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Atmospheric Pressure and Winds

deals with temperature, density, pressure, winds and humidity parameters of the atmosphere; Prssure gradient force, coriolis force, gravity force and friction force and winds and currents, ; pressure lows and highs, atmospheric circulation, winds.

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Atmospheric Pressure and Winds

  1. 1. Atmospheric Pressure and Winds Dr. Akepati S. Reddy Professor, School of Energy & Environment Thapar University, Patiala – 147 001 Punjab (INDIA)
  2. 2. Air Density, Temperature and Pressure Air or atmospheric density • Mass per unit volume (kg/m3) and indicated by ‘ρ’ • Mass subjected to gravity results in weight (same mass has different weights depending on varying gravity of different planets • At MSL, air density is 1.2 kg/m3 and it decreases with altitude • Air density is pressure and temperature dependent • At the same temperature and pressure conditions, density of the moist air is lower than that of dry air – Why? Air or atmospheric temperature • Temperature is a measure of average speed of moving molecules (kinetic energy) • Measured in Kelvin scale (K) – at 0 K, there is no kinetic energy • Temperature increases with increasing air density • Atmospheric temperature is affected by – Short-wave radiation from above and long-wave from below - further, sensible and latent heat received from below (earth surface) – Varying radiation absorption/emission properties of the atmosphere
  3. 3. Air Density, Temperature and Pressure Atmospheric temperature • Atmosphere has a distinctive temperature profile • Temperature profile of the lower atmosphere is very variable (due to the variable heating and cooling by earth surface (sensible and latent heat, and radiation) • Average temperature of the Earth is 288 K – in troposphere it decreases with altitude (average vertical lapse rate is 6.6°C/km) Atmospheric pressure • Pressure is force per unit area – force is mass multiplied by acceleration (kg.m/sec.2) – acceleration is change in velocity through time (m/sec./sec. or m/sce2) • Pressure units: N/m2 or Pa; Bars/millibars (1 mb = 100 Pa) –at MSL pressure is 1013 mb or 101.3 kPa or 10130 kg air over one m2 area • Atmospheric pressure decreases with altitude (700 mb at 3 km, 500 at 5.5 and 300 at 10) – mass of the overlying air reduces with height
  4. 4. Density, Temperature and Pressure, and Winds Atmospheric pressure • Atmospheric pressure also varies horizontally on the earth surface (results from unequal heating of the earth surface) – Recorded highest & lowest sea level pressures: 1084 mb and 870 mb (typical range is 950 mb to 1050 mb) – Surface pressure tendency over the fast several hours is useful in local short range weather predictions • Pressure (scalar quantity) is exerted in all directions – still an air parcel is in equilibrium (pressure exerted by it is balanced by the force -gravity pull- exerted by overlying air: hydrostatic equilibrium) • Air parcels with pressures different from the surroundings, have disturbed hydrostatic equilibria, and move in the atmosphere • Pressure gradient force is responsible winds – Despite very large vertical pressure gradients, because of the hydrostatic equilibrium, vertical movement of air is very limited – Vertical pressure gradient force operates opposite to gravity force – Hydrostatic equilibrium or balance is disturbed in case of convection currents and thunderstorms – Horizontal pressure gradient on the other hand causes winds, though it is many times lower than the vertical pressure gradient
  5. 5. Density, Temperature and Pressure, and Winds Winds • Movement of wind is due to the pressure gradient force from high pressure region to low pressure region • Divisible into surface winds and aloft or upper atmosphere winds and also into vertical currents • Winds carry and transport heat, moisture and pollutants, and wind create conditions for clouds formation/dissipation and precipitation • Wind is a vector quantity and has both speed and direction components – Increasing PGF (closer spacing of isobars) increases wind speed – Wind speed is influenced by friction force (slows down the wind) – Wind is named after from where it is blowing (west wind: wind coming from the west) – Wind direction (indicated in degrees in the clockwise direction from the north) is influenced by both friction and coriolis effect
  6. 6. Density, Temperature and Pressure, and Winds Winds • General wind pattern of the Earth ( also called general circulation, global circulation, or primary circulation) includes three circulation cells (Hadley cells, Ferrel cells and Polar cells) – Trade winds, Westerlies, and Polar Easterlies – Geostrophic winds, and Jet streams • Secondary circulation winds: Regional scale winds – Monsoon winds ? • Tertiary circulation winds: local winds (upto 100 km distance) – Sea level breezes and mountain and valley breezes • Wind is an important renewable energy source (indirect solar energy) – Wind speed matters a for wind energy (energy potential is proportional to the cube of wind speed) – Consistent winds are preferred (variability is not desirable)
  7. 7. Due to compressibility of air, atmospheric pressure decreases faster near the surface but less so aloft
  8. 8. International Standard atmosphere Pressure: 1013.25 hPa Density: 1.225 kg/m3 Temperature: 288.15 K Acceleration due to gravity: 9.80665m/sec.2 Speed of sound: 340.294 m/sec.
