first, let’s recall the 5+ basic elements of the atmosphere – the main ingredients of weather...
TRANSCRIPT
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