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A stable atmosphere. An
absolutely stable
atmosphere exists when
a rising air parcel is
colder and heavier (i.e.,
more dense) than the air
surrounding it. If given
the chance (i.e.,
released), the air parcel
in both situations would
return to its original
position, the surface. (In
both situations, the
helicopter shows that the
air is being lifted. In the
real world, this type of
parcel lifting, of course,
would be impossible.)
An unstable
atmosphere. An
absolutely unstable
atmosphere exists when
a rising air parcel is
warmer and lighter (i.e.,
less dense) than the air
surrounding it. If given
the chance (i.e.,
released), the lifted
parcel in both (a) and
(b) would continue to
move away (accelerate)
from its original
position.
The initial environmental lapse
rate in diagram (a) will become
more stable (stabilize) as the air
aloft warms and the surface air
cools, as illustrated in diagram (b).
The initial environmental lapse
rate in diagram (a) will become
more unstable (that is,
destabilize) as the air aloft cools
and the surface air warms, as
illustrated in diagram (b).
Conditionally unstable atmosphere. The atmosphere is conditionally unstable when
unsaturated, stable air is lifted to a level where it becomes saturated and warmer than the air
surrounding it. If the atmosphere remains unstable, vertical developing cumulus clouds can
build to great heights.
• Moist
adiabatic
lapse rate is
less than
the
environmen
-tal lapse
rate which is
less than
the dry
adiabatic
lapse rate
• Stable
below cloud
unstable
above cloud
base
• Atmosphere
usually in
this state
The ice-crystal
(Bergeron) process.
(1) The greater
number of water
vapor molecules
around the liquid
droplet causes water
molecules to diffuse
from the liquid
droplet toward the ice
crystal. (2) The ice
crystal absorbs the
water vapor and
grows larger, while (3)
the water droplet
grows smaller.
Mr Wind:
http://www.youtube.com/watch?v=2mTLO2F_ERY
We know this is the cause of decreasing air pressure with elevation
(vertical gradient):
Air pressure is a measure of the overlying air mass. (mass = vol/density)
• However, what causes air pressure changes in the horizontal
directions?
• Why does air pressure change at the surface?
Both columns are at the same elevation, both have the same
air temperature, and both have the same air pressue. There
are the same number of air molecules in each air column.
Because it takes a
shorter column of
cold air to exert the
same surface
pressure as a taller
column of warm
air, as column 1
cools, it must
shrink, and as
column 2 warms, it
must rise.
Which air column is
more dense?
Because at the same level in the
atmosphere there is more air
above the H in the warm
column than above the L in the
cold column, warm air aloft is
associated with high pressure
and cold air aloft with low
pressure. The pressure
differences aloft create a force
that causes the air to move
from a region of higher
pressure toward a region of
lower pressure. The removal of
air from column 2 causes its
surface pressure to drop,
whereas the addition of air into
column 1 causes its surface
pressure to rise. (The difference
in height between the two
columns is greatly exaggerated.)
The heating and
cooling of air columns
causes horizontal
pressure variations
aloft and at the surface.
These pressure
variations force the air
to move from areas of
higher pressure toward
areas of lower
pressure. In
conjunction with these
horizontal air motions,
the air slowly sinks
above the surface high
and rises above the
surface low.
Ideal Gas Law
“mm Hg”
Pressure = Temperature
X Density X Constant
P ~ Tρ
Station Pressure: barometer
reading at a particular location
and elevation
P ~ Tρ
ρ = mass/volume
http://chemwiki.ucdavis.edu/Physical_Che
mistry/Physical_Properties_of_Matter/Phas
e_Transitions/Phase_Diagrams_1
Phase change of water
from gas to a liquid:
http://www.youtube.com/watch?v=JsoE4F2Pb20
Barometers are adjusted to sea-level pressure.
The top diagram (a)
shows four cities (A, B,
C, and D) at varying
elevations above sea
level, all with different
station pressures. The
middle diagram (b)
represents sea-level
pressures of the four
cities plotted on a
sea-level chart. The
bottom diagram (c)
shows sea-level
pressure readings,
with isobars drawn on
the chart (gray lines)
at intervals of 4
millibars.
