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 the force exerted against a surface by the continuous collision of gas molecules - We can see its secondary effects --- storms; surge; inversion & smog

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

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)

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 (1/2 n per 5.6k)

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

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

Measuring Air Pressure, cont - 1 standard atmosphere: in of mercury millibars 101,325 newtons - Atmospheric pressure differs throughout the world according to temperature, elevation or altitude, etc… to be compared these pressure readings must be mathematically “corrected”

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

Factors Affecting Wind, cont - Factors and causes of horizontal motion: (a) thermal – temperature induced; “expansion” and “subsidence” of air … convection cell (b) dynamic – motion related; related to the motion of the Earth

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

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

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

Forces Affecting Air flow and Direction, cont … direction: 180 o 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

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

Forces Affecting Air flow and Direction, cont … direction: 90 o to wind flow … magnitude: directly proportional to wind speed and latitude

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

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

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