Ch19 FI

• The pressure exerted by the weight of air above.
• Avg air pressure at sea level: 1kg/cm².
• Air pressure is exerted in all directions --down, up & sideways.
• Air pressure pushing down on an object balances the air pressure pushing up on the object.

• Air pressure is exerted in all directions-down,up, and sideways. The air
• pressure pushing down on an object balances the air pressure pushing up
• on the object.

• A device used for measuring air pressure, design by Torricelli, student of Galileo.
• Standard sea-level pressure = 1013.2 millibars.

• When air pressure increases, the mercury in the barometer's tube rises.
• When air pressure decreases, so does the height of the mercury column

• Wind is the result of horizontal differences in air pressure.
• Air flows from areas of higher pressure to areas of lower pressure.

• The unequal heating of Earth's surface generates pressure differences.
• Solar radiation is the ultimate source of energy for most wind

presuure differences, Coriolis effect & friction

• created by differences in pressure:
• the greater the difference, the greater the wind speed.

lines on a map that connect places of equal air pressure.

The spacing of isobars indicates the amount of pressure change occurring over a given distance. These pressure changes are expressed as the pressure gradient

• Closely spaced isobars indicate a steep pressure gradient and high winds.
• Widely spaced isobars indicate a weak pressure gradient and low winds.
• Direction of force is always from higher pressure to lower pressure & at right angles to the isobar.
• Friction affects wind speed & direction.
• Coriolis effect affects wind DIRECTION only

Describes how the Earth's rotation effects moving objects. Strongest at the poles. Directed @ 90 degree angles to that of airflow

• describes how rotation affects moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere. In the Southern Hemisphere, they are deflected to the left.
• Earth rotates at 15^o/hr; therefore the Coriolis effect deflects at 15^o/hr.

• This deflection:
• 1) is always directed at right angles to the dirction of airflow
• 2) affects only wind direction and not wind speed
• 3) is affected by wind speed---th stronger th wind, th greater the deflection
• 4) is strongest at the poles and weakens toward the equator, bcoming nonexistent at the equator

15^o x 24 = 360^o

• effect of friction on wind is important only w/in a few kilometers of Earth's surface.
• friction slow air movement, which changs wind direction.

Friction over smooth ocean surface is low & angle of airflow is small

Friction over rugged terrain is high & winds move more slowly across the isobars at greater angles.

Fast-moving rivers of air near the tropopause that travel between 120 and 240 kilometers per hour in a west-to-east direction.

Because the object emits the same amount of pressure outward as the air above it

air pressure pushing down on an object balances the air pressure phushing up on the object.

The air pressure is decreasing.

If height of mercury column rises = pressure is increasing.

When column of mercury decreases = air pressure decreasingWhen column of mercury increases = air pressure increasing

the difference in the atmospheric pressure caused by the uneven heating of the earth by the Sun.

Air pressure travels from high to low, creating wind.

In the Northern Hemisphere it deflects things to the right. In the Southern Hemisphere it deflects things to the left.

The rate of deflection is 15^o/hr

in the tropopause or upper-level, jet stream flow can be almost parallel to the surface flow because of the Coriolis effect & pressure gradient forces.

Are centers of low pressure.

Are centers of high pressure.

• pressure gradient & Coriolis effect:
• wind blows from higher to lower & are deflected to right or left by Earth's rotation

In cyclones, the pressure decreases from the outer isobars toward the center. In anticyclones, just the opposite is the case-the values of the isobars increase from the outside toward the center

When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counterclockwise aroud a low. Around a high, they blow clockwise.

In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow of air outward an anticyclone.

• rising air = cloud formation & precipitation or unstable condition & stormy weather
• sinking air = clear skies, good weather

• low presssure:
• -cloudy conditions & precipitation or stormy weather

• high presssure:
• - clear skies & fair weather

• air spreads out (diverges) above the surce of cyclones
• air comes togehter (converges) above surface of anticyclones

• Northern Hemisphere:
• Low (cyclone) = counterclockwise
• High (anticyclone) = clockwise

• opposite in Southern Hemisphere:
• Low (cyclone) = clockwise
• High (anticyclone) = counterclockwise

• Northern Hemisphere:
• Low (cyclone) = counterclockwise
• High (anticyclone) = clockwise

• Cyclone = Low pressure system
• Anticyclone = High pressure sytem

the Trade winds, 2 belts of winds that blows almost constantly from easterly directions & are located b/t the subtropical highs & the equator.

The atmosphere balances these differences by acting as a giant heat-transfer system.This system moves warm air toward high latitudes and cool air toward the equator.

