Download I. What is air?

Air Pressure &
Wind
• Air pressure is linked
to temperature.
• Colder air is denser
than warm air.
• Air pressure exerted by
colder air > that by
warmer air.
• Lines on a map that
connect points of equal
temperature are called
isotherms.
Air Pressure &
Temperature
http://www.middleschoolscience.com/isotherms.htm
• Air pressure is measured
with a barometer.
• Originally measured in
inches of mercury using a
mercury barometer.
Air Pressure
Measurement
Homework: Make your own barometer
Materials:
1 large can, open at one end
1 plastic straw
1 large latex balloon
1 sheet of cardboard
tape
• cut the balloon to create a large sheet of
latex.
• on a day of average barometric pressure,
stretch the latex tightly over the open end
of the can and seal with tape.
• attach the straw and cardboard sheet as
shown.
• record barometric readings (high, low,
average) twice daily for 7 days.
• Currently measured with
an aneroid barometer.
• Meteorologists use units
called millibars, average
is 1013 mb.
Measruring
Air Pressure
High Pressure Systems
• Cool air is more dense, causes high pressure areas.
• Highs are associated with cool, drying, sinking
air—clear skies, fair weather.
Low Pressure Systems
• Warm air is less dense, causes low pressure areas.
• Lows are associated with warm, moist, rising air &
cloud formation—rain and storms.
Factors That Affect Wind
• Pressure differences
• Coriolis effect
• Friction
Wind is the result of unequal heating of
Earth’s surface.
Air moves from areas of high pressure to
areas of low pressure.
Greater pressure differences = higher wind
speeds.
• Pressure differences can exist due to temperature
differences causing differences in density.
Pressure Gradient
• Pressure changes across a given distance
occur over a continuous spectrum of values,
like a number line.
• Steep pressure gradient is like a steep hill –
causing greater acceleration of wind speed.
Isobars - lines connecting points of equal pressure.
Closely-spaced isobars = steep pressure gradient =
strong winds.
Widely-spaced isobars = light winds.
Activities -
Isotherm Activity
Isobar Activity
Isotherm Activity – draw your own isotherms every 10 degrees (40,
50, 60, etc. After you draw each one, select the isotherm you drew
under “Reveal Answer” to see how you did. Erase and redraw as
needed.
Isobar Activity - Weather maps
often abbreviate air pressure
readings to save space by using
only the last two whole digits plus
one decimal place. Ex: 1020.4 mb
would read 204. Click on the box
that says ‘Decode Pressures to see
the entire numbers. Then draw your
own isobars, checking and
redrawing each one as you did with
isotherms.
Coriolis effect causes moving objects to be
deflected to right in Northern hemisphere, left in
Southern Coriolis Effect 1 Coriolis Effect 2
Coriolis Effect 3 Coriolis Effect 4Coriolis Effect 5
http://www.youtube.com/watch?v=49JwbrXcPjc
Air Pressure & Weather Forecasting
• Air spirals into low pressure systems, causing
column of air to rise, increasing air pressure –
self-limiting.
• Rising air causes cloud formation & precipitation.
• Air pushes into warm, low pressure areas,
spiraling in counterclockwise cyclone (stormy
weather)
• From cold air mass, denser air pushes
downward and spirals out clockwise, causing
anticyclone (dry, fair weather)
Air Pressure & Weather Forecasting
• Surface conditions affect/reflect what is
going on in the atmosphere above.
Friction
• Friction slows air movement and can change its
direction.
• Only acts close to Earth’s surface.
• Above friction layer,
Coriolis Effect &
Pressure Gradient
balance each other,
causing air to move
parallel to isobars –
jet stream.
• Within friction layer,
slower wind speeds
decrease Coriolis
Effect.
Friction &
Coriolis Effect
Air Pressure & Weather Forecasting
• Weather systems generally move west-to-east
across North America.
• Observing changes in temperature, air pressure,
wind speed and direction, and cloud cover help
forecasters tell what is coming.
• Unequal
heating from
equator to
poles.
• Non-rotating
Earth Model:
cold air would
sink at the poles
and flow all the
way to the
equator, where
it would rise.
Global Winds
Global Winds
• Rotating
Earth Model:
results in 6
global wind
cells, 5 bands
of high/low
pressure, and
global wind
patterns due
to Coriolis
Effect.
Continental Influence
• Land masses interrupt global pressure belts.
• Develop seasonal highs in winter, seasonal lows
in summer.
• Result in seasonal wind changes – monsoons.
• Land & Sea Breezes
• Valley & Mountain
Breezes
• Sunny day, air of mt.
slopes heats more than
air of valley, less
dense, blows upslope –
valley breeze.
• At night, mt. slopes
cool, air’s more dense,
flows downslope –
mountain breeze.
Regional
Winds
Wind Measurements
• Speed – anemometer
– knots or mph
• Beaufort Scale
• Direction – named for
direction from which
they blow – compass
direction or degrees.
• Prevailing wind –
more often blows from
one direction than any
others.
El Nino
• Periodic abnormal
warming of ocean
water, pushes into
normally cold water.
• Disrupts jet streams
and normal weather
patterns – North
American winters may
be warmer in north,
wetter in south.
• El Nino
La Nina
• Surface temps. in
eastern Pacific
colder than average
• Sends colder air
over Pacific
Northwest & Great
Plains, warms rest
of U.S.