G304 – Physical Meteorology and Climatology Chapter Atmospheric pressure and wind By Vu Thanh Hang, Department of Meteorology, HUS 4.1 The concept of pressure • The atmosphere contains a tremendous number of gas molecules being pulled toward Earth by the force of gravity • These molecules exert a force on all surfaces with which they are in contact, and the amount of that force exerted per unit of surface area is pressure • The standard unit of pressure is the pascal (Pa) • Air pressure at sea level is roughly 1000 mb (100 kPa) or more precisely, 1013.2 mb Fig 4-1 • The enclosed air molecules move about continually and exert a pressure on the interior walls of the container (a) • Pressure can increase by increasing the density of the molecules (b) • Increasing the temperature (c) • If the air in the container is a mixture of gases, each gas exerts its own specific amount of pressure Æ partial pressure • The total pressure exerted is equal to the sum of the partial pressures Æ Dalton’s law 4.1 The concept of pressure (cont.) • In fact, atmospheric pressure is the mass of the air above being pulled downward by gravity • The pressure at any point reflects the mass of atmosphere above that point • The mass of atmosphere above necessarily decreases Æ pressure must also decrease • Air pressure is exerted equally in all directions: up, down, and sideways 4.1 The concept of pressure (cont.) • Surface pressure is the pressure actually observed at a particular location, whereas sea level pressure is the pressure that would exist if the observation point were at sea level • Sea level pressure allows us to compare pressure at different locations taking into account differences in elevation • To correct for elevation, add mb per 10 meters • For high-elevation sites, this method is unreliable because we must account for compressibility of the atmosphere 4.1 The concept of pressure (cont.) Pressure will be less at P2 than at P1 due to pressure decreasing with height 4.1 The concept of pressure (cont.) • Pressure does not decrease at a constant rate • Surface pressure also varies from place to place • Horizontal pressure differences are very small compared to vertical differences Fig 4-3 Pressure decreases with altitude by about half for each 5.5km 4.2 The equation of state • Temperature, density and pressure are ralated to one another • The Equation of State (Ideal Gas Law) p = ρRT where p is pressure (Pa), ρ is density (kg m-3), R = 287 (J kg-1 K-1), T is temperature (K) • If the air density increases while temperature is held constant, the pressure will increase, and at constant density, an increase in temperature leads to an increase in pressure Standard atmosphere: p0 = 101325 Pa, T0 = 288.15 K, ρ0 = 1.225 kg/m³ 4.5 Forces affecting the speed and direction of the wind (cont.) • The coriolis force (CF): • Æ magnitude proportional to: of deflection directly – Horizontal velocity – Sine of latitude • Æ effect is maximum at poles and zero at equator • deflection (turning) of the wind to the right in the NH and to the left in the SH • acting on any moving object, increases with the object’s speed • changes only the direction of a moving object, never its speed 4.5 Forces affecting the speed and direction of the wind (cont.) • Geostrophic balance: • In absence of friction (from surface) OR centripetal forces (arises from curve-isobars) • ONLY two equal & opposite forces acting on an air parcel • For steady flow: dP − = 2ωV sin φ ρ dn • PGF = CF Æ Geostrophic wind VG = − dP 2ω sin φρ dn In geostrophic balance air flows parallel to isobars with high pressure to the right in NH 4.5 Forces affecting the speed and direction of the wind (cont.) • The friction force (FrF): • Winds are slowed down by roughness of the surface over which it flows Æ friction • Friction: V ↓, CF ↓ Æ Imbalance & cross-isobaric flow • Friction is important within the lowest 1.5 km of the atmosphere (planetary boundary layer - PBL) 4.5 Forces affecting the speed and direction of the wind (cont.) • The Equation of Motion: dV / dt = PGF + CF + FrF where PGF stands for pressure gradient, CF stands for the Coriolis effect, and FrF stands for friction (acting on a unit mass of air) • The equation of motion says the acceleration of a mass of air is the sum of these three forces • The equation of motion is an expression of the conservation of momentum 4.6 Winds in the upper atmosphere - A stationary parcel of air in the upper atmosphere subjected to a south-to-north PGF (a) - The horizontal pressure gradient accelerates the parcel northward (b) Initially, when the wind speed is low, the CF is small - As the parcel speeds up, the strength of the CF increases and causes greater displacement to the right (c) - The wind speed increases the CF sufficiently to cause the air to flow perpendicular to the PGF (d) - The air flow becomes unaccelerated, with unchanging speed and direction known as geostrophic flow (or geostrophic wind) Æ occurs only in upper atmos 4.6 Winds in the upper atmosphere (cont.) Fig.4-13 In common pressure distributions the height contours curve and assume varying distances from one another In the absence of friction, the air flows parallel to the contours constantly changing direction and therefore undergoing an acceleration In order for the air to follow the contours, there must be a continual mismatch between the pressure gradient and Coriolis forces This movement is known as gradient flow (or gradient wind) 4.6 Winds in the upper atmosphere (cont.) Supergeostropic flow (a) occurs in the upper atmosphere around high-pressure systems As the air flows, it is constantly turning to its right This turning motion occurs because the Coriolis force has a greater magnitude than the pressure gradient force (as represented by the length of the dashed arrows) Observe the changing direction of the four solid arrows through Subgeostrophic flow (b) occurs in the upper atmosphere around lowpressure systems The pressure gradient force is greater than the Coriolis force and the air turns to its left in the Northern Hemisphere Fig 4-14 4.7 Near-surface wind • Geostrophic flow cannot exist