Transcript Air Pressure and Wind

[ORIGIN from Greek anemos ‘wind’]
[ORIGIN from Greek baros ‘weight’]
Beaufort
• ..\Activity
Source\Beaufort_Wind_Chart.pdf
1013.25mb= 1013.25hPa= 29.92 Hg
In a high pressure area,
air will (rise, sink)
because the air is
(less, more) dense.
This is because the air is
(cold, warm)
and (rises, sinks).
Therefore, clouds CANNOT form.
In a low pressure area,
air will (rise, sink)
because the air is
(less, more) dense.
This is because the air is
(cold, warm)
and (rises, sinks).
Therefore, clouds are
LIKELY to form.
Correctly draw the direction of wind flow
around both a high and a low pressure area
in the NORTHERN HEMISPHERE.
low
pressure
warm or cold air
air rising or sinking
clouds or no clouds
clockwise or
counterclockwise wind
direction
winds toward or
away from the center
high
pressure
On the diagrams below, label which one represents a
land breeze and which represents a sea breeze.
Correctly label on each diagram where the
high and low pressure areas would be found.
On the diagrams below, label which one represents a
land breeze and which represents a sea breeze.
Correctly label on each diagram where the
high and low pressure areas would be found.
On the diagrams below, label which one represents a
land breeze and which represents a sea breeze.
Correctly label on each diagram where the
high and low pressure areas would be found.
What’s the relationship
between wind and waves?
Using the chart on page 14, state the prevailing
wind direction for each latitude below:
45°N
45°S
75°N
20°N
Air stops rising when it
meets air of equal
density, then diverges at
high level to produce
more wind which
eventually sinks
elsewhere to complete
the circulation cell
Pressure, Wind and Weather Systems
WINDS are horizontal flows of air;
winds blow from areas of high
pressure to areas of low pressure
(nature tries to equalise pressure)
�
PRESSURE describes the
tendency of the air to rise or to sink
at any given place or time.
�
Air tends to rise or sink as a result
of its density.
�
Insolation
Air heated by
contact with ground
expands; becomes
less dense and rises
Denser air
drawn in at
low level to
replace rising,
less dense air
Air density varies with altitude
but, at the ground level, air density
is governed by its temperature.
�
Thus, variations in radiation and
temperature control pressure and
wind.
�
LOW
PRESSURE
Sun heats up ground
Denser air drawn in at low
level to replace rising, less
dense air
GLOBAL PRESSURE & WIND
North Pole 90°N
Arctic circle 66.5°N
HIGH
Tropic of Cancer
23.5°N
Equator 0°
LOW
Tropic of Capricorn
23.5°N
Antarctic circle 66.5°S
HIGH
North Pole 90°N
ZONE of least heating produces
HIGH PRESSURE
ZONE of greatest
heating produces
LOW PRESSURE
ZONE of least heating produces
HIGH PRESSURE
GLOBAL PRESSURE & WIND
Global circulation depends on differential heating over the globe. The system is driven by strong
equatorial heating, causing LOW PRESSURE. Equatorial air rises, diverges and descends over the
tropics, where HIGH PRESSURE dominates; where it diverges at ground level. This tropical air blows
towards the equator, completing the equatorial cell, or towards the mid-latitides where it meets cold,
dense polar air blown out from the polar HIGH PRESSURE. These contrasting tropical and polar air
masses meet at the POLAR FRONT LOW PRESSURE BELT, where the warmer air is forced upwards
by the polar air. At high level, this air again diverges towards the pole or to the tropic.
Rising air diverges at the
tropopause, where a
permanent temperature
inversion results in
warmer air above.
POLAR HIGH
POLAR FRONT (LOW PRESSURE)
TROPICAL HIGH
EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW
WIND DIRECTION & STRENGTH
Wind strength depends on
the difference in pressure
between the high and low
Farmers plant trees to protect orchards,
pressure systems, and the
houses, stock or prevent soil erosion
distance between them.
� This is called the PRESSURE
GRADIENT; it is a similar
concept to the physical slope
between two places, shown on
Locally, wind is channelled
a contour map. Pressure is
down streets (wind canyons).
shown by ISOBARS on a
weather map.
Strong winds also occur
� Pressure difference
in low latitudes due to
essentially depends on the
stronger heating and
temperature difference
steeper presure gradients.
between the two places.
Hurricanes and
A steep pressure gradient results
tornadoes are both
from a large pressure difference
Strong polar winds due to low
tropical phenomena.
or short distance between places
friction
and causes strong wind.
�
Hurricane in Florida
Tornado in USA
Beach windbreaks reduce windsped
by increasing friction
High
CORIOLIS FORCE
Theoretical wind
which would result
solely from pressure
gradient
Actual wind
which blows, as
diverted by
Coriolis Force
Low
Pressure gradient wind blows from
high presure towards low pressure.
