Transcript 105A L9
HORIZONTAL AIR MOTIONS HOW WIND ARISES AT THE LAND AND OCEAN SURFACE P/X =P DEFINITION: PRESSURE GRADIENT = DIFFERENCE IN PRESSURE FROM ONE POINT TO ANOTHER VERTICALLY, PRESSURE DECREASES EXPONENTIALLY, BUT HORIZONTALLY PRESSURE VARIES BY SMALL AMOUNTS, TYPICALLY 1 MILLIBAR PER 100 KM. VERTICAL ATMOSPHERIC PRESSURE PROFILE Z P Pressure Records around the Earth Note, Hurricane Wilma, Oct. 2005 set new Atlantic record at 882 mb. Method of pressure measurement Newton’s 1 st Law is applied here! NON-ROTATING EARTH MERIDINAL CIRCULATION TYPICAL WIND DIRECTIONS AND VELOCITIES UPPER LEVEL (TROPOPAUSE) ZONAL WINDS--WEST TO EAST (WITH NORTH-SOUTH UNDULATIONS) AT VELOCITIES OF 20-30 METERS/SEC... SURFACE WINDS ... WESTERLIES [WEST TO EAST AT MID-LATITUDES]... TRADE WINDS [EAST LATITUDES]... TO BOTH WESTERLIES AND METERS/SEC VELOCITIES WEST TRADES AT TROPICAL HAVE 5-20 MERIDIONAL WINDS: NORTH TO SOUTH WINDS AT VELOCITIES OF ABOUT 2 METERS/SEC... VERTICAL WINDS [RETURN IN CIRCULATION CELLS] VELOCITY OF ABOUT 0.2 METERS/SEC... Remove the Earth and the spacecraft travels tangentially away from the Earth at > 7 km/sec. MORE EXAMPLES OF ORBITS DEFINITION: VORTICITY IS DEFINED AS THE TOTAL AMOUNT OF ROTATION ASSOCIATED WITH AN AIR PARCEL. FOR THE ROTATING EARTH: VORTICITY = ROTATION LOW VORTICITY NEAR EQUATOR: TROPICAL WEATHER PATTERNS HIGH VORTICITY AT TEMPERATURE LATITUDES: MID-LATITUDE WEATHER PATTERNS AMONG EFFECTS OF VORTICITY: PRODUCTION OF MID-LATITUDE CYCLONES TORNADOES HURRICANES The Earth rotates once each day, that is Ώ x number of seconds per day! A CONSEQUENCE OF THE ROTATING EARTH: WIND ON A ROTATING PLANET (EARTH) AND NEWTON’S FIRST LAW EFFECT OF FRICTION ON DIRECTION OF WIND DEFLECTION OF WIND DUE TO THE CORIOLIS FORCE IS LATITUDE DEPENDENT BECAUSE THE FORCE ITSELF DEPENDS ON LATITUDE. WINDS ALOFT ARE DUE TO PRESSURE GRADIENT AND CORIOLIOS FORCE ACTING TOGETHER! JET STREAM IS GEOSTROPHIC WIND PLUS THERMAL WIND THERE ARE TWO COMPONENTS TO WINDS ALOFT. WE KNOW THAT TEMPERATURE DIFFERENCES INDUCE PRESSURE CHANGES WITH HEIGHT. JET STREAM OR ZONAL WINDS ARE SYNOPTIC AND UNDULATE ACTUAL GLOBAL CIRCULATION DEPENDS UPON ROTATION OF EARTH GRADIENT WINDS AROUND CURVED ISOBARS HIGH PRESSURE (CW ROTATION) ---> vG > vg; TYPICALLY REACHING ~ 12 METERS/SECOND LOW PRESSURE (CCW ROTATION) ---> vG TYPICALLY REACHING ~ 9 < vg; METERS/SECOND SUMMARIZING ORIGINS OF WINDS 1. PRESSURE GRADIENT FORCE. 2. CORIOLIS FORCE CURVING MOTION AND CAUSING ROTATION. 3. EXCHANGE OF EARTH AND ATMOSPHERE ROTATION DUE TO CONSERVATION OF ANGULAR MOMENTUM = VORTICITY. WIND TYPES AND ORIGINS 1. PRESSURE GRADIENT WIND ORIGIN: PRESSURE DIFFERENCES DIRECTION: ACROSS ISOBARS VELOCITY: TYPICALLY 10-20 MPH (5-10 MPS) 2. GEOSTROPHIC WIND ORIGIN: PRESSUREGRADIENT AND CORIOLIS FORCE DIRECTION: PARALLEL TO ISOBARS VELOCITY: SIMILAR TO PRESSURE GRADIENT WIND 3. GRADIENT WIND ORIGIN: PRESSURE GRADIENT AND CORIOLIS FORCE, BUT WITH AIR MOVING AROUND CLOSED ISOBARS DIRECTION: AROUND CLOSED ISOBARS VELOCITY: SIMILAR TO GEOSTROPHIC WIND, BUT HIGHER AROUND HIGH PRESSURE CENTER THAN AROUND LOW PRESSURE CENTER (EXCEPT FOR EFFECTSOF GREATER PRESSURE GRADIENTS IN THE CASE OF A TORNADO OR HURRICANE.