Chapter 6: Air Pressure and Winds

how do horizontal pressure differences generate wind?

*imagining one cold collumn of air and one warm collumn of air in equal altitudes -air molecules would move from the area of high pressure (warm air collumn) to low pressure (cold air collumn) - pressure gradient force

friction causes...

-winds at the surface to blow at an angle across the isobars and from areas of high pressure toward areas of low pressure -net inflow of air around a cyclone, and a new outflow around an anticyclone -convergence (when air moves from the smooth ocean to the land, increased friction can cause an abrupt drop in wind speed downstream and cause a pileup of air upstream, accompanied with converging winds and ascending air; leads to the lake-effect snow and cloudy conditions over humid-coastal regions) -divergence (when air moves from the land to the sea, general divergence and subsidence accompany the seaward flow of air due to reduced friction and increasing wind speed; results in clearing conditions)

millibar

100 pascals; made to simply pascal measurements

average atmospheric pressure at sea level

14.7 pounds per square inch 1013.25 millibars

aneroid barometer

An instrument that measures changes in air pressure without using a liquid, uses a partially evacuated metal chamber that is sensitive to changes in air pressure (compresses when pressure increases, expands as pressure decreases)

mercury barometer

An instrument that measures changes in air pressure, consisting of a glass tube partially filled with mercury, with its open end resting in a dish of mercury. The height of the column of mercury has a directly proportionate relationship with air pressure

coriolis force

The force that, owing to the rotation (spin) of the earth, deflects objects to the right in the Northern Hemisphere (counter rotation) and to the left in Southern Hemisphere (clockwise rotation). -modifies the direction (not speed) of airflow - strongest at the poles and weakens equator-ward, so the magnitude of the effect is dependent on latitude -always directed at right angles to the direction of airflow -the stronger the wind, the greater the deflecting force

pressure tendency (barometric tendency)

The nature of the change in the atmospheric pressure over the past several hours, which is a useful aid in short-range weather prediction.

wind sock

a cone shaped bag that is open at bond ends and free to change position with shifts in wind direction -its inflation indicates wind strength

closely space isobars indicate...

a steep pressure gradient and strong winds

friction

acts to slow moving objects and decrease wind (mainly surface) speed -greater at faster winds, 0 in calmness -depends on the surface; rough terrains have larger friction than flat barren lands or the ocean -friction reduces the Coriolis force by slowing airflow - affects the air aloft too

the net inward spiral in low pressure system cyclones causes...

air molecules to compact into a smaller area (convergence) - air piles up, creating a denser air column -paradox: low pressure systems cause a net accumulation of air, so surface cyclones should eradicate themselves quickly - could only maintain itself if divergence (spreading out of air) in the air aloft is equal or greater than the inflow below

barometric pressure

air pressure measured by a barometer

4 factors that contribute to changes in air pressure

altitude, temperature, humidity, and the movement of a mass of air from one location to another

aerovane

another type of anemometer which has propellers mounted on the end of a weather vane

the flow around a ridge is _____

anticyclonic

anticyclones

areas of high pressure that spiral clockwise in the Northern Hemisphere and counterclockwise in the southern hemisphere

why does water vapor reduce the density of air?

as the water content of an air mass increases, lighter water vapor molecules displace heavier nitrogen and oxygen molecules, making humid air less dense than dry air

high pressure systems (anticyclones)

associated with dry conditions

surface winds in weather maps blow ____from high pressure and ____ areas of low pressure

away, towards

station pressures

barometric pressures that have not been adjusted for the decline in air pressure with increasing elevation

convergence

causes air to be squeezed into a small space, resulting in an air column exerting more force on a surface, increasing air pressure (net inflow)

rising pressure is associated with

clearing conditions

500mb upper air charts

constructed to show the contour patterns of "hills" and "valleys" of a constant pressure surface - places that experience 500mb pressure at high altitudes are subject to higher pressures than places where the height contours indicate lower altitudes -high elevation contours indicate higher pressures

electronic barometers

converts an electrical signal into digital pressure values that are transmitted and stored

the flow around a trough is ____

cyclonic

an increase in altitude leads to a

decrease in atmospheric pressure and density

pressure gradient

demonstrated by the spacing of the isobars which indicate the amount of pressure change over a given distance - a steep, or strong, gradient causes greater acceleration of a parcel of air= strong winds -widely space isobars indicate a weak gradient and light winds