� The earth’s rotation diverts this
wind direction laterally. This force is
called the CORIOLIS FORCE.
� The Coriolis force diverts wind the
the right in the northern
hemisphere; to the left in the south.
� The effect is stronger at high
altitude where ground level friction
is less significant.
�
LOW
In the north, winds
blow clockwise out
from a high pressure.
(In the south, they blow
anti-clockwise).
HIGH
In the north,
winds blow anticlockwise into a
low pressure
system. In the
south, they blow
GLOBAL PRESSURE & WIND
POLAR HIGH PRESSURE
POLAR FRONT
MID-LATITUDE
LOW PRESSURE
INTER-TROPICAL
CONVERGENCE ZONE
-LOW PRESSURE
POLAR FRONT
MID-LATITUDE
LOW PRESSURE
TROPICAL HIGH
PRESSURE
TROPICAL HIGH
PRESSURE
POLAR HIGH PRESSURE
GLOBAL WIND BELTS (trade winds) are controlled by the major
pressure belts, which relate fundamentally to temperature. Regional
wind systems (eg the Indian Monsoon) relate to continental heating
effects, and seasonal changes. Local winds relate to smaller scale
temperature contrasts (ie Aspect, Albedo, Altitude etc).
HIGH PRESSURE
High Pressure means that air tends to sink. Sinking air is
compressed, warms up as a result and its relative humidity
falls below saturation. Any clouds evaporate. Rainfall is
unlikely, apart from occasional short, intense convectional
storms due to insolation with lack of clouds in daytime.
LITTLE
WIND
CLEAR SKIES
In Britain, high pressure systems have clear skies, little or
no wind, little rainfall and tend to be stable and slow moving.
� Visibility is intially good, but rapidly deteriorates as dust is
trapped by sinking air and is not washed out by rainfall.
� Cloud cover is slight, resulting in a high diurnal ranges of
temperature (hot days, cold nights). Due to the trapped dust
particles and cold nights, dew, frost, fog or smog are
common.
� Air quality is low as all forms of pollution are retained in
the lower atmosphere.
�
VISIBILITY IS
POOR
FEW CLOUDS
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN
MOUNTAINS - LESS POLLUTION
FOG & SMOG IS
COMMON
LOW-LATITUDE LOW PRESSURE SYSTEMS
Low pressure systems involve air that
tends to rise, thus causing clouds and
precipitation. Those near the equator
tend to be high energy due to strong
ground heating (convectional). Low
pressure systems may develop tornadoes
and sometimes develop into hurricanes,
fuelled by warm, very humid air
evaporated from tropical oceans in
summer. They tend to be fast moving,
with plenty of cloud cover that reduces
diurnal temperature range, strong winds
and high rainfall.
A convectional cumulo-nimbus
cloud results from strong ground
heating at the equator
Hurricane off USA
fuelled by hot
humid air over
the Caribbean
A tornado may
develop from a
cumulo-nimbus cloud
MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure
systems are called
depressions in Britain. They
also involve rising air, clouds,
strong winds and rainfall
and are fast moving.
Depressions result from
the convergence of warm air
from the tropical high
pressure belt with cold air
from the poles along the
Polar Front.
� The energy of the
depresion is a result of the
difference in temperature
and humidity between the
two air masses.
� This contrast varies with
the exact origin of the air
mass, the season and the
nature of the surface over
which they have passed.
POLAR
MARITIME
from
Greenland
Cool, humid.
ARCTIC
MARITIME
from Arctic Ocean
Cold, humid.
POLAR CONTINENTAL
from E.Europe
Cold, dry in winter
Warm, dry in summer.
�
POLAR
MARITIME
RETURN
Coolish, very
humid.
TROPICAL
MARITIME
from Atlantic near
tropic
Warm, humid
TROPICAL
CONTINENTAL
From N.Africa
Hot, dry
POLAR
FRONT
this shifts
polewards in
summer and
equatorward
s in winter,
hence British
seasonal
contrasts.
MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure
systems are called depressions in
Britain. They also involve rising
air, clouds, strong winds and
rainfall and are fast moving.
Depressions (L)
over Europe
showing FRONTS
OCCLUDED FRONT
Cold & warm fronts meet
COLD
FRONT
Depressions over
NW Europe
POLAR MARITIME
AIR
WARM
FRONT
COLD
FRONT
WARM
FRONT
COLD
FRONT
TROPICAL
MARTIME AIR
Depressions result from the convergence of warm air from the
tropical high pressure belt with cold air from the poles along the
Polar Front. The systems move rapidly across the Atlantic before
filling and drifting north-eastwards to Scandinavia from Britain.
� The energy of the depresion is a result of the difference in
temperature and humidity between the two air masses.
� This contrast varies with the exact origin of the air mass, the
season and the nature of the surface over which they have passed.
