Atmospheric Pressure and Wind

Definitions Atmosphere is held around planet by gravity –Gravity pulls molecules toward Earth causing them to have weight Atmospheric pressure (weight of air directly above an area) –Pressure: force per unit area –Air Pressure: weight of atmosphere per unit area- is equal in all directions 75% of Earth’s atmosphere is within 15 km of surface

Air pressure units and application 1 kg/cm² OR 14.7 lbs/in² at sea level –1 kg  2.2 lbs, 1 in  2.54 cm (e.g. if a box weighs 20 kg and is 20 cm wide and 10 cm long, CALCULATE the pressure underneath the box) –20 kg/(20 cm x 10 cm) = 20 kg/200 cm² =.10 kg/cm² Alternate units: at sea level at 0  C –760 mm Hg, 1005 mb, 1 atm

Air Pressure Units Inches of mercury in a barometer Millibars of pressure –1 in Hg  34 mb –.03 in Hg  1 mb Standard pressure = inches of mercury (ranges form inches) or millibars Typical sea level pressures (high: 1030 to 1050 mb) (low: 960 to 1000 mb) Isobars- lines of equal pressure on a weather map

Causes of Pressure Change Warm air is lighter than cold air because molecules are farther apart –Pressure at surface decreases when warmer rising air above replaces an equal volume of cold air –Pressure at surface increases when colder sinking air above replaces an equal volume of warm air Changes in humidity –More water vapor = Less pressure…….. WHY? –It displaces an equal volume of dry air (N 2 and O 2 both weigh more than H 2 O

Pressure Centers (anticyclones and cyclones)

Isotherms Isotherms are lines drawn on a weather map to connect places with equal temperatures –Isotherms shift positions throughout the year July is the warmest, January is the coldest in the N. Hemisphere

Isotherm Map

Isotherm Overhead Questions & Answers 1) Why do isotherms shift? 2) Why are the temperatures the hottest north of the Equator in July? 3) Do the isotherms shift more in the N. Hemisphere or the S. Hemisphere? 4) Why do isotherms shift more over continents than oceans? 5) Why are the isotherms more east-west in the S.Hemisphere than in the N. Hemisphere? 6) Besides Antarctica (temperatures not shown), where do you think it is the coldest? Where is it the coldest? 1) Changes in season cause temperature changes 2) The sun’s rays are most direct because it is the northern summer 3) Northern hemisphere- especially in winter 4) Over continents because land heats and cools faster and more readily than water 5) Temperatures are moderated due to more ocean coverage so there is not such a great range 6) Siberia

Isotherm Map and Pressure Map of same temperatures (shown by isotherms) Pressure mainly changes due to changes in temperature Warm air rising = lower pressure; Cold air sinking = high pressure

High and Low Pressure Centers High-pressure center- values increase toward a central area (winds blow clockwise out of high P areas) –Isobars surrounding high- pressure center is called a high- pressure area (1500 km across up to the whole U.S.) Low-pressure center- values decrease toward a central area (winds blow counter-clockwise into areas of low P) –Isobars surrounding a low- pressure center are called a low- pressure area

Pressure Gradient Air pressure (millibars per kilometer) –changes quickly between two places when isobars are close together Steep or strong gradient = strong winds –changes slowly between two places when isobars are far apart Gentle or weak pressure gradient = calm conditions or weak winds

Wind air that is moving horizontally- parallel to the earth’s surface labeled according to the direction it blows from –set in motion due to unequal heating around planet as warm air rises the pressure it exerts on earth’s surface decreases, thereby creating an area of low pressure- (sinking air = high pressure) blows from high to low pressure –permanent areas of high pressure at Poles (and at 30° latitude) and low pressure at equator

Why the Wind Blows Uneven heating of the atmosphere –Sets up rising warm air region (low pressure) Often land vs. water –Opposite the low pressure is a region of sinking air- high pressure –Local winds are set up this way Pressure-gradient is set up –The greater the temperature difference the greater the gradient--- faster winds

Land and Sea Breeze ALSO- land breezes are more common in winter than summer ALSO- sea breezes are more common in summer than winter

Coriolis Effect Large-scale, global wind systems are deflected to the right in the N. Hemisphere, and to the left in the S. Hemisphere due to Earth’s rotation west to east –TRY IT: Try tracing a straight line onto rotating piece of paper. What happens?

