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Section 04 Thermodynamics Adiabatic Processes Lesson 10/11

Section 04 Thermodynamics Adiabatic Processes Lesson 10/11. Atmospheric Water Vapour. The concentration of the invisible gas, water vapour varies greatly from place to place and from time to time. In warm tropical areas it may be 4% of the atmospheric gases while

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Section 04 Thermodynamics Adiabatic Processes Lesson 10/11

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  1. Section 04Thermodynamics Adiabatic ProcessesLesson 10/11

  2. Atmospheric Water Vapour • The concentration of the invisible gas, water vapour varies greatly from place to place and from time to time. • In warm tropical areas it may be 4% of the atmospheric gases while • in cold polar latitudes it may dwindle to a mere trace.

  3. Different states of water vapour • Equilibrium undeveloped, • i.e. air is unsaturated & water evaporates from the surface • State of Equilibrium • i.e. water molecules leave & return in equal numbers • Super-saturation, • i.e. air is saturated & water molecules return to liquid state

  4. Vapour Pressure • the total air pressure = the sum of the partial pressures for nitrogen + oxygen + water vapour + others

  5. Saturation vapour pressure • The amount of water vapour that can be held in the air depends upon the temperature

  6. Humidity • Water vapour exists in the atmosphere as an invisible gas. • The measure of the air’s water vapour content is referred to as it’s humidity and is measured in different ways.

  7. Humidity • Water vapour stores latent heat which is released during condensation and is one of the most important atmospheric sources of energy. • Condensation results in water droplet formation and hence cloud, fog and mist which obscure visibility.

  8. Humidity • The main source of atmospheric humidity is evaporation and as the earth’s surface is mostly covered by oceans the amount of evaporation is huge.

  9. Absolute Humidity • is a measure of the actual amount of water vapour present in a given volume of air. • It is expressed as g/m3 dry air.

  10. Humidity Mixing Ratio (HMR) • Humidity can also be expressed in ways that are not influenced by changes in volume. • The humidity mixing ratio is the ratio of the mass of water vapour present relative to the mass of dry air in the air parcel. • It is expressed as g/kg of dry air.

  11. HMR, cont’d • The HMR remains constant as long as the moisture content remains the same • warmer air has a greater capacity to hold water vapour than cold air

  12. HMR (cont.) • At the higher temperatures the water vapour molecules have a greater kinetic energy (average speed). Thus at high temperatures the molecules will have sufficient energy to remain as vapour. • As the temperature lowers, the average speed of the molecules decreases and hence fewer molecules have the energy to remain as vapour.

  13. Saturated Vapour Content Saturation Line Dew Point G/M3

  14. Saturation • When air at a particular temperature contains the maximum amount of water vapour possible, the air is said to be saturated. • Referring to the Vapour Capacity graph, it can be seen that at 30°C, the maximum vapour capacity is 30gm/m3. • The air is saturated.

  15. Saturated Humidity Mixing Ratio • Any cooling below the temperature and the air would be supersaturated and condensation would occur to establish a vapour capacity equilibrium at the lower temperature. • The air would be at its saturated humidity mixing ratio (SHMR). • The SHMR is the value of the humidity mixing ratio at a given temperature and pressure when the air is saturated.

  16. Dew Point • Is the temperature at which a sample of air would just become saturated with respect to a plane surface of water if cooled at constant pressure. • The water still exists as water vapour at saturation.

  17. Dew Point • Cooling below the dew point will initiate condensation in the presence of condensation nuclei. • If no nuclei are present the water content will remain as water vapour. • The air is then described as being “supersaturated.”

  18. Relative Humidity • Indicates the relative degree of saturation of the air. • At saturation RH = 100% Relative Humidity = Absolute Humidity Saturated Vapour Concentration

  19. RH Calculation • Referring to the Vapour Capacity curve: • Absolute Humidity at 20°C = 10 gm/m3 • Saturated Vapour Concentration at 20°C = 15 gm/m3 10 15 RH = x 100% = 67%

  20. Diurnal Variation of RH

  21. Diurnal Variation of RH Approx. RH % = DEW POINT x 100% DRY BULB TEMP

  22. Psychrometer (Wet & Dry Bulb Thermometers)

  23. Wet Bulb Temperature • Is the lowest temperature to which air can be cooled by the evaporation of water. • Note: Wet-bulb temperature must not be confused with Dew-point temperature. • Wet-bulb temperature is always between the Dew-point and Dry-bulb temperatures

  24. Wet Bulb (Cont.) • If air is cooled towards it’s dew-point; • The Relative Humidity will increase; • Dry Bulb and Wet Bulb temperatures will decrease until; • the Dew-point is reached at which time: • Dew-point = Wet-bulb = Dry-bulb Temperature.

  25. Points to note: • Dew-point is the best indicator as to the actual water vapour content of an air mass. • Dew points change very little during the day unless: • there is evaporation from wet ground or; • precipitation occurs or; • there is a change of air mass.

  26. Hygrometer • Used to measure humidity • Electrical • Used in radiosondes • Infrared

  27. Latent HeatLesson 11

  28. Change of State • Water exists in three states, namely: • ice(solid), • water(liquid) or • water vapour(gas). • Every time a substance changes state, latent heat is involved and it is important to understand its role in weather processes.

  29. Heat Input/Output • Heat input/output appears in 2 ways: • Sensible Heat which involves: • An observable change in temperature. • Latent Heat which involves • no observable change in temperature:

  30. Heat Input/Output. Cont’d • Latent Heat involves a change of state. • Solid  Liquid. • Liquid  Gas. • Latent heat is also involved when change of state is directly from: • Solid  Gas. • In which case the process is known as • ‘Sublimation.’

  31. Heating/Cooling of a Liquid • Heat input/output is usually expressed in • Joules, • 1 joule is the amount of heat necessary to raise the temperature of 1 kg of water 1°C, or; • Calories, • 1 calorie is the amount of heat necessary to raise the temperature of 1 gram of water 1°C.

  32. Latent Heat Evaporation/Freezing

  33. Latent Heat of Boiling

  34. Latent Heat of Sublimation (No liquid phase) SOLID GAS Deposition(e.g. Hoar Frost)

  35. Latent Heat Summarised DEPOSITION

  36. Deposition/Sublimation • Hoar frost comes from a deposition process • Airframe ice may reduce slowly in sub-zero air by sublimation

  37. Latent Heat (cont.) • Most of the energy in the atmosphere is stored in the form of latent heat of evaporation. • This is released during atmospheric cooling as as latent heat of condensation. • Latent heat release is thus the major energy source in all weather systems especially thunderstorms and tropical revolving storms.

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