�
A FRONT is the boundary betwen two air masses. A depression
has two, a warm (the front of the warm air) and a cold.
WARM FRONTS
1
TROPICAL MARITIME AIR
POLAR MARITIME AIR
The warm front is
angled gently due to
ground level friction
which slows the air at
low level as the whole
system moves
eastwards.
As the warm tropical maritime air moves eastwards towards Britain, it is forced upwards by colder,
denser polar maritime air. The speed of uplift depends on the relative temperature of the two air
masses. Uplift causes expansion, cooling, falling relative humidity until dew point temperature is
reached when condensation starts to occur on particles. The amount of precipitation depends on the
hunidity and temperature of the warm air mass, and the particles available.
2
MAINLY STRATUS
CLOUDS
POLAR
MARITIME
COLD FRONTS
TROPICAL MARITIME
As the depression moves
eastwards, the warm
tropical air continues to be
forced upwards by the
colder, denser polar air
mass.
The cold front is steeper, also due to ground level
friction slowing the lower air, so uplift is more rapid than
along the warm front.
� This causes cumulo-nimbus clouds and possible
thunderstorms rather than thick stratus cloud.
� Eventually, the two fronts meet, forcing the warm air
off the ground. This is an OCCLUDED FRONT
(occlusion), and happens to all depressions as they ‘fill’.
� The whole system takes about 24 hours to pass.
�
OCCLUSION
Tm air forced up
Pm air
meets at
ground level
Wind
WIND CAN OCCUR ON A SMALL SCALE AND LARGE
SCALE
1.LAND AND SEA BREEZE IS AN EXAMPLE OF SMALL
SCALE
2. MONSOON WINDS AND PLANETARY ARE EXAMPLE OF
LARGE SCALE.
LAND BREEZE AND SEA BREEZE
Planetary Winds
Planetary winds are winds that blow out
from high pressure belt to low pressure
belts.
There are three types of Planetary winds
1.Trade Winds
2.The Westerlies
3.The Easterlies
The planetary winds blow in curved/deflected direction due
to the rotational of the earth.
The wind deflect to the right in Northern Hemisphere and
left to the Southern Hemisphere
Equator is an area of low pressure and the 30◦ N and 30◦S
is the high pressure area.
Wind deflect to the right from the equator is called the
North East Trade in the Northern Hemisphere
Winds deflect to the left from the equator is called South
East Trade in the Southern Hemisphere.
Monsoon Winds
Monsoon winds is another example of large scale wind which
occurs on a regional level and blows over large land masses.
Monsoon ‘ mausin’ in arabic means season
The unequal heating and cooling of large land masses and the
oceans causes a difference in pressure. This causes in the
movement of large scale winds whose direction changes with each
season.
Monsoon winds blow over Southern and Eastern Asia and
influence the climate in these regions.
In the Philippines
A "monsoon" is a weather pattern; a monsoon has a different
name in each country that it affects. In the Philippines, the
Summer Monsoon (West or southwest) is called the Habagat(habag-at) and the Winter Monsoon (North or northeast) is called
the Amihan (a-me-han). The word 'monsoon' is believed to
originate from the Arabic word mawsim (season), via the
Portuguese and then Dutch monsun.
A "monsoon" is a consistent wind pattern generated by a large
weather system, that lasts for a period of months aand affects a
large area of the planet.
Summer Southwest Monsoon Habagat
• Summer Monsoon weather is characterized by a strong, generally
West or southwest breeze that is responsible for bringing significant
rainfall to the Asian subcontinent and to South and East Asia.
•
The significant southwest monsoon rainfall is a by-product of air
passing over large areas of warm equatorial ocean, stimulating
increased levels of evaporation from the ocean's surface; the
southwest monsoon air, now laden with water vapour, cools as it
moves north and as it rises over land; at some point the air is no
longer able retain its moisture and precipitates copious volumes to
irrigate rice fields and drench rainforests, sometimes causing severe
flooding below hillsides that have been stripped of forest cover.
•
The Summer Monsoon (West or southwest) is the predominant
weather pattern from late April through to early October each year,
throughout most Asian tropical destinations.
Winter Northeast Monsoon Amihan
• Winter Monsoon weather features a generally less strong, East or
northeast breeze that is cool and dry (compared to the Summer
Monsoon weather) with prolonged periods of successive cloudless
days.
• The Winter Monsoon (North or northeast) features cool and dry air
that originates in a vast anticyclone - a weather system with a
high barometric pressure - which forms over Siberia, Mongolia and
northern China during each northern winter.
• The Winter Monsoon air from the anticyclone pushes outward in a
clockwise motion from its centre and competes with the Summer
Monsoon over a period of a week or two, usually starting in late
September and early October, before finally dominating the
weather with a cooler and drier northeast monsoon, in most Asian
tropical and sub-tropical destinations, through to the following
April.