1 Pressure gradient force Coriolis force Centrifugal Force Frictional Force The force that causes wind to blow MOVES FROM HIGHER Pressure gradient force PRESSURE TO LOWER PRESSURE Coriolis force Larger the Higher the Altitude and Speed the larger the Coriolis force is Force due to the rotation of the Earth o to the right of its intended path in the northern hemisphere o to the left of its intended path in the southern hemisphere o Only influences wind direction wind direction no effect on wind speed o zero at equator and maximum at pole o deflect parcel to the right of its motion in NH Centrifugal Force important only during high winds that curve sharply those that have a small turning radius such as hurricane and tornado Frictional Force strong near the earth surface o Surface objects create frictional resistance to wind flow and slows the wind diminishing the CF and enhancing the effect of PGF o Surface winds cross isobars blowing out from H and in toward L o Angle at which wind crosses isobars depends on surface roughness average 30 degrees 2 Isobars wind direction speed upper atmosphere close to the surface contours of same pressure Winds blow clockwise around highs called anticyclones in northern hemisphere and are Winds blow anti clockwise around lows called cyclones in northern hemisphere and are outward from the center of the H inward to the center of the L On a weather map closely spaced isobars or isoheights represent a steep pressure gradient a strong PGF and high winds while widely spaced isobars or isoheights represent a gentle pressure gradient a weak PGF and light winds 3 Highs Lows wind around them and between them E y R H I 4 Vertical winds in Highs Lows descending rising Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules These updrafts may cause cloud formation Likewise diverging air molecules from a high pressure area ar e replenished by downward winds 5 Microscale mesoscale synoptic scale planetary scale Microscale o PGF CeF FF o Turbulent eddies o Lifetimes of minutes o CF becomes important o Land sea breezes mountain valley winds thunderstorms tornadoes Lifetimes of o Geostrophic balance between PGF and CF o Hurricane tropical storms lows and highs fronts everyday weather Lifetimes of days Mesoscale hours to a day Synoptic Scale to weeks Planetary Scale o Geostrophic balance between PGF and CF o last for weeks long waves in westerlies 6 Wind rose indicates statistical distribution of wind directions 7 Land sea breeze valley mountain breeze chinook monsoon El Ni o Description of process What Why When Where Valley Mountain Breeze o Daytime Solar heating of hill slopes o Solar heating of hill slopes creates air that is warmer and less dense o less dense than air at the same elevation over the valley and generates rising valley breezes o Nighttime Nocturnal Nocturnal radiational radiational cooling cooling of the mountain slope creates relatively denser relatively denser air that sinks as a mountain breeze or gravity wind o Winds which lead to condensation and precipitation of their moisture when ascending mountains and then compressional warm and dry when descending Orientation of mountains important Westerly winds over the Rockies Chinook Southerly winds over the Alps F hn o warm dry winds that warm by compressional heating as they descend the leeward side Chinook of mountains Monsoon El nino o winds are those that change direction seasonally They usually blow from water onto land during the warm wet season and from the land out over the water during the cool dry season o A major El Ni o event is a condition where warm surface water covers vast areas of the tropical Pacific When the water in the equatorial Pacific turns colder than normal this condition is called La Ni a o An extensive ocean surface warming that begins along the coast of Peru and Ecuador 4 C Doldrums Subtropical Highs o The increase in ocean surface temperature causes changes in the pressure wind and precipitation patterns in tropical Pacific that impacts regions far from the tropical Pacific o Occurs at irregular intervals once every 2 7 years o Varies in strengths with sea surface temperature changes from 0 5 1 C to more than 8 Doldrums subtropical Highs Trade winds ITCZ Polar Front Subpolar Low Polar Easterlies Description of zones What latitudes Winds Precipitation Temperature o regions near the equator characterized by regions near the equator characterized by weak winds strong rising motion towering convective weak winds strong rising motion towering convective clouds and thunderstorms Trade Winds o belt of high pressure near 30 belt of high pressure near 30o horse latitude latitude caused by cooling of equatorial air as it moves caused by cooling of equatorial air as it moves northward and convergence pile up of air aloft Sinking northward and convergence pile up of air aloft Sinking motion clear sky major deserts warm surface motion clear sky major deserts warm surface temperature o low level northeasterly winds southeasterly level northeasterly winds southeasterly in SH that occupy most of the tropics as the in SH that occupy most of the tropics as the Coriolis o Coriolis force deflects the air flowing back from the deflects the air flowing back from the horse latitudes horse latitudes towards equator towards equator ITCZ Polar Front Subpolar Low Polar Easterlies o the boundary the boundary where the northeasterly trade winds in NH converge with where the northeasterly trade winds in NH converge with the southeasterly trade winds in the SH o a semi permanent front that separates a semi permanent front that separates tropical air masses from polar air masses tropical air masses from polar air masses o a zone of low pressure near the polar front o shallow layer of northeasterly flows shallow layer of northeasterly flows formed when the southward moving cold polar air behind formed when the southward moving cold polar air behind the polar front is deflected by the the polar front is deflected by the Coriolis Coriolis force to the force to the right of its path right of its path