2Weather patterns are caused by differential heating of the Earth’s surface, both on a local and a global scale
3Atmospheric PressureDefined as the force per unit area exerted on any surface by the weight of the overlying air column. At sea level, the atmosphere exerts an average of 14.7 pounds of force on a one-by-one inch area (slightly bigger than a postage stamp).Atmospheric pressure decreases with altitude, since gravity pulls air molecules towards the earth.
4Atmospheric pressure units Atmospheric pressure is often measured in millibars (1000 mb = 1 bar).Standard pressure at sea level is bar or mb, often rounded to 1000 mb.American pilots and TV weatherman express atmospheric pressure in “inches mercury” where 29.9 inches = standard pressure at sea level.Atmospheric pressure is often called barometric pressure, and it is measured with a barometer.
5Pressure and windWind – movement of air from high to low pressure areas.Wind is caused by pressure differences due to unequal heating of Earth’s atmosphere
9Isobars-Lines of Equal Pressure Drawn at the earth’s surface (there are upper-level charts as well) Corrected to “sea level” so only effects of weather shown, not elevation Units are millibars (mb) where 1013 mb is standard sea-level pressure.Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.
10Isobars-Lines of Equal Pressure Wind flags: Winds blow away from the flags and towards the dots Winds blow from high pressure to low pressure Winds blow towards the L, away from the H Although temperature infuences pressure, no real correlationNote that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.
12Santa Ana Conditions High pressure to our east and northeast drive winds from east to west
13As these winds get pushed over the mountains, they undergo adiabatic compression, becoming hot and dry
14Global Wind Patterns and the Coriolis Effect Wind blows from high to low pressureWind blows straight in one direction, but earth turns underneathWe experience the wind as curving, not us moving!This apparent “turning force” is the Coriolis Effect.
15Global Wind Patterns and the Coriolis Effect If you stand behind the wind and watch it move away from you in the N. Hemisphere, it appears to curve to the rightIf you stand behind the wind and watch it move away from you in the S. Hemisphere, it appears to curve to the left.
16Cyclones and Anticyclones Regions of local pressure highs and lows have characteristic circulation patternsCyclones: local low pressure centers: Air spirals inward and upwardAnticyclones: local high pressure centers: Air spirals outward and downward
17Air Movement at a Cyclone (Low) Warm (or humid) air is less dense than cold air and therefore exerts lower pressureRising warm air undergoes expansional (adiabatic) cooling which causes clouds and rain.Low surface pressure associated with unsettled weather and rain.
18Pressure and wind Cool (or dry) air is denser than warm This air exerts a higher pressure than warm air and will tend to sink,Compressional (adiabatic) warming prevents saturation and cloud formationHigh pressure often associated with good weather
19Isobars-Lines of Equal Pressure Wind flags: Winds blow away from the flags and towards the dots. Wind spirals in a counterclockwise direction around the L. Wind spirals in a clockwise direction around the H.Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.
20Global Circulation and the 3-cell model Global circulation patterns are created by differential heating and modified by the Coriolis Effect.Idealized atmospheric model: 3 convection cells in each hemisphere:Hadley Cell (tropical)Ferrel Cell (mid-latitude)Polar CellNote warmer air at surface for all cells
21Global atmospheric circulation ITCZ The equatorial low pressure is due to rising warm equatorial airAdiabatic expansions causes the frequent rainfall. It rains a lot in the tropics!Returning air from the Hadley Cell converges at the Intertropical Convergence Zone (ITCZ).Since air is rising up after converging at the ITCZ, there is little wind, hence the sailor’s term: “the “doldrums”.
22Global atmospheric circulation –Trade Winds Leg of Hadley Cell closest to Earth’s surface is pushed west by Coriolis Effect.results are winds that curve in from the east and converge at the ITCZ.These are the easterly trade winds (coming from NE in the northern hemisphere; from SE in the southern hemisphere).Trade winds drive surface equatorial ocean currents in the tropics.
