Wind
Atmospheric Circulation
(22:39min)
Wind
• The horizontal movement
of air from an area of high
pressure to an area of
lower pressure.
• Caused by the uneven
heating of Earth’s surface
by the sun.
– Cooler, more dense air has
higher air pressure and
sinks.
– Warm, less dense air has
lower air pressure and
rises.
Isobar Maps
• Isobar Maps show air pressure changes over an
area.
– The closer the isobars = higher the wind speeds.
– The farther the isobars = slower the wind speeds.
H
L
Fast Winds
Slow Winds
H
Wind
• Winds are classified by their direction and speed
– Direction is described by the direction the wind is
coming from.
• Arrows point in the direction that the wind is blowing to.
– Ex. South wind blows from the south toward the north.
– Speed is measured in miles per hour (mph) or a knot
(kn).
• 1 knot = 1.151 mph
– ½ feathers = 5 knots
– Full feathers = 10 knots
Determining Wind Speed & Direction
Direction: From the Southeast
Speed: 25 knots
Wind Types
• Winds are classified by how large of an area
they affect.
1. Local Winds - blow over short distances.
2. Global Winds - blow steadily from specific
directions over long distances.
Local Winds
• Blow from any direction and usually cover
short distances
• Causes:
– The sun heats the land faster than the water.
– Land loses heat faster than water.
• 2 Types:
– Land Breeze
– Sea Breeze
Land Breeze
• Land Breeze – The flow of air from the land to
the sea.
– Occurs during the night when the land cools off
faster than the sea. (Nighttime ONLY)
• Convection currents drive the cooler air out over the sea.
Sea Breeze
• Sea Breeze – the flow of air from the sea to the
land.
– Occurs during the day when air over the land is
warmer than the air over the sea. (Daytime ONLY)
• Convection currents drive the cooler air in over the land.
Global Winds
• Winds that blow steadily from specific directions
over long distances.
– Warm air rises at the equator and cold air sinks at the
poles thus producing global winds.
• Due to Earth’s rotation, global winds do not
follow as straight path but curve – Coriolis effect.
–
–
–
–
Northern Hemisphere = curve to the right
Southern Hemisphere = curve to the left
Equator = doldrums, calm to no wind
30°N & S Latitude = Horse Latitudes, calm to no winds
Doldrums & Horse Latitudes
Northern Hemisphere
Horse Latitudes
Doldrums
Horse Latitudes
Southern Hemisphere
Equator
Global Wind Belts
• Trade Winds
– Winds that blow between the equator and 30° N and S
latitude, east to west.
– Belt of warm, steady wind.
• Prevailing Westerlies
– Winds that blow between 30° and 60° N and S latitude,
west to east.
– Very strong winds!
• Polar Easterlies
– Winds that blow between 60° N and S latitude and the
poles, east to west.
– Extremely cold but weak.
Global Wind Belts
Polar Easterlies
Prevailing Westerlies
Trade Winds
Equator
Trade Winds
Prevailing Westerlies
Polar Easterlies
Jet Streams
• Approximately 10km above Earth’s surface are
bands of high-speed winds that blow from west
to east.
– Form at boundaries of adjacent air masses with
significant differences in temperature.
• 2 Types:
– Polar Jet Stream – 30-60° N and S latitude
• Arctic cold meets warm air mass
– Strongest jet stream
– Subtropical Jet Stream – 30° N and S latitude
• Tropical hot meets warm air mass
– Weaker jet stream
Jet Streams
Subtropical
front