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

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.

Saturated Vapour ContentG

/M3

Saturation Line

Dew Point

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 =

Diurnal Variation of RH

Diurnal Variation of RH

Approx. RH % = DEW POINT x 100% DRY BULB TEMP

Psychrometer

(Wet & Dry Bulb Thermometers)

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.

Hygrometer

Used to measure humidity

ElectricalUsed in radiosondes

Infrared

Latent Heat

Lesson 11

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.

Latent HeatEvaporation/Freezing

Latent Heat of Boiling

Latent Heat of Sublimation

GASSOLID

Deposition(e.g. Hoar Frost)

(No liquid phase)

Latent Heat Summarised

DEPOSITI

ON

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.