  9. 9. Measurement/Monitoring and Analysis • Temperature – Bimetallic thermometers (differential expansion of two different metals by temperature is the basis of measurement) – Electronic thermometers (elec. resistance changes with temp.) • Pressure – Mercury barometer (Evangelista Torricelli, 1643) – Aneroid barometer – no liquid is used – air pressure deforms an evacuated chamber and this is the basis of measurement • Winds – Wind velocity is measured by Anemometer – Wind direction by Wind Vane (aerovanes for both speed and direction) • Humidity (dew point monitoring!) – Dew point: Temp. to which air needs cooling for saturation moisture – Humidity is measured by hygrometer (Filamentous hygrometers – hair expands/contracts with humidity variation; Electrical hygrometers – chemicals absorb moisture and change resistance) – Sling psychrometer (measures cooling by evaporation) and Wet bulb and dry bulb thermometers
  10. 10. Measurement/Monitoring and Analysis • Monitored at the surface and at higher altitudes and use – Surface weather stations – Doppler radar (detects precipitation types and amounts , and measures wind velocities) – Radiosondes • Package of instruments (thermometer, barometer, hygrometer and transmitter) • Launched twice daily (at 0000 and 1200 Universal Time Coordinate on balloons from earth stations • As the balloon ascends, temperature, dew point and wind are measured and reported as a function of pressure (radiosonde telemetry) – Geostationary satellites and aeroplanes • Data analysis and calculations for the parameters through indirect measurements
  11. 11. Measurement/Monitoring and Analysis • Wind roses from wind data analysis – Wind speed, direction and frequency are pictorially presented – How to construct wind roses and how to read them? – Wind energy potential assessment • Theoretically available power of a wind is expressed as – Density of air decreases with temperature and altitude – Wind velocity is the major factor in power generation (20% increase in velocity increases power output by 73%) • Potential temperature: Temperature that a parcel of air (at pressure P) would acquire if adiabatically brought to a standard pressure P0 – It is denoted by θ and given by P = 1/2 ρ A v3 P = power (W) ρ = density of air (kg/m3) A = area perpendicular to the wind (m2) v = wind velocity (m/s) T is current temperature (in K) of air parcel R is the gas constant of air Cp is specific heat capacity at constant pressure R/ Cp for air is 0.286
  12. 12. Atmospheric Pressure Gradient Force • Horizontal pressure differences are mapped in the Average Sea Level Pressure Charts (constant height charts) using isobars – Pressure differences in the upper atmosphere are mapped in the Constant Pressure Charts using iso-hypse (iso-heights) • Iso-hypse gradient and horizontal pressure gradient influence the speed of surface winds and upper atmospheric winds respectively • Atmospheric pressure patterns are controlled by – Temperature changes (thermal air pressure lows and highs) – Earth rotation also creates air pressure systems (dynamic air pressure lows and highs) • Moving air masses (winds) affect changes in atmospheric pressures • Temperature changes can be – Latitudinal (high temperature at equator and decreasing temperature with higher latitudes – Land and ocean surfaces (land surfaces rapidly heated and rapidly cooled when compared to oceans) – Elevation/Topography of the surface
  13. 13. January July Isobars and Mean Sea Level Pressure Maps Depicts how pressure changes while holding the height constant Good weather analysis tool Surface/station pressure is reduced to sea level and depicted by isobars (lines of equal MSLP) – station pressure is adjusted for elevation to obtain the SLP Wind speed is proportional to distance between isobars These maps show low and high pressure centres, and troughs and ridges Troughs: curved isobars forming elongated regions of low pressure Ridges: curved isobars forming elongated regions of high pressure   ratiopressuretotaltopressureourwater vapis 61.01Texp v 10 10 rhere rThere TR zg PP vd       
  14. 14. Isohypse (Isoheights) Constant Pressure Charts Pressure is held constant. Used to describe upper air conditions. Prepared twice a day at 0000 and 1200 UTC for several mandatory pressure levels (925, 850, 700, 500 mb, etc.) Temperature, humidity and wind data required is provided by radiosonde - data is supplemented from other sources (aircraft and satellites) Forecast data is also depicted on constant pressure charts
  15. 15. 2211PP ConstantPVor 1 VPV V P    Pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature Boyle’s law Avogadro number and molar volume The number of elementary particles (molecules and/or atoms) per mole of a substance (6.022×10 23 mol -1) – it is expressed by the symbol NA Molar volume: volume occupied by one mole of ideal gas – Its value at STP is 22.414 L/mol and at NTP is 22.414 L/mol - it is same for all the gases or mixture of gases Volume of an ideal gas at constant pressure is directly proportional to the absolute temperature. V1 = original volume and V2 = new volume T1 = original temperature and T2 = new temp. Charle’s law 2 2 1 1 T V T 1 Constant T V or T V TV     2 2 1 1 T P Constant T P or T P TP Gay-Lussac’s law
