Coriolis Effect (see book chapter 2)
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However, there is a wrong, or at least misleading, statement: The narrator says several times that points near the equator spin faster than points near the poles, and that the 'spinning' momentum (angular momentum??) is preserved. This is INCORRECT! Every point on the solid Earth spins with the same spin rate (revolution speed) around Earth's axis, no matter where on Earth this point is. But the distance travelled is greater for points near the equator, hence the VELOCITY (= distance/time) is greater near the equator than near the poles. This excess velocity is what makes objects deflect when the move between lines of latitudes.
Adiabatic Changes
adiabatic cooling: air moves from high air pressure to low air pressure
- example in the air column: air moves from high surface pressure to low pressure aloft
- when air rises, it expands and cools at 6-10oC/km
- cooler air can hold less water vapor so the relative humidity increases
- condensation takes place when the rel. humidity reaches 100%; latent heat is released during condensation, so this is no longer an adiabatic process
- rising air causes a low surface pressure relative to the surroundings
- examples: rising air at the equator, rising air in a developing storm system
- example in the air column: air moves from low pressure aloft to high surface pressure
- when air sinks, it contracts and warms
- warm air can hold more water vapor so the relative humidity decreases
- evaporation leads to the take-up of more water vapor and latent heat; this is then no longer an adiabatic process
- sinking air causes a high surface pressure relative to the surroundings
- examples: sinking air over subtropical latitudes, sinking air in stable air masses
Low- and High Pressure Systems, Cyclonic and Anti-Cyclonic Air Flow
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- Earth's spin deflects moving objects -> Coriolis forces make the movement to become a spiral movement
- use right-hand rule to determine wind direction in high-/low-pressure system: make a fist with thumb sticking out; thumb: direction of air flow; fingers then point in direction of winds
- low-pressure system (cyclones): air rises at center; winds blow inward and counterclockwise in northern hemisphere (clockwise in southern hemisphere)
- high-pressure system (anti-cyclones): air sinks at center; winds blow outward and clockwise (counterclockwise in southern hemisphere)
Example for a High Pressure System affecting local climate: Santa Ana Weather
- caused by high pressure system in the Southwest (e.g. Nevada)
- typically in fall and early winter
- lasts a few days
- winds blow clockwise, so winds in San Diego come from northeasterly directions
- in mountains, extremely strong winds (30km/h or 20mph, with gusts up to 80 km/h or 50 mph)
- very dry, with relative humidity below 20%, sometimes reaching into the single digits
- effect in San Diego county enhanced by orographic lifting and additional loss of moisture as air has to pass Laguna Mountains
- people may get dehydrated; sweat evaporates quickly and there is no feeling of sweating
- EXTREME FIRE RISK! Low humidity and high temperatures dry out potential fuel (e.g. dead wood) and gusty winds can whip fires out of control
- also see special Santa Ana page
Example for changes of Low Pressure Systems and Global Climate Change: El Niño and La Niña
- (Spanish for "Christ Child") fishermen in Peru named the El Niño phenomenon after a time near Christmas when they sometimes catch less fish than normal.
- the main modern, satellite-based observation is that during an El Niño, equatorial waters in the East Pacific ocean are up to 5°C warmer than normal, whereas during a La Niña, surface waters are colder than normal
- for more details on observations see lecture on climate change (Lecture 22)
- the exact cause not entirely understood but phenomenon is linked to weakening trade winds and shifts in the low pressure system in the western Pacific Ocean
- Normally, a major equatorial low-pressure cell in the western Pacific and high-pressure over East Pacific create a large east-west convection cell, causing deep water to ascend in the east Pacific bringing up nutrients.
- During El Niño conditions, the low-pressure area moves eastward, causing high rainfall, and a high-pressure cell to develop in the western Pacific. West-east convection cell inhibits nutrient-rich water to ascend from depth in the east Pacific.
- El Niño oscillations also cover parts in the Southern Pacific, affecting weather patterns in Australia in a similar way as Indonesia; hence the term ENSO is used for El Niño southern oscillation.
- ENSO events have a periodicity of 4-7 years but the strength of events varies. A possible cause is the interaction with other oscillations (e.g. the Pacific Decadal Oscillation with a periodicity of 20-30 years.)
