WATER VAPOUR

Question 1

How does water vapour enter the atmosphere?

Answer 1

Mainly by evaporation from exposed water surfaces (e.g. oceans,lakes).
Also via transpiration from trees & plants
Called EVAPOTRANSPIRATION

Question 2

Where are the greatest concentrations of water vapour found

Answer 2

near the surface of the earth, however over vast continents, central-most countries can experience extreme dryness as they are further from sources of water vapour.

Question 3

What happens to water vapour concentration with altitude?

Answer 3

Decreases.

Question 4

What states can water exist in the atmosphere?

Answer 4

Solid, (clouds, icing, frost, snow, hail)
Liquid, (rain, clouds)
Gas (vapour) - factor that controls air temperature

Question 5

General RoT wrt the relationship between air temperature and water vapour?
(Analogy)

Answer 5

Air appears to behave like a sponge.
Increase in air temp = increase in sponge size = increase in water vapour content. (And vice versa)

THE WARMER THE AIR PARCEL, THE MORE WATER VAPOUR IT CAN HOLD

Question 6

Define Relative humidity (RH)

Answer 6

The amount of water vapour present in the air, expressed as a percentage of the amount needed for saturation to occur.

Question 7

Equation for RH?

Answer 7

RH = (amount of water vapour the air IS holding/the amount the air CAN hold at that temperature) x 100/1

Question 8

What causes a change in RH in the atmosphere (2)

Answer 8

1. A change in water vapour content in a parcel
2. By a change in temperature

Question 9

Define ‘dew point’

Answer 9

The temperature to which a parcel of air must be cooled (at a constant pressure) to become fully saturated.

Question 10

What is ‘saturation’

Answer 10

occurs when the parcel of air contains the maximum amount of water vapour possible for a given temp.
RH = 100%
Air temp = dew point/wet bulb temp

Question 11

What is meant by ‘saturation vapour pressure’

Answer 11

Closed container example; as more water molecules evaporate to become water vapour, the vapour pressure in the air increases. Due to the increase in vapour pressure, molecules are forced to return back to the liquid. Eventually this reaches equilibrium.
At this point, the air is saturated (can no longer take any more water vapour molecules). The pressure exerted by the water vapour at this point is called
SATURATION VAPOUR PRESSURE.

Question 12

High temps vs. saturation point?

Answer 12

Air parcels at higher temps can hold more water vapour, therefore, far more water vapour is required for the air parcel to become saturated (e.g. 35deg C can hold 40g, whereas -15deg C can only hold 1g)

Question 13

Effect of moisture content of air on dew point?
General RoT?

Answer 13

more water added to air, dew point temp goes up & vice versa.
Air temp is always greater than, or equal to, the dew point temperature.
The closer the two numbers, the closer we are to saturation (condensation)

Question 14

Why is dew point better than RH for aviation?

Answer 14

RH is effected (will increase/decrease) even if no water vapour is added/taken away.
So when RH changes, we can’t tell if it was due to a change in the amount of water vapour, or just due to a change in air temp.

Question 15

Why, if the dew point has been steady, would we suddenly see a degree drop an hour before the air temp matches the dew point temp?

Answer 15

There is less water vapour in the air, due to dew depositing on the ground (hence ‘dew point’).
Radiation fog is always pre-empted by heavy dew, then formation of fog 1-2hrs later.

Question 16

Define condensation

Answer 16

Invisible water vapour changes to its visible liquid water state.
Air temp must cool to meet dew point temp & there must be an aerosol for the vapour to condense on to.

Question 17

Define evaporation

Answer 17

Reverse process to condensation. liquid > vapour.

Question 18

Define deposition

Answer 18

Process where water vapour deposits directly to ice without liquid phase. (e.g. rime icing)

Question 19

Define Sublimation

Answer 19

Reverse process to deposition. Solid > vapour

Question 20

Define melting

Answer 20

Ice melts to liquid water at 0deg C (only if pressure is 1013.25)

Question 21

Define freezing

Answer 21

Process where liquid water changes to solid ice (at 0deg C (only if pressure is 1013.25))

Question 22

Define Latent heat

Answer 22

When water changes between its three states, latent heat is released or taken in.
It is the heat energy required to convert between the stages.

Question 23

Latent heat: released vs. taken in?

Answer 23

Released: deposition/freezing/condensation
Taken: Melting/Evaporation/Sublimation

Question 24

How does air & water temperature effect the rate of evaporation?

Answer 24

Increased water temp = greater rate of evaporation due to the increased energy of molecules in the water.
Higher air temps = air can hold more water, so will readily accept a greater rate of evaporation

Question 25

How does moisture content of air effect rate of evaporation?

Answer 25

The drier the air is initially, the greater the rate of evaporation.

Question 26

How does wind speed effect rate of evaporation?

Answer 26

Faster the movement of the air, the greater the evaporation rate as molecules that are escaping the water are whisked away by the wind = lower water vapour pressure above the liquid water surface.

Question 27

What is a wet bulb thermometer

Answer 27

A thermometer whose bulb is covered with a moist muslin wrapping.
Water evaporates off the muslin by absorbing latent heat from the bulb and the air surrounding.
This is a cooling process for the air that results in a lower temp value.