Download An unstable atmosphere. An absolutely unstable

10/1/2012
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.)
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).
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.
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
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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.
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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.)
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? 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
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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.
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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, less‐
dense 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.
Pressure Gradient = difference in pressure/distance
Newtons Second Law
Force= Mass X Acceleration
F = ma
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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.)
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.
Causes of Wind
http://www.youtube.com/watch?v=uBqohRu2RRk&list=FLA8rzatZrvuFZS5jkp0XvIg&index=5&feature=plpp_video
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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
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Coriolis Force varies with latitude:
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).
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.
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.
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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.
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?
Winds and air motions associated with surface highs and lows in the Northern Hemisphere.
http://www.weatherquestions.com/What_causes_wind.htm
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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?
Fig. 6‐23, p. 169
Global Air Circulation:
http://www.youtube.com/watch?v=DHrapzHPCSA
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