Agenda - WHAT YOU LEARN

• What is pressure and how is it measured?
• What is wind and what is its role in the global ecosystem?
• How does unequal heating sets up pressure differences
• What are high pressures and low pressures
• What are the three factors that affect wind direction
• How do winds circulate around highs and lows?

WHAT IS PRESSURE?

• Pressure is the weight of the atmosphere
• Average sea level pressure = 15 lbs

HOW IS PRESSURE MEASURED?  Using a Mercury Barometer

• Air pressure pushes mercury up the tube
• The mercury rises and falls as pressure changes
• Average sea level height is 30 inches (76 cm)

CHANGES IN PRESSURE

• Typical sea level pressure varies 28.9 to 31 inches
• Pressure decreases with increasing altitude above sea level

PRESSURE VARIES WITH WEATHER CONDITIONS

• Fair, clear weather is typical of high pressure
• Stormy weather means low pressure
• Pressure also varies with elevation

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WHAT IS WIND?

• A horizontal movement of air
• Vertical movements are currents or updrafts and downdrafts

WHAT IS THE ROLE OF THE WIND?

• Wind moves excess heat from the tropics toward the poles
• Wind moves water vapor from ocean to land
• Winds move ocean currents
• Wind creates waves on the ocean and other water bodies

WHAT CAUSES WIND?

• Unequal heating
• Unequal heating causes pressure differences
• Winds blow from high to low

BRITISH LIVING ROOM IN WINTER

HEAT ENGINE IN MID AFTERNOON

HEAT ENGINE AT SUNRISE

HOW CAN YOU SHOW PRESSURE ON A MAP?

• Meteorologists use imaginary lines to show changes in pressure across the surface of a map
• An isobar is a line joining places with the same air pressure (adjusted to sea level)

AIR PRESSURE AND ISOBARS

• Isobars are usually plotted in the metric system of millibars
• Average sea level pressure is 1013.2 millibars
• Isobars are shown on maps at 4 millibar intervals

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THREE FACTORS AFFECT WIND DIRECTION

• Pressure Gradient Force
• Coriolis Effect
The Coriolis effect deflects winds and ocean currents:
• To the right in the northern hemisphere
• To the left in the southern hemisphere
• Friction slows surface wind speed and weakens the Coriolis effect

WHICH WAY DOES THE WIND BLOW AROUND HIGH AND LOW PRESSURE CENTERS IN THE NORTHERN HEMISPHERE?

• Clockwise and outward around a high pressure
• Counterclockwise and into a low pressure

HIGH PRESSURE AND LOW PRESSURE

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REALITY CHECK

• Pressure is:

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• Pressure is measured using a:

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• Wind is a:

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• When hot air rises, it creates:

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• When cold air sinks, it creates:

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• Winds blow from:

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• Pressure is shown on maps using:

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• High pressure is associated with weather that is:

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• Low pressure is associated with weather that is:

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• Unequal heating along the coastline creates breezes.
  During the afternoon, wind blows from:

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GLOBAL HEAT ENGINE

THE EARTH IS HEATED UNEQUALLY

2.5 times more insolation at the equator than at the poles

UNEQUAL HEATING CAUSES PRESSURE DIFFERENCES

• Cold, heavy air sinks
• Warm, expanding air rises

A SIMPLE CONVECTION SYSTEM WOULD SHOW THREE MAIN PRESSURE BELTS
(NON-ROTATING EARTH)

• Polar High
• Equatorial Low
• Polar High

 
Winds would blow from high to low
From poles to the equator

BUT THE EARTH ROTATES

• The Coriolis effect complicates this pattern of global circulation.
• Warm air rising at the equator turns back towards the earth’s surface at 30 degrees north and south of the equator.
• Subsiding air creates a warm subtropical high-pressure area.

PRESSURE BELTS AND WIND BELTS

Atmospheric circulation - three convection cells
• Hadley Cell
• Ferrel Cell
• Polar Cell

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SURFACE WINDS ON A UNIFORM EARTH FROM NORTH POLE TO THE EQUATOR

• Polar High
• Polar Easterlies
• Polar Front
• Westerlies
• Subtropical High
• Northeast Trade Winds
• Intertropical Convergence Zone

GLOBAL PRESSURE AND WIND BELTS MIGRATE WITH THE SEASONS

Shift 5 degrees north and south with the migration of the sun's direct rays

POLAR FRONT - BATTLEGROUND OF CONTRASTING AIR MASSES

• Storms are born along the Polar Front
• Rossby Waves develop where warm and cold air in contact

JET STREAMS DEVELOP IN UPPER AIR WAVES

• Form narrow zone of high velocity winds
• 200-250 mph at the core

MONSOONS - A SEASONAL REVERSAL OF WINDS

Monsoon in India
WINTER - THE DRY NORTHEAST MONSOON
High Pressure over Siberia causes dry outblowing winds over India.

SUMMER - THE WET SOUTHWEST MONSOON
• Low pressure over Northern India sucks in moist air from the Indian Ocean.
• Some years, the monsoon brings torrential rains and floods.

THE WORLD'S RAINY PLACES COINCIDE WITH GLOBAL PRESSURE AND WIND BELTS

THREE MAJOR GLOBAL RAIN BELTS

• The Northern Hemisphere Polar Front
Midlatitude Cyclonic Storms - low pressure
• The Intertropical Convergence Zone (ITCZ)
Trade winds converge along the equatorial low
Convectional lifting produces cumulus clouds
• The Southern Hemisphere Polar Front
Midlatitude Cyclonic Storms - low pressure

FIVE KEY IDEAS ABOUT PRESSURE AND WINDS

• The Polar Front is a zone of changeable weather and the birthplace of midlatitude cyclones.
• The Westerlies are the dominant wind system across the United States and Europe.
• The drought-causing subtropical highs create deserts and dry summers in California.
• The ITCZ (Intertropical Convergence Zone) brings cumulonimbus clouds and convectional rain to the equatorial belt.
• Pressure and wind belts migrate 5°N and 5°S of the equator with the overhead sun.

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