near the surface • Friction slows the wind, so that the Coriolis force is less than the pressure gradient force Æ the wind in BL not flow parallel to the isobars • The air flows at an angle to the right of the pressure gradient force in the NH (a) and to the left in the SH (b) 4.8 Cyclones, anti-cyclones, troughs, and ridges • Enclosed areas of high pressure marked by roughly circular isobars or height contours are called anticyclones • The wind rotates clockwise around anticyclones in the NH, as the Coriolis force deflects the air to the right and the PGF directs it outward • In the boundary layer, the air spirals out of anticyclones (a), while in the upper atmosphere it flows parallel to the height contours (b) • In the SH, the flow is counterclockwise (c) and (d) Fig 4-16 4.8 Cyclones, anti-cyclones, troughs, and ridges (cont.) • Closed low-pressure systems are called cyclones • Air spirals counterclockwise into surface cyclones in the NH (a) and rotates counterclockwise around an upper-level low (b) • The flow is reversed in the SH (c) and (d) Fig 4-17 4.8 Cyclones, anti-cyclones, troughs, and ridges (cont.) Elongated zones of high and low pressure are called ridges (a) and troughs (b), respectively 4.8 Cyclones, anti-cyclones, troughs, and ridges (cont.) Maps depicting troughs, ridges, cyclones, and anticyclones 4.9 Measuring wind • Direction is always given as that from which the wind blows, so that a “westerly” wind is one from the west • It is often expressed by its azimuth, the degree of angle from due north (0o or 360o), moving clockwise • A simple device for observing wind direction is the wind vane • Wind speeds are measured with anemometers that have rotating cups mounted on a moving shaft • Looking like an airplane without wings (right), an aerovane indicates both wind direction and speed • Upper-level wind measurements are obtained by rawinsondes, radiosondes whose movement is tracked by radar 4.9 Measuring wind A wind vane An aerovane 4.9 Measuring wind (cont.) Wind direction [...]... 4- 7 4. 4 The distribution of pressure (cont.) Fig 4- 8 The gradual poleward decrease in mean temperature results in denser air occurring at high latitudes As indicated by the hydrostatic equation, pressure drops more rapidly with height at high latitudes and lowers the height of the 500 mb level The dashed lines depict the height of the 500 mb level as they would be drawn on a 500 mb weather map 4. 4... speed 4. 4 The distribution of pressure (cont.) • The vertical pressure gradient force and the force of gravity are normally of nearly equal value and operate in opposite directions, a situation called hydrostatic equilibrium • The Hydrostatic Equation dp/dz = -ρg where dp refers to a change in pressure, dz refers to a change in altitude, and -ρg refers to density and the acceleration of gravity 4. 4 The... length of the dashed arrows) Observe the changing direction of the four solid arrows 1 through 4 Subgeostrophic flow (b) occurs in the upper atmosphere around lowpressure systems The pressure gradient force is greater than the Coriolis force and the air turns to its left in the Northern Hemisphere Fig 4- 14 4.7 Near-surface wind • Geostrophic flow cannot exist near the surface • Friction slows the wind,... In the SH, the flow is counterclockwise (c) and (d) Fig 4- 16 4. 8 Cyclones, anti-cyclones, troughs, and ridges (cont.) • Closed low-pressure systems are called cyclones • Air spirals counterclockwise into surface cyclones in the NH (a) and rotates counterclockwise around an upper-level low (b) • The flow is reversed in the SH (c) and (d) Fig 4- 17 4. 8 Cyclones, anti-cyclones, troughs, and ridges (cont.)... proportional to the air pressure • Aneroid devices that plot continuous values of pressure over extended periods are called barographs 4. 4 The distribution of pressure • An isobar is a line that connects points having exactly the same sea level pressure drawn at intervals of 4 mb on surface weather maps • The spacing of the isobars indicates the strength of the pressure gradient, or rate of change in pressure... the distribution of sea level air pressure The pressure is relatively low over the northeastern U.S and eastern Canada, and the highest and lowest pressure on the map are only within about 4 percent of each other 4. 4 The distribution of pressure (cont.) • If the air over one region exerts a greater pressure than the air over an adjacent area, the higher-pressure air will spread out toward the zone of... of the wind - friction force Æ slows the wind 4. 5 Forces affecting the speed and direction of the wind (cont.) • The pressure gradient force (PGF): • Horizontal pressure gradient force per unit mass: 1 dP PGF = ρ dn • ρ = air density (1.2 kgm-3 at sea level) • dP/dn = horizontal gradient of pressure (SI units) – mb/km Æ Pa/m – 1mb = 100Pa; 1km = 1000m 4. 5 Forces affecting the speed and direction of... direction of the wind (cont.) • The coriolis force (CF): • Deflective force (per unit mass): CF = 2ωVsinφ • ω = angular velocity of spin (Earth: 2π/ 24 rad/hr = 7.29*10-5 rad/s) • V = velocity of mass (wind speed) • φ = latitude • Coriolis parameter f = 2ωsinφ 4. 5 Forces affecting the speed and direction of the wind (cont.) • The coriolis force (CF): • Æ magnitude proportional to: of deflection directly... with high pressure to the right in NH 4. 5 Forces affecting the speed and direction of the wind (cont.) • The friction force (FrF): • Winds are slowed down by roughness of the surface over which it flows Æ friction • Friction: V ↓, CF ↓ Æ Imbalance & cross-isobaric flow • Friction is important within the lowest 1.5 km of the atmosphere (planetary boundary layer - PBL) 4. 5 Forces affecting the speed and... perpendicular to the PGF (d) - The air flow becomes unaccelerated, with unchanging speed and direction known as geostrophic flow (or geostrophic wind) Æ occurs only in upper atmos 4. 6 Winds in the upper atmosphere (cont.) Fig .4- 13 In common pressure distributions the height contours curve and assume varying distances from one another In the absence of friction, the air flows parallel to the contours constantly