  16. 16. Ideal gas law for dry air Ideal Gas Law expresses the relation between pressure, temperature and volume or density in an ideal or perfect gas – expressed as P V = n Ru T or p V = m R T or P = (m / V) R T or P = ρ R T Ru (universal gas constant) = 8314.47 (J/kmol/K) n is number of moles R (individual gas constant)(R = Ru / Mgas) = 8314/29 = 287 J/Kg/K V = volume of gas (m3) p = absolute pressure (N/m2, Pa) m = mass of gas (kg) T = absolute temperature (K) ρ = density (kg/m3) (ρ = m / V) Mgas = molecular weight of the gas Ideal gas law for moist air • Daltons Law states that the total pressure exerted by a mixture of gases is the sum of the partial pressures of the individual gases pt = pa + pw pt is total pressure pa is dry air partial pressure pw is water vapour partial pressure • Dry Air Partial Pressure is pa = ρa (286.9 J/kg K) T • Water Vapor Partial Pressure is pw = ρw (461.5 J/kg K) T • For moist the ideal gas law can be written as P = ρ R Tv Here R should be for moist air rather than for dry air – Instead correction is made to temperature, T (as virtual temperature,Tv) Tv = T (1 + 0.61 r) Here r is volume ratio of water vapour in the moist air
  17. 17. Pressure Gradient Force and Wind Systems • Consider a warm column of air and a cold column of air separated by 3000 kM distance, and assume – 1005 mb pressure at near sea level and 600 mb pressure at 5500 m altitude for the warm air column – 1020 mb pressure at near sea level and 400 mb pressure at 5500 m altitude for the cold air column – 500 mb pressure is measured at 5880 m altitude for worm column and at 4800 m altitude for cold column • Difference in pressure at 5500 m altitude between the two columns will initiate horizontal flow of air from warm column to cold column – Height gradient indicates the magnitude of force causing the air movement aloft – Iso-heights for the warm column and the cold column can be used in estimating the height gradient • Similarly at the sea level, horizontal flow of air from the cold column to the warm column is initiated – Iso-bars between the warm air column and the cold air column can be used in finding the pressure gradient – Sea level pressure maps can be used for computing the HPGF
  18. 18. Horizontal pressure gradient between the two columns is The height gradient (difference in height of a particular pressure value, 500 mb, for upper altitudes) between the two columns is mb/kM005.0 kM3000 )10051020(    mb HPGF m/kM36.0 kM3000 )48005880( gradientHeight    m Higher pressure is usually associated with fair weather and clear skies, and lower pressure with storms (tornadoes and hurricanes) Pressure Gradient Force and Wind Systems Vertical air movement of the air is the result of net force of vertical pressure gradient force and the vertical gravity force (acts opposite to the VPGF – disturbed hydrostatic equilibrium) Vertical motion of air masses produce winds ZgP  Hydrostatic equation:
  19. 19. Low Pressure & High Pressure Wind Systems • Atmospheric pressure patterns are controlled by – Temperature changes created by differential heating (thermal air pressure lows and highs) – Dynamic air pressure lows and highs created by upper level winds and earth rotation • Temperature changes can be – Latitudinal (high temperature at equator and low at higher latitudes – Land and ocean surfaces (land surfaces are rapidly heated and rapidly cooled when compared with ocean surfaces) • The horizontal pressure gradients on the earth surface and the height gradients of the upper atmosphere are highly dynamic – Average Sea Level Pressure charts and Constant Pressure Charts are used to show the dynamism • Wind movement on the surface, in the upper atmosphere and vertical movement of winds influence the pressure gradients – Surface low pressure wind systems are maintained or intensified by the divergence aloft of air
  20. 20. Colder earth surfaces on the other hand develop cold air columns – Characterized by surface thermal highs and by upper altitude lows, and by surface divergence and upper altitude convergence of air Warmer earth surfaces through heating develop warm air columns – Characterized by surface thermal lows and by upper altitude highs and by surface convergence and upper altitude divergence of air Surface high pressure Cool sinking air Surface low pressure Warm rising air