Coriolis Effect on Wind At surface wind blows from high to low pressure at angles to isobars due to friction Wind blows parallel to isobars from high to low pressure 1 to 2 km above surface where there are no mountains

Rotational Questions and Facts Earth rotates ???  longitude in one hour? –15  How, from where, and in what direction would winds flow if the earth didn’t rotate –High pressure to low pressure, poles toward equator, N-S and S-N Due to the Coriolis Effect, winds blow from what direction at the Equator? Is the directon the same in both hemispheres? –East to West –yes Why is the latter question above true? (sketch) –Coriolis Effect deflects to right in N. Hemi. And to left in S. Hemi

Problem Solving the farther wind travels, the more it is affected by Coriolis force –faster = more force- local wind is affected little Earth rotates in ? hours? Does a person on equator travels farther or less far than someone at 45  latitude? –Draw 2 circles, one inside the other (latitude) to help Does the person travel faster or slower at equator?

Planetary Wind Belts part of global air circulation –affect large areas over extended time periods – driven by convection As warm air rises over equator it cools, stops rising and spreads toward poles, most of it sinking at 30  latitude Patterns of rising and sinking air creates pressure belts- wind moves between them –Seasonal solar heating differences cause pressure belts to shift 10  N in summer and 5  S in winter High P at Poles, low P at 60  N and S, high P at 30  N and S, low P at equator

Doldrums and Horse Latitudes warm regions with little wind 10  N and S Intertropical Convergence Zone (ITCZ)- where winds from two hemispheres come together- rainfall in rising, cooling and condensing air 30  N and S sinking air, high pressure fair weather (dry), and calm or no winds –desert belts on land

Trade Winds and Westerlies air at Horse Latitudes spreads N and S as planetary winds wind in each hemisphere from 30  to 10  blows toward equator (flow northeasterly in N.H. due to Coriolis effect) are warm and steady from 30  to 60  Westerlies (southwesterly at surface in N.H.)- affect air travel speed increases w/ latitude direction changes frequently

Polar Easterlies Blow from northeast to southwest in N. Hemisphere near North Pole; SE to NW in S. Hemi. near S. Pole –Come from sinking, cold air in Polar High regions –Cold, dry winds on land; cold, moist winds near large bodies of water

Equatorial (low) & Subpolar (lows) 0  latitude Trade Winds collide here from opposite directions and force the air upward –Winds do NOT cancel each other and stop– they go up Rainy, warm conditions –Creates the Doldrums 60  N and S latitude in middle latitudes polar easterlies meet westerlies –warmer air of westerlies is forced up creating low pressure belts subpolar lows- mixing of cold and warm air is responsible for mid- latitude weather in N. Hemisphere (forms polar front –cold and stormy weather

Subtropical High & Polar High Air from Subpolar low and Equatorial low sink, compress warm, and dry out to form Subtropical high pressure area –Occurs around 30° N and S of the Equator –Forms boundary between Westerlies and Trade Winds –Horse Latitudes result air at poles sinks- is very dense- creates high-P systems very cold weather air moves toward poles (polar easterlies)

Wind and Pressure Shifts As sun’s rays shift seasonally, so do the pressure and wind belts –Suns shifts between 23.5° N and S –Wind and pressure shifts about 10° north of average location in summer and 10° S of average location during winter Florida has Westerlies in winter but trade winds during summer

Jet Streams & Measuring Wind narrow “rivers of wind” near the top of the troposphere (usually from 6000 to meters altitude) –60 km/h in summer to 300 km/h in winter – blow from SW to NE –form above subpolar lows and form loops that might extend to the tropics –do not follow regular patterns around globe –jets fly in this stream Wind vane- wind direction Anemometer- wind speed –Windiest place on earth: Commonwealth Bay, Antarctica –300 km/h »cold air drains rapidly off the high Antarctic ice sheet