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.) Figure 5.3: 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 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. Figure 5.6: 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).

Stable below cloud unstable above cloud base 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 Figure 5.9: 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.

The ice-crystal (Bergeron) process 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. Figure 5.22: 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

Why does air pressure change at the surface? 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? Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) 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. (c) 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.)

Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) 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. (c) 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.) 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.

Which air column is more dense? 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? Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) 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. (c) 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 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.) Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) 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. (c) 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. Figure 6.3: 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.

Pressure = Temperature X Density X Constant 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_Chemistry/Physical_Properties_of_Matter/Phase_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. Figure 6.8: 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.

Figure 6. 9: (a) Surface map showing areas of high and low pressure Figure 6.9: (a) Surface map showing areas of high and low pressure. The solid lines are isobars drawn at 4-mb intervals. The arrows represent wind direction—the direction from which the wind is blowing. Notice that the wind blows across the isobars. (b) 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.

Arrows indicate the wind direction (vectors). Surface Weather Map Figure 6.9: (a) Surface map showing areas of high and low pressure. The solid lines are isobars drawn at 4-mb intervals. The arrows represent wind direction—the direction from which the wind is blowing. Notice that the wind blows across the isobars. (b) 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. Arrows indicate the wind direction (vectors). Isobars are at 4-mb intervals. Wind blows across isobars.

Constant Pressure Chart Figure 6.9: (a) Surface map showing areas of high and low pressure. The solid lines are isobars drawn at 4-mb intervals. The arrows represent wind direction—the direction from which the wind is blowing. Notice that the wind blows across the isobars. (b) 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 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. Figure 2: 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.

Force= Mass X Acceleration F = ma 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. Figure 6.10: The higher water level creates higher fluid pressure at the bottom of tank A and a net force 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? Figure 6.11: The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure gradient force.

Figure 6.12: 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. 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.

Figure 6. 13: Surface weather map for 6 a. m Figure 6.13: Surface weather map for 6 a.m. (CST), Tuesday, November 10, 1998. Dark gray lines are isobars with units in millibars. The interval between isobars is 4 mb. A deep low with a central pressure of 972 mb (28.70 in.) is moving over northwestern Iowa. The distance along the green line X-X′ is 500 km. The difference in pressure between X and X′ is 32 mb, producing a pressure gradient of 32 mb/500 km. The tightly packed isobars along the green line are associated with strong northwesterly winds of 40 knots, with gusts even higher. Wind directions are given by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (A wind indicated by the symbol would be a wind from the northwest at 10 knots. See blue insert.) The solid blue line is a cold front, the solid red line a warm front, and the solid purple line an occluded front. The dashed gray line is a trough.

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: Figure 6.15: Except at the equator, a free-moving object heading either east or west (or any other direction) will appear from the earth to deviate from its path as the earth rotates beneath it. The deviation (Coriolis force) is greatest at the poles and decreases to zero at the equator.

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. Figure 6.16: Above the level of friction, air initially at rest will accelerate until it flows parallel to the isobars at a steady speed with the pressure gradient force (PGF) balanced by the Coriolis force (CF). Wind blowing under these conditions is called geostrophic.

Figure 6.17: 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). 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).

Figure 6.18: 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. 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? Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above sea level. Dashed red lines are isotherms in °C.

Figure 6.20: (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. (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.

Figure 6.22: Winds and air motions associated with surface highs and lows 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

Figure 6.23: An onshore wind blows from water to land, whereas an offshore wind blows from land to water. 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? Figure 6.26: 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. The prevailing wind is NW and the wind direction of least frequency is NE.

Global Air Circulation: http://www.youtube.com/watch?v=DHrapzHPCSA

http://ww2010. atmos. uiuc. edu/%28Gl%29/guides/maps/sfcobs/cntr/wind 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