  21. 21. Coriolis Force and Friction Force, and Wind Systems Winds created by the pressure gradient force are modified by Coriolis force and friction force Coriolis force • Proportional to the speed of wind and varies with altitude (because of friction force) and latitude – Coriolis force changes only the wind direction but not speed (the level of coriolis deflection is influenced by the wind speed) – The amount of Coriolis deflection increases with latitude (zero at equator and the maximum at poles) – It acts at right angles (towards right in northern hemisphere and towards left in southern hemisphere) to the direction of wind • Moves air counter-clock-wise around a low pressure system and clock-wise around a high pressure system in northern hemisphere
  22. 22. Wind Systems of Atmospheric Lows and Highs In the northern hemisphere – Air moves out (diverges) from a surface high in the clockwise direction and it moves in (converges) into the surface low in the counter- clockwise direction due to Coriolis effect and friction – In the higher atmosphere, with no friction and with only pressure gradient force and Coriolis effect in action, air masses (winds or geostrophic winds) run parallel to isotherms (clockwise direction around the highs, and counter- clockwise direction around the lows) In the southern hemisphere the wind direction of convergence and divergence is opposite to that of the northern hemisphere
  23. 23. Coriolis Force and Friction Force, and Wind Systems Friction force • It is Earth’s surface drag and it is limited to planetary boundary layer (1 to 3 kM) – reduces with altitude – surface roughness increases • Experienced by surface winds (through slowing down the wind speeds it is reduces the Coriolis deflection) In the upper atmosphere beyond the boundary layer, friction force is negligible and Coriolis force is exactly equal and opposite to PGF • Wind moves parallel to the isobars at constant speed (these winds are called geostrophic winds) – The air masses run parallel to the isobars (clockwise direction around the highs, and counter-clockwise direction around the lows) – Sub-geostrophic flow occurs around low pressure centres and super- geostrophic flow occurs around high pressure centres • Gradient winds: winds that blow at constant speed parallel to the curved isobars (around highs/lows of the upper atmosphere)
  24. 24. Global Pressure Patterns and General Circulation • Temperature difference between the equator and the poles generates global general circulation – Thermal lows at the equator and thermal highs at the poles are generated – Large scale vertical air movement generates pressure differences across the Earth and assist in the development of surface winds – 60% of the heat energy redistribution is by the atmospheric circulation) • For an ideal earth (non-rotating and all oceanic earth) a single circulation cell is expected – Air raises near the equator, moves towards the poles in the upper atmosphere, descends near the poles and moves towards the equator on the earth surface • What makes global air circulation very complicated? – Latitude (radiation received varies) – Earth’s rotation and tilt of the earth’s rotational axis (seasonality) – Positions of continents and oceans - northern hemisphere has more land than the southern hemisphere – Altitude and roughness of the earth surface – Cloud cover (during daytime clouds reflect radiation and during nights clouds prevent escape of radiation
  25. 25. Global Pressure Patterns and General Circulation • Due to the rotation, instead of a single cell, three cells of large-scale circulation (of rising and descending air) are existing – Hadley cells, Ferrel cells and Polar cells • Continents gain and loose heat much more quickly than oceans – Become warmer during day time and colder during nights (land-sea breezes) – Become much warmer in summers and much colder in winters - low pressure cyclones are developed during summer and high pressure anticyclones during winters - (monsoons!) – Coastal land areas stay warmer in winter and cooler in summer – Because of the land-sea distribution differences, mid-latitude cyclonic depressions are rapidly developed in the northern hemisphere oceans
  26. 26. Atmospheric pressure patterns Thermal highs and lows • Equatorial thermal lows at 5 N to 5 S (intertropical convergence zone –ITCC) • Polar thermal highs at 90 N and S: Characterized by descending air • Monsoon lows • Highs and lows associated with land-sea breezes and Mountain- valley breezes Rotation induced or dynamic air lows and highs • Earth rotation causes accumulation of air at certain latitudes (highs) and air divergence at other latitudes (lows) • Sub-tropical highs (at 25 to 40 N and S) characterized by descending dry air and clear skies • Sub-polar lows (at 55 to 70 N and S) characterized by ascending air and storm centers (warm air from low latitudes is lifted up by the cold polar air)
  27. 27. Global Pressure Patterns & Climate Zones Tropical climate: • Intertropical convergence zone (ITCC) • Equatorial thermal lows near the equator (between 5 N to 5 S) • Characterized by high sun angles, long days, high surface temperatures, ascending air, heavy precipitation, cloud cover and thunder storms • This zone shifts to the north of the equator in summer and to the south in winter Subtropical climate: • This zone is between 25-40° N and between 25-40 S latitudes • Air rising at the equator spreads out, cools, and descends here • This zone is associated with clear skies, low rainfall, and high day time temperatures (>40°C) • Many of the hot deserts are found here • This zone expands towards higher latitudes during summer
