PRESSURE, WINDS AND CIRCULATION PATTERNS C.J. Cox/ Air Pressure & Winds PRESSURE, WINDS AND CIRCULATION PATTERNS Geography Dept. (916) 789-2862

5+ Basic Elements of the 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 + 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 atmosphere 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) – 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

Mercurial Barometer 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 C.J. Cox/ Air Pressure & Winds 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 Geography Dept. (916) 789-2862

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

C.J. Cox/ Air Pressure & Winds Idealized World Pressure Belts Geography Dept. (916) 789-2862

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

The Coriolis Effect 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 Upper Air Winds Wind Belts Surface Winds Earth

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 Seasonal Winds --- The Monsoons

South AsiAn / INDIAN Monsoon

Movement of Pacific High and California Weather

LOCAL/Surface Winds Land & Sea Breeze

Local / Surface Winds Mountain & Valley Breeze

H L Santa Ana Winds Other Similar Local Winds: Chinook (Rockies) Foehn (Alps) L

Chinook Wind Adiabatic Cooling and Heating

Upper Air Winds

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