• Two belts of winds that blow almost constantly from easterly directions.
• Trade winds are located b/t the subtropical highs & the equator.

stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin

They make up the dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs. They usually move the weather west-to-east

Winds that blow from the polar high toward the subpolar low

The stormy belt in the middle latitudes.

The Polar front is where the westerlies and polar easterlies meat.

Landmasses break up the ocean surface & large seasonal tempreature differences disrupt the global pattern of pressure zones in the atmosphere.

• In the winter in Northern Hemisphere, large land masses become cold in winter when a seasonal high-pressuresystem develops, causing surface airflow off the land.
• In the summer, landmasses are heated & develop low-pressure cells, which permits air to flow onto the land.

Seasonal changes in wind direction.

Regional Wind Systems

Small-scale winds produced by a locally generated pressure gradient.

The local winds are caused either by topographic effects or by variations in surface composition-land and water-in the immediate area

• Land & Sea breezes
• Valley & mountain breezes

During the day, land heats up faster, causing the air to rise & thus lower pressure on surface, this cause the higher pressure & cool air of the water to push onto land, giving a cool breeze.

At night, the land cools faster, causing higher pressure near surface, causing cool wind to blow out onto lower pressure water.

• valley breeze (dominant in warm season):
• During daylight hours, air along the slopes of mountains heated more intensely than air at same elevation over valley floor, causing the warm, less dense air to glide up the slopes., generating a valley breeze.

• mountain breeze (dominant in cold season):
• At night, rapid cooling of air along the mountain slopes produces a layer of cooler mor dense air next to the ground, causing it to move downslope

Mountain Breeze

the westerlies consistenly move weather from west to east across the continent.

When the wind consistently blow more often from one direction than from any other

Common device used to measure wind speed

• El Niño and La Niña result from interaction between the surface of the ocean and the atmosphere in the tropical Pacific. Changes in the ocean impact the atmosphere and climate patterns around the globe. In turn, changes in the atmosphere impact the ocean temperatures and currents.
• The system oscillates between warm (El Niño) to neutral (or cold La Niña) conditions with an on average every 3-4 years.

At irregular intervals of three to seven years, these warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters

when surface temperatures in the eastern Pacific are colder that average, a La Nina event is triggered that has a distinctive set of weather patterns

are extreme phases of a naturally occurring climate cycle referred to as El Niño/Southern Oscillation. Both terms refer to large-scale changes in sea-surface temperature across the eastern tropical Pacific

West-to-east

El Nino happens.

when surface temperatures in the eastern Pacific are colder than average.

To the NW

• unusually strong countercurrents accumulate large quantities of warm water tht block the upwelling of colder, nutrient-filled water, causing anchovies to starve & devasting the fishing industry.
• causes abnormal weather patterns that drastically affect the economise of Ecuador & Peru.

when surface temperatures in the eastern Pacific are colder than average.

El Nino & El Nina

Global Circulations explain how air and storm systems travel over the Earth's surface. The global circulation would be simple (and the weather boring) if the Earth did not rotate, the rotation was not tilted relative to the sun, and had no water.

In a situation such as this, the sun heats the entire surface, but where the sun is more directly overhead it heats the ground and atmosphere more. The result would be the equator becomes very hot with the hot air rising into the upper atmosphere.

That air would then move toward the poles where it would become very cold and sink, then return to the equator (above right). One large area of high pressure would be at each of the poles with a large belt of low pressure around the equator.

However, since the earth rotates, the axis is tilted, and there is more land mass in the northern hemisphere than in the southern hemisphere, the actual global pattern is much more complicated.

• Instead of one large circulation between the poles and the equator, there are three circulations...
• Hadley cell
• - Low latitude air movement toward the equator that with heating, rises vertically, with poleward movement in the upper atmosphere. This forms a convection cell that dominates tropical and sub-tropical climates.

• Ferrel cell
• - A mid-latitude mean atmospheric circulation cell for weather named by Ferrel in the 19th century. In this cell the air flows poleward and eastward near the surface and equatorward and westward at higher levels.

• Polar cell
• - Air rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the polar highs. At the surface air diverges outward from the polar highs. Surface winds in the polar cell are easterly (polar easterlies).
• Between each of these circulation cells are bands of high and low pressure at the surface. The high pressure band is located about 30° N/S latitude and at each pole. Low pressure bands are found at the equator and 50°-60° N/S.

• Usually, fair and dry/hot weather is associated with high pressure, with rainy and stormy weather associated with low pressure. You can see the
• results of these circulations on a globe. Look at the number of deserts located along the 30°N/S latitude around the world. Now, look at the region between 50°-60° N/S latitude. These areas, especially the west
• coast of continents, tend to have more precipitation due to more storms moving around the earth at these latitudes.
• http://www.srh.noaa.gov/srh/jetstream/global/circ.htm