Understanding Air Pressure
Air Pressure
• Is exerted in all
directions: down, up,
sideways
• Sea level pressure =
1 kilogram per square
inch.
Measuring Air Pressure
• Barometer – used to
tell the air pressure
around you (in
millibars)
• Increase in AP =
Mercury rises
• Decrease in AP =
Mercury lowers
Wind
• A result of horizontal
differences in air
pressure.
• Flows from high to
low pressure
• Solar radiation =
ultimate source for
wind.
Control Wind
• Three factors:
1.) Pressure
Differences
2.) Coriolis Effect
3.) Friction
Pressure Differences
• Pressure Gradient –
read from isobars
• Close isobars = high
winds
• Far apart isobars =
low winds
Coriolis Effect
1.) A right angle to the
direction of air flow
2.) Affects wind
direction and wind
speed.
3.) Strong wind = strong
deflection
4.) Strong at poles and
weak at equator
Friction
• Slows down air
movement which
changes direction.
• Above layer = parallel
to isobars (low
friction)
• Jet streams = fast
moving “rivers” of air
Pressure Centers and Wind
Fancy for: Cyclones
High Pressure vs. Low Pressure
• Anticyclones
• Cyclones
• Pressure increases
from outside to center
• Pressure decreases
from outer to center
• Winds blow outward
and clockwise
• Winds blow inward
and counterclockwise
• Net flow = outward
• Net Flow = inward
Weather and Air Pressure
• Rising air = cloud
formation and
precipitation.
• Sinking air = clear
skies
• Have to have
convergence and
divergence!
Weather Forecasting
• Low pressure system
= bad weather!
Global Winds
• Atmosphere = Air
conditioner
• Warm air goes toward
high latitudes
• Cool air towards
equator.
Non – Rotating Earth Model
• Where hot air goes to
the poles when it
reaches the
troposphere.
Rotating Earth Model
• 4 pockets:
1.) Trade Winds
2.) Westerlies
3.) Polar Easterlies
4.) Polar Front
Influence of Continents
• Southern Hemisphere
= continuous
pressure system.
• Land + Water =
different pressure
systems
Monsoons
• Seasonal changes in
the wind direction.
• LOTS OF RAIN!!!
Regional Wind Systems
Section 19.3
General Info
• Circulation in the middle latitudes is
complex and does not fit the convection
system described for the tropics.
Local Winds
• Caused by either topographic effect or by
variations in surface composition.
– Land and Water
Valley and Mountain Breezes
• Valley Breeze – heat during the day =
warm air that rises
• Mountain Breeze – cooling at night = air
moving into the valley
Measuring Wind
• Two basic measurements
– Direction and Speed
• Labeled by the direction they flow
– Ex. North wind – north towards the south
– Measured by a wind vane
• Wind Direction
– Prevailing wind –
blows fro the same
direction consistently
• Ex. Westerlies (US)
• Wind Speed
– Measured by an
anemometer
El Nino
• Warm
countercurrents that
become unusually
strong and replace
cold offshore water
with warm water.
• 3 – 7 years
La Nina
• When surface
temperatures in the
eastern Pacific are
colder than average
• Distinctive set of
weather patterns.