  28. 28. Global Pressure Patterns & Climate Zones Temporate climate: • Between 50-60 N and between 50-60 S latitudes • Some of the descending air of subtropical zone (warm surface air) and the cold surface air from the poles moves towards this zone • The warm surface air and the cold polar collide in this zone, and rises up, developing a low pressure zone • This zone is cyclonic in nature, and is associated with the development of frontal depressions (more dominant in winters) • Sometimes, during summers, the subtropical highs expand into, and the temperate zone experience calming influence on weather Polar zone of climate: • Polar thermal highs at 90 N and S • Characterized by very low temperatures, descending and heavy air, and creation of highs • This zone has permanent, thick snow and ice cover • This zone can be as dry as hot deserts of subtropical climate zone
  29. 29. Winds and Currents • Horizontal motion of air is considered as wind and vertical motion as current • Wind is a constituent of weather and wind is also a determinant of other elements of the weather (temperature and precipitation) • Air movement is because of the interacting forces imbalances – Interacting forces include real forces (horizontal & vertical PGFs, gravity force and friction force) and apparent forces (Coriolis force) – Air movement is meant for the balancing or equilibriating the forces – Since the factors causing the force imbalances are constantly changing, the equilibriating process is an unending process • Vertical movement of air occurs only when gravity is not balanced by VPGF (at lows and highs) – Air moves up when the vertical pressure gradient force is stronger then the gravity force, and moves down when it is weaker • Moving winds experience no Coriolis deflection at the equator and the deflection increases with latitude (the maximum at poles) • Winds in the upper atmosphere experience no surface drag (frictional force) and hence have higher wind speeds (geostrophic)
  30. 30. Winds and Currents Surface winds • Intertropical convergence zone (ITCZ): the equatorial (between 5 N and 5 S) belt of variable winds and calm (also known as doldrums) • Trade winds: North-East and South-East winds seen at 5 to 25 N and S latitudes; the winds are steady and persistant • Horse latitudes: subtropical belt of variable winds and calm; seen in 30 to 35 N and S latitudes • Weterlies: seen at 35 to 60 N and S latitudes; these winds are neither persistent nor steady. • Polar front: seen in 60 to 65 N and S latitudes; it is a zone ofconflict among different air masses – the boundary between the polar easterlies and the westerly winds of the mid latitudes – it separates the cold polar air from the warm temperate air • Polar easterlies: seen at 65 to 80 N and S latitudes; more prevalent in the southern hemisphere than in the northern hemisphere • Polar zone: seen in 80 to 90 N and S latitudes; variable winds and calm are characteristics
  31. 31. Winds and Currents Winds aloft • Surface lows/highs have matching upper atmosphere highs/lows • In the upper atmosphere air moves under the influence of PGF and Coriolis force and parallel to the isobars • Upper atmosphere has westerly jet streams - Rivers of extremely high speed winds - occurs in the zones of strong temperature contrasts – polar and sub-tropical jet streams Winds of the upper atmosphere • Upper level westerlies: seen between 25-90 latitudes • Tropical high pressure belt (5 to 20 N and S latitudes) • Equatorial easterlies Jet streams: • Polar jet: moves from west to east at ~10 kM altitude – when the cold air from poles meets the warm air from the other side, strong temperature and pressure gradient is developed - seasonally shifts • Subtropical jet (westerly jet) – located at 13 kM altitude above the subtropical high zone – this jet is relatively weaker because of the weeker latitudinal temperature and pressure gradients
  32. 32. Regional Winds and Local winds Regional winds • Monsoons Local winds are caused by contrasts in heating of the atmosphere at the surface • Water and land, at coastlines of seas and lakes • Urban and rural areas • Vegetated and unvegetated areas • Wet and dry areas • Snow-covered ground and bare ground Day time sea breeze Night time land breeze
  33. 33. Summer monsoon

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deals with temperature, density, pressure, winds and humidity parameters of the atmosphere; Prssure gradient force, coriolis force, gravity force and friction force and winds and currents, ; pressure lows and highs, atmospheric circulation, winds.


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