PRESSURE, WINDS
AND CIRCULATION PATTERNS
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 + Circulation
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




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
ATMOSPHERIC PRESSURE

Pressure is the weight
of the atmosphere
Mercurial Barometer
Atmosphere as a whole has considerable
weight and exerts average pressure
of about 14.7 lbs. /sq. in.


A device used to measure air
pressure is called a barometer
– hence, barometric pressure
Pressure is usually measured
in millibars (mb) –
MERCURIAL BAROMETER
14.7 lbs. of pressure causes mercury
in a barometer to rise 29.92 inches,
which is translated into 1013.2 millibars.
And this Standard sea-level pressure
is also considered to be “normal”
pressure.
 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
PRESSURE also describes the tendency of
air to rise or sink at any given place or time
[as we will see later]
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
LOW PRESSURE

Pressure
below 1013.2 mbs.,
characterized by:


warm moist air
air movement is up
and in


“convergent air”
Also called Cyclone
– a low pressure
center
HIGH PRESSURE

Pressure above
1013.2 mbs.,
characterized by:


Cold dry air
Air movement is down
and out
 “divergent

air”
Also called
Anticyclone – a high
pressure center
PRESSURE VARIATIONS . . .
 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
PRESSURE VARIATIONS . . .

Horizontal Variations – grouped into two:

Thermal (determined by temperature):
As earth’s surface heats the air in contact
with it, the air expands in volume and
decreases in density – when this warm air
rises, there is less air near the surface, resulting in decrease in
surface pressure  Such low pressure occurs regularly along the
Equator  The opposite occurs in the Polar areas.

Dynamic or Mechanical (due to motions of the atmosphere):
The dynamic causes are more complex and related to Earth’s
rotation and broad patterns of circulation  Both the Subtropical
High and Subpolar Low pressure regions are dynamically induced.
MAPPING PRESSURE DISTRIBUTION
• Isobar Maps ► Pressure is mapped using isobars, every 4 mb
• Pressure Gradient ► Winds blow from higher to lower pressure
• Global Pressure Belts and Pressure Maps
Idealized World Pressure Belts
Actual
Global Pressure
Maps –
Seasonal Variations
WIND AND CIRCULATION PATTERNS

Wind is the horizontal movement of air in response
to differences in pressure



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
Basic Wind Patterns
induced by Pressure configurations


Linear -- Pressure Gradients and Winds
Circular -- Cyclones (L), Anticyclones (H) and Winds
Pressure Gradients and Winds
Cyclones (L),
Anticyclones (H)
and Winds
Movement of
Surface Winds
associated with
Cyclones and
Anticyclones
in the Northern
and Southern
Hemispheres
 Wind Modifiers:
 Surface Friction (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
Geostrophic Wind
Upper-level winds in which the Coriolis effect and pressure gradient are balanced,
resulting in wind flowing parallel to the isobars.
EARTH’S WIND SYSTEMS – VERTICAL LAYERING
WIND BELTS:
GLOBAL SURFACE WIND SYSTEMS
Idealized Model of Atmospheric Circulation
Trade Winds
ITCZ + Doldrums
Subtropical Highs
+ Horse Latitudes
Westerlies
Polar Easterlies
SUBGLOBAL SURFACE WIND SYSTEMS
SOUTH ASIAN /
INDIAN MONSOON
Movement of Pacific High and California Weather
LOCAL/SURFACE WINDS

Land & Sea Breeze
 Mountain & Valley Breeze
Santa Ana Winds
H
Other Similar Local Winds:
Chinook (Rockies)
Foehn (Alps)
L
Chinook
Wind
Adiabatic Cooling
and Heating
Subtropical Jet Stream
Polar Front Winter Jet Stream
OCEAN-ATMOSPHERE RELATIONSHIPS
 Ocean Currents – broad circulatory patterns
(Gyres)
 Clockwise in Northern H.
 Counterclockwise in S.H.
 Gyres do not cross the
equator
 Upwelling
 Oscillations
Major Ocean Currents
 Anomalies – Pressure Oscillations

El Nino - Southern Oscillation (ENSO) and La Nina
• These are linked atmospheric and oceanic phenomena of pressure
and water temperature in equatorial Pacific.
• Southern Oscillation refers to a periodic seesaw of atmospheric
pressure in the tropical southern Pacific Ocean basin.
• El Nino involves the weakening or reversal of the trade winds and
the warming of surface water off the west coast of South America.
• La Nina is often described as the opposite of El Nino; it is associated
with cooler than usual water off the west coast of South America.
• Importantly, these events influence weather and climate patterns
across the globe.

North Atlantic Oscillation (NAO)

Pacific Decadal Oscillation (PDO)

Arctic Oscillation (AO)
Thermal Infrared Satellite Images of
El Nino and La Nina episodes in the Tropical Pacific
El Nino
La Nina