PRESSURE, WINDSAND CIRCULATION PATTERNS

First, Let’s Recall the5+ 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

PRESSURE

Pressure is the weight of the atmosphereAtmosphere 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) –

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.

PRESSURE also describes the tendency of air to rise or sink at any given place or time [as we will see later]

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

At sea level a column of air one inch

square weighs 14.7 lbs

This 14.7 lbs

of weight exerts

pressure forcing

mercury in a tube to rise

29.92 inches or 1013 mb

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

Pressurebelow 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 perbreath – 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

Movement of Surface Windsassociated with Cyclones and Anticyclones in the Northern and SouthernHemispheres

Cyclones (L), Anticyclones (H) and Winds

The Coriolis EffectDeflection 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 Effecthttp://www.youtube.com/watch?v=mcPs_OdQOYU&feature=related

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)

Geostrophic WindUpper-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

Trade Winds

ITCZ +

DoldrumsSubtropical Highs + Horse Latitudes

Westerlies

Polar

Easterlies

Idealized Model of Atmospheric Circulation

SUBGLOBAL SURFACE WIND SYSTEMS

SOUTH ASIAN /INDIAN MONSOON

Movement of Pacific High and California Weather

LOCAL/SURFACE WINDS

Land & Sea Breeze

Mountain & Valley Breeze

H

L

Santa Ana Winds

Other Similar Local Winds:Chinook (Rockies)Foehn (Alps)

Adiabatic Cooling and Heating

ChinookWind

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 atmosphere 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