Section 04 Thermodynamics Adiabatic Processes Lesson 10/11
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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.
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
Vapour Pressure
the total air pressure =
the sum of the partial pressures for nitrogen + oxygen + water vapour + others
Saturation vapour pressure
The amount of water vapour that can be held in the air depends upon the temperature
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.”
Relative Humidity Indicates the relative degree of saturation of
the air. At saturation RH = 100%
Relative Humidity =
Absolute HumiditySaturated Vapour Concentration
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
x 100% = 67%RH =
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
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.
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.
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.
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:
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.’
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.
Deposition/Sublimation
Hoar frost comes from a deposition process
Airframe ice may reduce slowly in sub-zero air by sublimation
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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