- La Niña events are between El Niño events and have the opposite effect compared to El Niño events (the textbook phrases this a little differently).
Observations:
Mechanisms:
Global Air Circulation and Air Pressure At the Surface
- equatorial areas receive more insolation than higher latitudes
- excess heat transported toward poles
- air rises (adiabatically) at the equator (leaving a low pressure system at the surface); adiabatically rising air cools; can hold less moisture, water vapor condenses and eventually rains down -> rain forest
- air sinks (adiabatically) near the poles (creating a high pressure system at the surface); adiabatically sinking air warms; can hold more moisture -> extremely dry climate (Antarctica is driest place on Earth)
- Earth's spin deflects moving air (see Lecture 15). This leads to additional low- and high-pressure belts at the surface:
- high pressure at 30o latitude: air sinks and heats adiabatically; captures moisture, therefore it rains more rarely -> deserts
- low pressure near 60o adiabatically rising air cools; can hold less moisture, water vapor condenses and eventually rains down
Global Air Circulation and the Coriolis Effect
- An idealized non-rotation Earth (i.e. if it would not revolve once a day) has a large convection cell going from the equator to the poles.
- Earth's spin deflects moving air (Coriolis effect). Earth's spin adds west-east component to motion. When air travels to poles or equator, it now experiences additional forces than make air sink/rise earlier along way to the pole/equator.
Coriolis forces break convection cell into three sub-cells
- polar: from +/-60° to poles
- Ferrel: from +/-30° to +/-60°
- Hadley: from equator to +/- 30°
Prevailing Winds
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NB: The trade winds received their name because historical trading ships used these winds to get from Europe to the Americas. The horse latitudes got their name because early trading ships got stuck in windless belts and crew disposed of horses for easier journey. |
Intertropical Convergence Zone
The ITCZ and Monsoons: An example of large-scale, seasonal changes
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major reversal in wind direction that causes a shift from very dry to very wet season (southern Asia);
- winter: stable high pressure cell over central Asia due to cold, dry air -> ITCZ gets pushed out into Indian Ocean; no rain in India
- summer: central Asia heats up dramatically creating pronounced low-pressure area -> ITCZ moves north allowing moisture rich marine air to approach and bring rain the India.
- a similar process occurs in the sub-Sahara Sahel Zone
The Jet Streams
- high-altitude winds near the top of the troposphere
- basic mechanism starts with air rising at equator, raising the troposphere relative to the poles (the latter having greater air pressure)
- air then flows north; become high-altitude Westerlies because of Coriolis effect (deflected to the east but coming from the west)
- wind speeds particularly high where pressure gradients are steep; strong winds at 200-400km/h; these are the jet streams
- 2 special places where pressure gradient is particularly steep: over the polar front (convergence zone between Polar and Ferrel cells) and over the horse latitudes (divergence zone between Ferrel and Hadley cells)
- Jet streams control weather. Their path depends on the season and it tends to be farther south in the winter.
- jet streams undulate around high- and low-pressure systems, so their path depends on the large-scale arrangement of air masses
San Diego's Marine Layer and June Gloom
- usually occurring during spring, when land heats up faster than ocean
- condition especially strong along cold currents that are found near the eastern rims of oceans (e.g. Cold California Current, Humboldt Current off Peru, Canary Current off Northwestern Africa, Benguela Current off Namibia)
- air above ocean is cooled above ocean surface by cold ocean current
- water vapor condenses -> fog
- in second half of day, air heated above land pushes out the sea, above cold air over ocean
- cold air and fog can't ascend because it is cooler than layer above -> inversion layer
- onshore flows during night time brings fog toward coast
- fog often does not burn off until afternoon, when inversion layer is broken down by strong sun
- in areas with high pollution, smog can develop
- the condition is more likely to occur during La Niño years when sea surface temperatures are lower check out SIO's page on the June Gloom
Weather Links
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Government Sites:
- National Weather Service
- National Weather Service (Climate Prediction Center)
- National Weather Service (Storm Prediction Center)
- San Diego weather at NWS
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Commercial Sites:
- AccuWeather
- The Weather Channel