Surface Weather Map
Arrows indicate the wind direction (vectors).
Isobars are at 4-mb intervals.
Wind blows across isobars.
Constant Pressure Chart
The upper-level (500-mb) map for the same day as the
surface map.
Solid lines on the map are contour lines in meters above
sea level. Dashed red lines are isotherms in °C.
Arrows show wind direction. Notice that, on this upper-air
map, the wind blows parallel to the contour lines.
The area shaded
gray in the diagram
represents a surface
of constant pressure.
Because of the
changes in air
density, a surface of
constant pressure
rises in warm, lessdense air and lowers
in cold, more-dense
air. These changes in
height of a constant
pressure (500-mb)
surface show up as
contour lines on a
constant pressure
(isobaric) 500-mb
map.
Newtons Second Law
Force= Mass X Acceleration
F = ma
•
•
•
•
Pressure gradient force (due to difference in pressure)
Coriolis force (due to rotation of earth)
Centripetal force (due to circular paths of wind)
Friction (like with the ground surface, trees, etc.)
Causes of Wind
http://www.youtube.com/watch?v=uBqohRu2RRk&list=FLA8rzatZrvuFZS5jkp0XvIg&index=5&feature=plpp_video
Pressure Gradient = difference in pressure/distance
The net force
is directed
toward the
lower fluid
pressure at the
bottom of tank
B. This net
force causes
water to move
from higher
pressure
toward lower
pressure.
What is the pressure gradient between Point 1 and Point 2?
The closer the spacing of the isobars, the greater the pressure gradient. The greater
the pressure gradient, the stronger the pressure gradient force (PGF). The stronger
the PGF, the greater the wind speed. The red arrows represent the relative magnitude
of the force, which is always directed from higher toward lower pressure.
Coriolis Force = 50,000 views:
http://www.youtube.com/user/OSUOC530?feature=CAgQwRs%3D
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
Coriolis Force varies with latitude:
Above the level of
friction, air initially
at rest will
accelerate until it
flows parallel to the
isobars at a steady
velocity with the
pressure gradient
force (PGF)
balanced by the
Coriolis force (CF).
Wind blowing under
these conditions is
called geostrophic.
The isobars and contours on an upper-level chart are like the banks
along a flowing stream. When they are widely spaced, the flow is
weak; when they are narrowly spaced, the flow is stronger. The
increase in winds on the chart results in a stronger Coriolis force (CF),
which balances a larger pressure gradient force (PGF).
Winds and related forces around areas of low and high pressure above
the friction level in the Northern Hemisphere. Notice that the pressure
gradient force (PGF) is in red, while the Coriolis force (CF) is in blue.
What does this map show?
(a) The effect of surface friction is to slow down the wind so that,
near the ground, the wind crosses the isobars and blows toward
lower pressure. (b) This phenomenon at the surface produces an
inflow of air around a low and an outflow of air around a high. Aloft,
the winds blow parallel to the lines, usually in a wavy west-to-east
pattern. Both diagram (a) and (b) are in the Northern Hemisphere.
Winds and air motions associated with surface highs and lows in the
Northern Hemisphere.
http://hsc.csu.edu.au/primary_ind/prim_ind_240/compulsory/RTE2503A/3264/pressure_cells.htm
Which maps are for the
Northern Hemisphere and
which maps are for the
Southern Hemisphere?
http://www.weatherquestions.com/What_causes_wind.htm
Fig. 6-23, p. 169
This wind rose represents the percent of time the wind blew from
different directions at a given site during the month of January for the
past ten years. What is the prevailing wind direction?
Global Air Circulation:
http://www.youtube.com/watch?v=DHrapzHPCSA
http://ww2010.atmos.uiuc.edu/%28Gl%29/guides/maps/sfcobs/cntr/wind.rxml
Legend for the map: http://www.hpc.ncep.noaa.gov/html/fntcodes2.shtml
Specific Heat, Air Pressure, Winds, Sea Breeze:
http://www.youtube.com/watch?v=BpQ5_uyUmfY
http://emissioncontrolcenter.com/2weeks/howHurricanesWork.aspx