23Global atmospheric circulation – subtropical high pressure ~30o N/S latitude, air from the Hadley Cell lost some heat and much moisture, so fallsAdiabatic compression causes hot dry air and high pressure at the surface – a subtropical high pressure zone in both northern and southern hemispheres.Much of the world’s deserts are located in this part of the world.San Diego’s latitude is approximately 33°N. It’s climate is influenced by the high pressure belt.
25Global Circulation in the Mid-latitudes The boundary between Hadley and Ferrel Cells is located at about 30° N/S latitude.Warm, dry air descending at this junction divergesNE/SE trade winds go towards the Equator.Other branch goes towards the N/S pole and is deflected to the East by the Coriolis Force.Since they blow from the west, these winds are called the Westerlies.
27Global Circulation– The Polar Cell The cold pole creates permanent high pressure at the N/S Pole)Polar easterlies from descending polar airRising air from the junction of the Ferrel and Polar Cells create a region of stormy, unsettled weather at about 60° N/SPolar jet stream– forms along the polar front.
29Upper Level Circulation: Winds aloft – 30-40,000 ft agl Upper-level wind patterns:Weak equatorial easterliesTropical high pressure beltsUpper-air westerliesPolar lowPolar front – boundary between upper-air westerlies and the polar easterlies (low pressure)Rossby Waves – refer to the meandering waves made by the upper-air westerlies along the polar front
30Winds in the Upper Atmosphere Rossby Waves and the Jet Stream
31Winds in the Upper Atmosphere Rossby Waves and the Jet Stream
32Winds in the Upper Atmosphere Rossby Waves and the Jet Stream
33Winds in the Upper Atmosphere Rossby Waves and the Jet Stream
34Winds in the Upper Atmosphere Jet Streams Wind streamsAt high altitudeIn a narrow corridorSpeeds are maximum toward the centerThe jet stream located closest to each pole is the polar-front jet stream
35Air masseslarge body of air with uniform temperature and moisture characteristics over a large area.retain integrity for several days before mixing.Source regionorigin of the air massgive air mass its characteristics
37FrontsSharply-defined boundary between a 2 air masses with different characteristicsFronts may be warm, cold, occluded (closed) or stationaryCause weather systems – cyclonic, or frontal precipitation
38Warm FrontsMoving warm air mass overtakes a stationary or slow-moving cold air mass.Warm air rises over the cooler air and cools adiabaticallyLifting process is called frontal wedging.
39Warm FrontsCommonly, warm fronts are slow moving, producing stable conditionsstratus type clouds (no vertical development)steady precipitation
40Warm Frontsif the warm air mass is unstable (pushed up too quickly), cumulonimbus clouds and thunderstorms result (not shown here).
41Map symbol for Warm Front – half circles on the side of rising air – move generally north in n. hemisphere
42Cold FrontCold front – fast-moving cold air overtakes warm air and shoves underneath it, creating a steep contact (frontal wedging, again)Warm air rises rapidly, causing unstable conditions
43Cold FrontResult is cumulus and cumulonimbus cloud formation (clouds with significant vertical development)Showery precipitation/ thunderstormsCold clear weather after the front passes.
44Map symbol for cold front – triangle on the side of descending air – move south and east in N. hemisphere
45Occluded Front (Cutoff Low) Faster moving cold air mass traps a warm air mass against a second cold (or at least cool) air mass.Note the more gently-sloping warm front compared to the more steeply-sloping cold front and the different weather patterns resulting from them.
46Occluded Front (Cutoff Low) Warm air is completely cut off from the surface.Precipitation occurs along both frontal boundariesnarrow band of heavy, possible convective precipitation along former cold frontwider band of steady precipitation at warm frontNet result is large zone of inclement weather.Continues until “cutoff” warm air mass runs out of moisture.
