BottomESRT’s Page 14 BottomLink to global_circ_anim.swfLink to global_circ_anim.swf

3-D Version of the ESRT’s3-D Version of the ESRT’sPlease note the deflection of the wind to Please note the deflection of the wind to

ITS right in the N.H. due to CFITS right in the N.H. due to CF

This is what the air circulation would look This is what the air circulation would look like if the Earth DID NOT ROTATE – NO like if the Earth DID NOT ROTATE – NO

CF!!!!!!CF!!!!!!