050 03 00 00 THERMODYNAMICS

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050 03 00 00 THERMODYNAMICS

 

050 03 01 00 Humidity

050 03 01 01 Water vapour in the atmosphere

 

(01) State that the density of moist air is less than the density of dry air.
Yep, the mass of water vapour is less than air.

 

(02) Describe the significance for meteorology of water vapour in the atmosphere.
Without water vapour, there is no weather

 

(03) Indicate the sources of atmospheric humidity.
Oceans, seas etc. Anywhere with water…

 

(04) Define ‘saturation of air by water vapour’.
Where relative humidity is 100%, the parcel of air can contain no more water vapour.

 

 

050 03 01 02 Intentionally left blank

 

 

050 03 01 03 Temperature/dew point, relative humidity

 

 

(01) Define ‘dew point’.
…is the temperature at which the air will become saturated

 

(02) Define ‘relative humidity’.
Expressed as a percentage, it is a measure of how much water vapour is present compared to how much is could hold.

 

(03) Explain the factors that influence the relative humidity at constant pressure.
The amount of water vapour present and the temperature of the air. As the temperature goes up, it can hold more air therefore the relative humidity goes down.

 

(04) Explain the diurnal variation of the relative humidity.
During the day the temperature rises, therefore it can hold more water vapour, so the RH reduces and vice versa at night.

 

(05) Describe the relationship between temperature and dew point.
It describes the relative humidity.When close together, god and low clouds are to be expected, when far apart, the opposite.

Rough cloud base in feet = (T – DP) x 400

 

(06) Estimate the relative humidity of the air from the difference between dew point and temperature.

For example: 20/15 is 75% RH

 

 

050 03 02 00 Change of state of water

 

050 03 02 01 Condensation, evaporation, sublimation, freezing and melting, latent heat

 

 

(01) Define ‘condensation’, ‘evaporation’, ‘sublimation’, ‘freezing and melting’ and ‘latent heat’.
  • Condensation is where gas changes to a liquid.
  • Evaporation is where liquid changes to a gas.
  • Sublimation or de-sublimation is where a gas changes from solid to a gas skipping the liquid bit and of course vice versa.
  • Freezing and melting is to/from a liquid and a solid.
  • Latent heat is the mechanism by which all the above phase changes happen, it is energy released or absorbed without any change in temperature.

 

 

 

(02) List the conditions for condensation/evaporation.
  • Condensation – the air must be cooled and there must be a nucleus of matter for the water vapour to stick to.
  • Evaporation – An increase of temperature and/or pressure.

 

(03) Explain the condensation process.

Water droplets combines with each other when they no longer have enough energy to stay apart.

 

(04) Explain the nature of and the need for condensation nuclei.
They provide a suitable surface for the water droplets to stick to and begin the condensation process.

 

(05) Explain the effects of condensation on the weather.
Clouds basically

 

(06) List the conditions for freezing/melting.
A freezing nucleus is needed and a temperature from 0°C and below.

 

(07) Explain the process of freezing.
Latent heat is released.

 

(08) Explain the nature of and the need for freezing nuclei.
Like condensation.

 

When water exists in liquid form below 0°C because of a lack of freezing nuclei.

 

(Refer to Subject 050 09 01 01)”

 

(10) List the conditions for sublimation.
Nuclei required

 

(11) Explain the sublimation process.
From solid to gas, latent heat is absorbed. Deposition or de-sublimation is a gas to a solid.

 

(12) Explain the nature of and the need for sublimation nuclei.
Something to hold on to.

 

(13) Describe the absorption or release of latent heat in each change of state of water.
If it gets colder latent heat is released, if it gets warmer latent heat is absorbed.

 

(14) Illustrate all the changes of state of water with practical examples.
  • Evaporation – Increasing humidity, performance decrease because of the reduction in density.
  • Condensation – Clouds, reduced visibility.
  • Freezing – Ice on aircraft etc.
  • Sublimation – Ice on cold airframe, high altitude contrails.

 

 

050 03 03 00 Adiabatic processes

050 03 03 01 Adiabatic processes, stability of the atmosphere

 

(01) Describe the adiabatic process in an unsaturated rising or descending air particle.

Adiabatic, warming or cooling with no heat transfer.

The dry adiabatic lapse rate is 3°C/1000′ or 1°C/100m. It is linear through the atmosphere.

 

 

(02) Explain the variation of temperature of an unsaturated rising or descending air particle.

The higher the temperature the dryer the air.

 

(03) Explain the variation of humidity of an unsaturated rising or descending air particle.

A more humid particle will have less far to rise before it becomes saturated.

 

(04) Describe the adiabatic process in a saturated rising or descending air particle.

The SALR is 1.8°C/1000′ or 0.6°C/100m. It changes its temperature by a smaller amount because of the release of latent heat with condensation.

 

(05) Explain the variation of temperature of a saturated air particle with changing altitude.

The rate varies with temperature and is a non-linear curve up through the atmosphere, the warmer it is the slower it cools. This means a slower rate in the tropics about 0.4°C/100m and 0.9°C/100m in the polar regions or at higher altitudes.

 

(06) Explain the static stability of the atmosphere using the actual temperature curve with reference to the adiabatic lapse rates.

Static stability is when the parcel of air concerned matched that of its surroundings, I think. One definition I found,

“Static stability is defined as the stability of the atmosphere in hydrostatic equilibrium with respect to vertical displacements.”.

 

(07) Define qualitatively and quantitatively the terms ‘stable’, ‘conditionally unstable’, ‘unstable’ and ‘indifferent’.
  • Stable – will sink back down after a lifting trigger, i.e remains colder than its surroundings.
  • Conditionally unstable – Where the ELR sits between the SALR and DALR – this means the saturated air is unstable.
  • Indifferent – rubbish word, neutral stability is better, it will stay where it is left when disturbed. When ELR = DALR and Dry air is disturbed or ELR = SALR and saturated air is disturbed.

 

 

(08) Illustrate with a schematic sketch the formation of Foehn.

Its a local, dry wind:

Moist air blown against a mountain, it is lifted orographically cooling at the DALR, the condenses and continues at the SALR loosing moisture with cloud being lower on the windward side obviously.

In stable conditions, the air sinks on the other side warms adiabatically and becomes much dryer.

 

 

(09) Explain the effect of the advection of air (warm or cold) on the stability of the air.

Warmer air advecting over a colder surface, increases stability because the lower levels will be cooled reducing the ELR or the difference in temperature.

Opposite is true.

 

Remark: Dry adiabatic lapse rate = 1 °C/100 m or 3 °C/1 000 ft; average value at lower levels for saturated adiabatic lapse rate = 0.6 °C/100 m or 1.8 °C/1 000 ft (values to be used in examinations).