air movement

ENVS 09-26-08

what makes air move ?

• air moves from areas of high atmospheric pressure to areas of low atmospheric pressure

• low pressure areas – troughs

• high pressure areas - ridges

http://www.dnr.sc.gov/climate/sco/Education/wxmap/wxmap.gif

• isobars = equal lines of pressure• distance between isobars → change in pressure

over distance = pressure gradient• isobars close together → pressure varies rapidly

→ air flows rapidly form high to low pressure → strong winds

pressure gradient force

• describes how rapidly air pressure changes between two points

• steep gradient (isobars close together) → air moves rapidly → pressure gradient force is large

• shallow gradient → pressure gradient force is small

but…

• most atmospheric patterns are swirls of clouds and winds …

• very few straight patterns

The Rotating Earth → Coriolis “Force”

• Earth is a rotating system

• On earth things that move straight appear to move on curved path as earth rotates beneath

• swirly cloud patterns on satellite images, zone of westerlies etc.

Coriolis Effect does NOT

determine the way your sink drains !

Coriolis Effect with (almost) no Math

a) movement to and from the poles

N-S moving air:

• air has larger tangential velocity at equator than at pole (equator = max, pole = 0)

• as air moves polewards it maintains its tangential velocity

• since it is encountering slower moving air it “overtakes” these higher latitude air masses

• apparent deflection towards the right (in N – hemisphere)

• air moving towards the equator seems to lag lower latitude air masses → again, deflection to the right

from: www.bom.gov.au/info/ftweather/ page_14.shtml

Coriolis Effect Summary:

• earth is a rotational system

• standing on earth, objects that move on straight line seem to be following curved paths (because we are rotating)

• Coriolis effect deflects air masses– towards right on northern hemisphere– towards left on southern hemisphere

Coriolis Effect - Magnitude

• effect is very small

• only affects large scale movements (Hurricanes, ocean currents etc.)

• it will not affect small, household scale currents

air currents II

influences on air currents

• pressure gradient force: air flows from areas of high pressure to areas of low pressure – the more rapid the change in pressure the stronger the flow

• Coriolis effect: due to rotation of the earth air masses are deflected to the right on northern hemisphere and deflected to the left on southern hemisphere

• resulting flow: geostrophic flow

Coriolis effect

• rotating earth → air masses are moving on curved paths across earth

• on northern hemisphere: deflection to the right

• on southern hemisphere: deflection to the left

coriolis effect

http://www.physics.umd.edu/lecdem/services/demos/demosd5/d5-11.htm

Coriolis effect demonstrations

stationary disk

rotating disk – stationary observer

rotating disk, rotating observer

one last time:

• on northern hemisphere – air is deflected to the right

• on southern hemisphere – air is deflected to the left

due to the rotation of the earth

close to ground:

• friction between atmosphere and surface slows air down

• winds are stronger at high elevations

three influences on air movement

• pressure gradient force

• Coriolis effect

• friction

conservation of angular momentum

• combination of pressure gradient force and Coriolis effect cause air to spin

• as air is sucked into low-pressure area it spins faster and faster due to the conservation of angular momentum

at high altitudes

• friction between atmosphere and earth’s surface is negligible

• air currents are influenced by balance between pressure gradient force and Coriolis effect

→ geostrophic winds

development of geostrophic flow

• pressure gradient force gets air moving• Coriolis effect deflects to right and keeps

deflecting until…• effects of pressure gradient and Coriolis effect

cancel out→ now wind flows almost parallel to isobars !

airflow symbols

http://www.atmos.washington.edu/~mcmurdie/oct96/300mb_geownd_17oct.gif

two ways to depict air pressure

• lines of equal pressure (isobars)

• equal pressure surfaces

http://www.physicalgeography.net/fundamentals/7d.html

constant pressure levels

• are at higher elevation for high pressure areas

• are at lower elevation for low pressure areas

http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg

Effect on Hurricanes:

• balance between Coriolis force and pressure gradient

• causes rotation of storm system

• counterclockwise in N-hemisphere

• clockwise in S-hemisphere

surfacewinds

http://www.srh.noaa.gov/hun/stormsurveys/2008-02-06/weather/12z_500mb.jpg

surface winds or friction layer winds

• friction between atmpsphere and earth’s surface prevents air masses to reach equilibrium between coriolis effect and pressure gradient force

• friction layer winds blow at an angle to the isobars

local wind systems

• land and sea breezes

• Föhn winds (Chinook)

land and sea breezes

• differential heating between sea and land

• land heats up faster and warm air rises (low pressure area over land)

• process reverses at night

Föhn winds

• orographic rain – air masses lose most of their moisture when they cross a mountain range

• on the other side the air is warm and very clear

for all of you who still haven’t enough…

Global Atmospheric Circulation

ENVS 110 - 10-03-2008

http://en.wikipedia.org/wiki/Image:IntertropicalConvergenceZone-EO.jpg

Vertical air movement in the atmosphere

http://en.wikipedia.org/wiki/Image:Omega-500-july-era40-1979.png

Seasonal Changes

link

Polar Front

Planetary frontal zone

Mixing of warm and cold air exchange of heat from subtropics to higher latitudes

Local effects

• Asian monsoon

• North American monsoon

http://img.timeinc.net/time/photoessays/2007/india_monsoon/india_monsoon_01.jpg

Differential warming between continents and oceans• Summer – Indian sub continents heats up• Develops low-pressure zone over continent• Sucks in moist air from surrounding oceans

• India:– Warm subtropical oceans extremely moist

oceanic air– Orographic rain on southern edge of Himalayas