47Map symbol for occluded front – alternating half circles and triangles on the same side
48Mid-latitude Wave Cyclones Initial conditions: Along the polar front, cold polar air mass from the north (cP or mP) meets warm humid subtropical air mass from the south (mT).No relative movement between the air masses – yet.
49Mid-latitude Wave Cyclones (1) An undulation or disturbance causes cold air to push southeast and warm air to push north. This results in 2 fronts and a counter-clockwise circulation pattern.
50Mid-latitude Wave Cyclones (1) An undulation or disturbance causes cold air to push southeast and warm air to push north. This results in 2 fronts and a counter-clockwise circulation pattern. This is the case for the location marked #1.
51Mid-latitude Wave Cyclones (2) A low pressure region (warm air rising) and cyclonic circulation develops. Cold front usually moves faster.Characteristic precipitation zones along the both fronts.
52Mid-latitude Wave Cyclones (2) The cold front is moving east (mP and/or cP air mass, north and west of frontal boundary), while the warm front is still moving north (mT air mass, south from Gulf of Mexico).
53Mid-latitude Wave Cyclones (3) The cold front catches up to the warm front, “pinching” the warm air and pushing it up above the colder air. An occluded front forms and precipitation continues over a large area.
54Mid-latitude Wave Cyclones (3) The northern part of the cold front catches up to the western part of the warm front, cutting off a portion of the warm air from the surface. Location 3 is an occluded front with widespread precipitation.
57Fronts Identify and label the cold front in blue. Identify and label the warm front in red.What air mass causes the cold front? Where is it on the map?What air mass causes the warm front? Where is it on the map?
58Precipitation DataThe colored section shows us where it’s raining. The red boxes are thunderstorm activity. It may help to sketch the fronts in. Note most of the precip is north of the fronts, but the T-storm activity is a narrow band to the east.
60Boiling water at altitude Why does water boil at a lower temperature at a higher elevation?Less air pressure allows the water to change state (from liquid to gas) without being so “energetic”.Since boiling water at altitude is not as hot, cooking times must be altered.
61Atmospheric Pressure - force exerted by atmospheric gas molecules on a given area When air masses move around the earth due to differential heating, this value can change.Elevation also changes the value of atmospheric pressure.Atmospheric pressure is often called barometric pressure, as it is measured with a barometer.
62Mean sea-level pressure When atmospheric pressure is corrected for elevation effects, it is reported as mean sea-level (msl) pressure.When using msl pressure, any change from 1013 mb can be attributed to weather systems, and not just elevation.The weatherman reports msl pressure, whether San Diego or on Mt. Everest.
63Measuring atmospheric pressure – the barometer If you evacuate a tube (i.e. remove all the air) and put it in a dish of liquid, the liquid will fill the tube as the air pressure pushes on the liquid in the dish.If you tried this with a dish of water, the water would rise up to about 33 feet in the tube!
64Measuring atmospheric pressure Using mercury, a very heavy liquid, we find that at normal sea-level barometric pressure, the liquid in the tube rises to a height of 760 mm (or inches).This apparatus is the original form of the barometer, a device used for measuring barometric pressure.American TV weather reports are usually given in terms of inches of mercuryOverseas reports use millibars.
65Measuring barometric pressure – the modern way Mercury barometers are dangerous and difficult to use.Modern aneroid barometers use changes within a partially evacuated chamber to move the pointer to the correct value.
66Winds in the Upper Atmosphere Rossby Waves and the Jet Stream geostrophic wind: theoretical wind that would result from an exact balance between the Coriolis Effect and the pressure gradient.The true winds at upper levels of the atmosphere (30-40 thousand feet) outside the tropics can be approximated as geostrophic winds. Upper air winds are westerlies.Rossby Waves: Wave like undulations in the circulation patterns where warm tropical air interacts with the cold polar air.Jet Stream: narrow bands of high-speed air flow within upper air westerlies, usually at the boundaries of warm and cold air.