Download Pressure Wind

PRESSURE, WINDS
AND CIRCULATION PATTERNS
Obvious Next Questions:
How do insolation and heat energy transfer
processes relate to Weather and Climate
phenomena on Earth?
WEATHER AND CLIMATE
Weather: refers to the condition of atmospheric
elements at a given time, and for a specific area.
Climate: average of weather conditions for 30+
years (including atmospheric anomalies).
Weather and Climate are of prime interest to the
Physical Geographer because they affect and
are interrelated with all of Earth’s environments.
Earth’s Weather and Climate are the results of the
interrelationships between Earth and the Sun, and
between our Atmosphere:
First, Let’s Recall the
5+ Basic Elements of the Atmosphere
– the main ingredients of weather and climate
-- Also called Elements of Weather and Climate
• Solar Energy
-- Insolation and Heat Energy Transfer
• Temperature
In this segment, we’ll discuss the next two elements:
•
•
•
+
Pressure
Wind
Precipitation
Air Masses (and Fronts)
PRESSURE, WINDS AND CIRCULATION PATTERNS

Atmospheric Pressure




Basic Pressure Systems – High & Low Pressure
Pressure Variations – Vertical and Horizontal
Mapping Pressure Distribution – Isobars, Pressure
Gradients, Global Pressure Belts
Winds and Circulation Patterns



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Pressure – Wind Relationships
Wind Modifiers – The Coriolis Effect, Friction
Surface Wind Systems – Global, Upper Air, Seasonal, Local
Ocean – Wind Relationships – Ocean Currents, El Nino,
La Nina and the Sothern Oscillation
PRESSURE

Pressure is the weight of the atmosphere
Atmosphere as a whole has considerable weight and exerts average
pressure of about 14.7 lbs. /sq. in.





at sea level a column of air one inch square weighs 14.7 lbs
this 14.7 lbs of pressure exerts pressure forcing mercury in
a tube to rise 29.92 inches
a device used to measure air pressure is called a barometer
– hence, barometric pressure
Pressure is usually measured in millibars (mb) –
29.92 inches is translated into 1013.2 millibars
Atmospheric Pressure is important because it directly
affects atmospheric circulation –
the differences in pressure create our wind systems,
and the movement of the winds drive our ocean currents
BASIC PRESSURE SYSTEMS

There are two basic types of pressure systems:


the Low, or Cyclone, or “L”  Convergent, ascending air
the High, or Anticyclone, or “H”  Divergent, descending air
PRESSURE also describes the tendency of air to rise or sink at any given place or time
LOW PRESSURE

Warm moist air
below 1013.2
millibars

air movement is in
and up
 convergent

air
Cyclone – a low
pressure center
HIGH PRESSURE

Cold dry air above
1013.2 millibars

air movement is
down and out
 divergent

air
Anticyclone – a high
pressure center
PRESSURE VARIATIONS 1

Vertical Variations –

Air pressure decreases with elevation – pressure on
top of Mt. Everest is only about 1/3rd of that at sea level

The higher we go, air density decreases as the air
molecules become more diffused and widely spaced

Air tends to rise or sink as a result of its density – at ground
level, air density is governed by its temperature –
As air is heated, it expands, becomes less dense, thinner,
and rises


Reduced air pressure also means less oxygen per
breath – that’s why airplane cabins are pressurized
MAPPING PRESSURE DISTRIBUTION
• Isobar Maps
• Pressure is mapped using isobars, every 4 mb
• Winds blow from higher to lower pressure
Pressure Gradients and Winds
GLOBAL PRESSURE & WIND
North Pole 90°N
Arctic circle 66.5°N
HIG
H
Tropic of Cancer
23.5°N
Equator 0°
LOW
Tropic of Capricorn
23.5°S
Antarctic circle 66.5°S
HIG
H
South Pole 90°S
ZONE of least heating
produces HIGH
PRESSURE
ZONE of
greatest
heating
produces LOW
PRESSURE
ZONE of least heating
produces HIGH
PRESSURE
Idealized World Pressure Belts
PRESSURE, AND WIND
Air stops rising when it
meets air of equal
density, then diverges at
high level to produce
more wind which
eventually sinks
elsewhere to complete
the circulation cell
WINDS are horizontal flows of
air; winds blow from areas of
high pressure to areas of low
pressure (nature tries to equalise
pressure)
�
PRESSURE describes the
tendency of the air to rise or to
sink at any given place or time.
�
Air tends to rise or sink as a
result of its density.
�
Isolation
Denser air
drawn in at
low level to
replace
rising, less
dense air
Air heated by
contact with
ground
expands;
becomes less
dense and rises
LOW
PRESSUR
E
Sun heats up
Air density varies with altitude
but, at the ground level, air
density is governed by its
temperature.
�
Thus, variations in radiation and
temperature control pressure and
wind.
�
Denser air drawn in at
low level to replace
rising, less dense air
WIND AND CIRCULATION PATTERNS

Wind is the horizontal movement of air in response to
differences in pressure



Basic Wind Patterns -- induced by Pressure differences



Winds are the means to balancing:
(a) the uneven distribution of pressure, and
(b) radiational heating and cooling
Winds also influence the rate and distribution of evaporation, and
thus, precipitation
Linear -- Pressure Gradients and Winds (Fig. 5.5)
Circular -- Cyclones (L), Anticyclones (H) and Winds (Fig. 5.9)
Wind Modifiers:


Friction and Wind (effective up to about 1000 m above the
surface – reduces wind speed)
The Coriolis Effect and Wind – deflection due to rotation (Fig. 5.6)
The Coriolis
Effect
Deflection of
winds and
ocean currents
to the right in
the northern
hemisphere
and to the left
in the southern
hemisphere
Caused by
earth’s rotation
below
Coriolis Effect
http://www.youtube.com/watch?v=mcPs_OdQOYU&feature=related
AIR PRESSURE AND WINDS
SUBGLOBAL SURFACE WIND SYSTEMS
SURFACE WINDS

Land & Sea Breeze
 Mountain & Valley Breeze
H
Santa Ana Winds
L
Adiabatic Cooling and Heating