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CHAPTER 6 AIR PRESSURE AND WINDS

CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

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Page 1: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

CHAPTER 6

AIR PRESSURE AND WINDS

Page 2: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Understanding Air Pressure

- Air pressure is a very abstract term. We cannot actually see it or touch it.--- It is the force exerted against a surface by the continuous collision of gas molecules

- We can see its secondary effects--- storms; surge; inversion & smog

Page 3: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Understanding Air Pressure, cont

- Pressure is important in the mechanics of weather

- We are generally aware of it for its results – particularly as the driving force of the winds--- simplified, but… it is the uneven heating of the Earth’s surface that produces pressure differences, and therefore the winds

Page 4: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Understanding Air Pressure, cont

- Differences in pressure are generally the result of changes in:

(1) density(2) temperature

--- closely related concepts as temperature change will produce pressure change through increased molecular motion

… temperature up / pressure down (real world)… temperature up / pressure up (closed container)

Page 5: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Understanding Air Pressure, cont

- Earth atmosphere adds:(3) altitude… creates

difference in air pressure- In the atmosphere, the pressure at

any altitude is equal to the “weight” of the air directly above that point

- This numerical change in pressure is not constant as altitude increases (1/2n per 5.6k)

Page 6: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Understanding Air Pressure, cont

- Associated with the pressure-altitude relationship is related Dalton’s Law--- total pressure of a gas is equal to the sum of the partial pressures of the individual gases chemically --- partial pressure of a gas is the pressure

it would exert at the same temperature as the mixture if it alone occupied the volume that the mixture does

--- partial pressure will equal ATA for an altitude

Page 7: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Measuring Air Pressure- Atmospheric pressure is measured from

sea level and is expressed in terms of a standard atmosphere or atmospheres absolute (ATA)[I have problems with concept of a standard atmosphere]

- General public knows it as inches of mercury

- Meteorologists generally measure it in millibars

- Other measures include: atmospheres, dynes, newtons, pascals

Page 8: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Measuring Air Pressure, cont

- 1 standard atmosphere:29.92 in of mercury1013.25 millibars101,325 newtons

- Atmospheric pressure differs throughout the world according to temperature, elevation or altitude, etc… to be compared these pressure readings must be mathematically “corrected”

Page 9: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Factors Affecting Wind

- Wind (advection)- horizontal movement of air… nature’s attempt to equalize air pressure --- why:

difference in horizontal air pressure between locations … air flow from high to low

pressure … unequal global heating

produces

Page 10: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Factors Affecting Wind, cont

- Factors and causes of horizontal motion:

(a) thermal – temperatureinduced; “expansion” and “subsidence” of air

… convection cell(b) dynamic – motion related;

related to the motion of the Earth

Page 11: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction

(1) Pressure Gradient Force – rate of pressure change across spatial distance

… pressure differences must create a force in order to

drive the wind - PGF … pressure patterns are illustrated by isobar change… magnitude: from isobar spacing… direction: perpendicular to

isobars

Page 12: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

- The closer the spacing of the isobars, the stronger the PGF

- The stronger the PGF, the stronger the wind

- Because of the unequal heating between the Tropics and the Poles, a PGF is created (thermal / global convection cell)

- If the Earth did not rotate (dynamic) PGF would be the only force driving wind

Page 13: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

(2) Friction – as soon as air flow begins, it is affected by contact with the surface of the Earth… negligible above a few kms

… may not consider friction to be a force, but is a significant

constraint to air flow… the rougher the terrain, the

greater the frictional force

Page 14: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

… direction: 180o to air flow … magnitude: affected by

surface characteristics and speed of air flow

… as frictional force decreases wind speed it (a) decreases Coriolis Effect, (b) does not effect PGF

Page 15: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

(3) Coriolis Effect- (CF)“apparent” force deflecting items that

move latitudinally on the Earth

… deflected to the right (north) and to the left (south) hemisphere

Page 16: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

… direction: 90o to wind flow … magnitude: directly proportional to wind speed and latitude

Page 17: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

- For high altitude winds, CF turns airflow until winds are blowing parallel to isobars, effectively balancing PGF (Fig 6.15)… termed gradient winds or geostrophic winds

(“turned by the Earth”) … lack of friction allows faster winds

Page 18: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

- For surface winds: Friction weakens CF, and PGF is strengthened, winds blow across isobars at approx 30o … atmosphere influenced by friction is the planetary boundary layer

Page 19: CHAPTER 6 AIR PRESSURE AND WINDS. Understanding Air Pressure - Air pressure is a very abstract term. We cannot actually see it or touch it. --- It is

Forces Affecting Air flow and Direction, cont

Influence of CF, PGF and friction result in patterns of air flow around high and low pressure (Fig 6.17)

- Around a low, PGF attempts to pull air inward. It is balanced by CF to curve in a counter-clockwise flow in the northern hemisphere

- Around a high, PGF attempts to pull air outward. It is balanced by CF to curve in a clockwise flow in the northern hemisphere

- Within the planetary boundary layer friction reduces CF and PGF “spirals” air