US standard atmosphere model

depicts the idealized vertical distribution of atmospheric pressure at various altitudes

troughs

elongated areas of low pressure that sweep equator-ward and are associated with cool air moving towards the equator

ridges

elongated high pressure areas that extend towards the poles and are associated with warm air moving poleward

boundary layer

friction significantly reduces airflow only in the first mile of the earth's atmosphere. any air above that is negligible to the effect of friction

anticylonic flow

has the opposite direction of rotation as earth: clockwise in the north, counter in the south

cyclonic flow

has the same direction of rotation as Earth: counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere

Air flows from areas of _______________ pressure to areas of _________________ pressure.

high pressure to low pressure

low pressure systems (cyclones)

in mid latitude areas, produce stormy weather

geostrophic wind

in regions where isobars are relatively straight and evenly spaced, the resulting winds flow in relatively straight lines, parallel to the isobars - generated when a balance is reached between the opposite Coriolis and pressure gradient forces: geostrophic balance

falling pressure is associated with

increasing cloudiness and potential precipitation

hydrostatic balance

is the balance between the upward push of air from the surface toward space, since higher pressure moves towards lower pressure, and the downward pull of gravity

isobars

lines that connect places of equal air pressure

differences in air pressure generate

local and global winds that bring us weather (since it causes air to move, which then influences temperature and humidity)

wind vane

measures wind direction, often seen in tops of buildings - cup anemometer and a wind vane

cup anemometer

measures wind speed. The faster it spins, the faster the wind speed.

by measuring and mapping the pressure field (orientation ad spacing of isobars)...

meteorologists can determine both wind direction and speed

Air pressure drops ________ with altitude in a column of cold (dense) air than in a column of warm (less dense) air.

more rapidly (due to differences in KE)

winds aloft (isolines) tend to flow ______ to the lines of constant height

parallel

wind is controlled by a combination of forces including

pressure gradient force Coriolis force friction

wind rose

provides a way to represent prevailing winds by indicating the percentage of time the wind blows from various directions - the length of the line indicates the percentage of time

rawinsondes

radiosondes tracked by radar that allow us to calculate airflow at several levels of the atmosphere

water vapor _____ the density of air

reduces

friction ____ the coriolis force (proportional to wind speed) and ____ the pressure gradient force

reduces, doesn't affect

divergence

regions where there is a net outflow of air where surface pressure drops

surface level convergence and upper level divergence cause a ____motion

rising

surface level divergence and upper level convergence cause a _____ motion

sinking

sea level equivalents

station pressures converted into barometric pressures that account for altitude by removing its influence, in order to plot on a surface radar map - temp is also considered a bit since it affects air density

atmospheric pressure

the force per unit area on a surface exerted by the weight of the air above; exerted in all directions

pressure gradient force

the force produced when air is subjected to greater pressure on one side than on the other, directed from areas of high pressure towards areas of low pressure - this imbalance created acceleration; the greater this imbalance the greater the wind speed - always directed at right angles to the isobars

in a low pressure system

the inward directed pressure gradient force is opposed by the outward directed Coriolis force, resulting in a counter clockwise flow in the north (cyclonic) -the coriolis force must be weaker in order to balance the pressure gradient and produce centripetal acceleration (winds must be slower)

centripetal acceleration

the inward turning of air, present in low pressure system cyclonic air flow

in a high pressure system...

the outward directed pressure gradient force is opposed by the inward directed Coriolis force, resulting in a clockwise flow in the north (anticyclonic) - the coriolis force must be stronger in order to balance the pressure gradient (winds must be faster)

when the isobars are curved

the pressure gradient force radiates out from the area of high pressure and toward areas of low pressure

surface winds being affected by friction move at an angle across the isobars towards areas of low pressure since...

the pressure gradient force wins against the coriolis force, which is affected by the friction - the rougher the surface, the more friction, the slower the wind will move, the greater the angle from the isobar and, thus, the more that wind will move towards low pressure as pressure gradient wins

wind

the result of horizontal differences in air pressure

what is the main cause of pressure differences, caused by daily temp differences, and, thus, wind?

unequal heating of the earth's surface

supergeostrophic wind

upper air winds around a high pressure that are faster than geostrophic wind -present in the anticyclonic flow of high pressure systems

prevailing wind

when the wind consistently blows more often from one direction than from any other

subgeostrophic winds

winds around a low pressure system in the upper atmosphere that are slower than geostrophic winds (flow straight) -present in the cyclonic flow of low pressure systems

gradient winds

winds around cells of high or low pressure that follow highly curved paths and blow roughly parallel to curved isobars where we see pressure gradient forces and Coriolis forces acting

cyclones

winds around centers of low atmospheric pressure that rotate counterclockwise in the north and clockwise in the south