Download Winds

Winds
Notes
What is Wind?
• Wind is the sideways movement of air.
The air always moves from an area of
HIGH pressure to an area of LOW
pressure.
• Differences in air pressure are caused
by differences in how places are heated
by the sun. Warm air has less pressure
than cooler air.
Remember that cities typically have higher
temperatures, and therefore lower pressure than
surrounding areas.
PGF is Pressure Gradient Force. In other words, which
direction is the pressure going?
What is Wind?
•
•
•
The name of a wind tells you where the
wind is coming from. Land breeze, sea
breeze, westerlies, polar easterlies, etc.
Wind speed is measured with an
anemometer.
Blowing winds can reduce body heat, this
is known as a wind-chill factor. Higher
winds equal lower wind-chills. You’ve
probably seen a weather forecast stating
that it’s 42°F, with a wind-chill of 36°F.
Anemometers work
just like the
speedometer in your
car.
The faster the winds
are, the faster the cups
will move.
Remember that
speed = distance/time.
Local Winds
• Local winds are winds that blow over a
short distance. A cool breeze blowing in
from the water over a beach is an
example of a local wind.
• Unequal heating often happens near
lakes and ocean coasts, as well as in
densely packed cities.
Local Winds
• A sea breeze is a local wind that blows
from an ocean. A lake breeze is a local
wind that blows from a lake.
• Sea breezes occur during the day due
to the land heating up much faster than
the water (water has a high specific
heat) and creating rising air a low
pressure.
Local Winds
• A land breeze is a local wind that blows
from the land over onto a lake or ocean.
• Land breezes occur at night as the land
cools off very rapidly, creating sinking
air and high pressure, whereas the
water will be releasing the energy
stored from the daylight, creating rising
air and low pressure.
Global Winds
•
•
•
A global wind is a wind that blows steadily
over very large distances. A global wind
blows from the same direction every time.
The sun heats Earth unequally due to Earth
being a sphere. Sunlight strikes most directly
near 0° latitude, and strikes at less direct
angles as the latitude becomes greater.
Due to this, air has a tendency to rise at the
equator (Low Pressure) and sink at the poles
(High Pressure) just like ocean currents, but
this is occurring in the troposphere.
Global Winds
• Of course, we can’t forget that Earth
also rotates towards the East. This is
going to curve winds just like it does
ocean currents. This is the Coriolis
effect.
• Northern hemisphere winds generally
curve to the right.
• Southern hemisphere winds generally
curve to the left.
Global Wind Belts
• There are 6 “belts” of wind that circle
the entire planet. Three belts in each
the northern and southern
hemispheres.
• Between each of these belts, there are
areas of very calm, very little winds.
•
•
•
•
•
Global Wind Belts
From the equator (0°) to 30° in each hemisphere,
there are the trade winds.
From 0°-30°N, it’s the NE trade winds.
From 0°-30°S, it’s the SE trade winds.
Remember that the names of winds tell you where
they come from.
Where these two trade winds meet, winds are
rapidly warmed by the sun and rise, creating the
doldrums. Doldrums is an Old English word for
“foolish,” for those who got stuck in this area of no
winds.
Global Wind Belts
• From 30°N - 60°N, as well as 30°S - 60°S, winds
always blow from the west. These two prevailing
wind belts are known as westerlies.
• Where the westerlies meet the trade winds at
30°N and S, the area of calm winds form known
as the horse latitudes. This is because as
sailors got stuck in this windless latitude, they
would either have to eat the horses, or throw
them overboard.
Global Wind Belts
• From 60°N to 90°N, as well as from 60°S to
90°S, a prevailing wind will always blow directly
from the poles outwards.
• Remember that air drops quickly over the poles
due to the temperature/density/pressure
change. As the winds blow away from the
poles, they will curve in response to the
Coriolis effect. They will always curve towards
the west.
• These winds are the polar easterlies.
Global Wind Belts
• About 10 km above Earth’s surface
(250mb), there are bands of high-speed
winds that travel without friction to slow
them down. These are jet streams.
• These winds are hundreds of kilometers
wide, blow from west to east, reach
speeds of 200-400 kph, and control the
paths of storms in the United States.