Water Vapour

Question 1

Explain the sources of water vapour in the atmosphere.

Answer 1

• Evaporation/sublimation from any wet surface
• Transpiration = plants releasing water vapour
• Most water vapour in troposphere
• 4% at the equator and almost 0% at the poles

Question 2

Describe ‘vapour pressure’ and ‘saturation vapour pressure’

Answer 2

• Vapour pressure = partial pressure of water vapour in air
• Does not change with temperature
• Saturation vapour pressure = Vapour pressure when it saturates/maximum it can hold
• Changes with temperature
• Increase temperature = higher vapour pressure is required
• Decrease temperature = lower vapour pressure required

Question 3

Define ‘latent heat’.

Answer 3

• Heat energy released/absorbed when substance changes state
• Does not change its own temperature

Question 4

Describe the condensation process.

Answer 4

• Water vapour changing to liquid state
• Cooled to dew point/saturation vapour pressure
• Releases latent heat

Question 5

Describe the freezing and melting processes with reference to latent heat.

Answer 5

• Freezing = temperature below 0degrees
  Liquid to solid change in state
  Releases latent heat
• Melting = temperature above 0degrees
  Solid to liquid change in state
  Absorbs in the liquid

Question 6

Describe the diurnal variation of relative humidity and dew point.

Answer 6

• Relative humidity and dew point decreases during the day (lowest mid afternoon)
• Increases at night (highest at sunrise)
• Due to change in temperature from warm to cold
• Relative humidity = vapour pressure/saturated vapour pressure
• RH is maximum approximately 30 minutes after sunrise when the temperature is minimum.

Question 7

Describe the effects of moisture content on the density of the air.

Answer 7

• Water vapour is a light gas compared to oxygen and nitrogen
• Water is H2O
• Hydrogen = lightest element on periodic table
• Increase moisture content = decrease in density