5.2 - Global Water Cycle

Question 1

What different ways is water vapour in the atmosphere quantified?

Answer 1

1) Specific humidity
2) Mixing ratio
3) Vapour pressure
4) Relative humidity
5) Column-integrated water vapour

Question 2

What is the specific humidity? q[g/kg]

Answer 2

It is the mass of water vapour divided by total mass of air (includes vapour in total mass)

Question 3

What is the mixing ratio? R[g/kg]

Answer 3

Mass of water vapour divided by the mass of dry air (excludes vapour in total mass)

Question 4

What is the vapour pressure? Pv [pascal = n/m2 = kg m s-1/m2]

Answer 4

The portion of total atmospheric pressure due to water vapour only (much smaller than total atmospheric pressure)

Question 5

What is the relative humidity? RH = 100 x pv/pv* [%]

Answer 5

The amount of water vapour in the air relative to the maximum amount possible

Question 6

What is the column-integrated water vapour? CWV [kg/m2]

Answer 6

The total mass of water vapour per metre2 integrated through full depth of the atmosphere

Question 7

What is the maximum quantity of water that air can hold controlled by?

Answer 7

The Clausius-clapeyron relationship

Question 8

What is the Clausius-Clapeyron relationship?

Answer 8

Saturation vapour pressure, pv, is the pressure exerted by the maximum amount of water an air parcel can hold.

Pv is an exponential function of temperature: the Clausius-clapeyron relationship

• Saturation Vapor pressure increases exponentially with temp

Question 9

Saturation vp equation

Answer 9

RH= 100 x Pv/P*v

• When air is saturated, RH= 100 and actual vapour pressure = saturation vapour pressure

Question 10

What are the 3 ways to bring air to saturation (where RH=100%) and forms clouds, fog and rain?

Answer 10

1. Add water vapour to the air (increase pv)
2. Cool the air (decrease pv, this is how rain and clouds form)
3. Mix cold air with warm, moist air

Question 11

What is the relationship between humidity and temperature?

Answer 11

Temp and relative humidity are inversely proportional to one another

Question 12

How does surface relative humidity vary over the world

Answer 12

Surface relative humidity is quite uniform over oceans (approx 80-90%) but can be much lower over land where water is limited

Question 13

How does water vapour in the atmosphere vary around the world

Answer 13

There is much more water vapour at the equator than the poles (where atmosphere is warm)

It is warmer near the earths surface so the air has more moisture and is more humid

Water vapour moves in the atmosphere moves around with the winds, and is modified by precipitation and evaporation

Question 14

What is net precipitation?

Answer 14

It is precipitation minus evaporation

Dry regions where P-E < 0
Wet regions where P-E > 0

Question 15

What is precipitation minus evaporation balanced by?

Answer 15

The transport of water vapour in the atmosphere

D(q)/dt = E-P

(Dt = change in total amount of water vapour in the atmosphere column per unit time)

Water Vapour fluxes by atmospheric winds: wind x specific humidity

D(q)/dt = E-P + (Fin - Fout)/L

Question 16

What happens if F out is > F in

Answer 16

If F out> F in, the atmosphere is diverging water away from a region and must have P-E < for balance

Question 17

What happens if F out is < than F in

Answer 17

The atmosphere is converging water into a region and must have P-E > 0 for balance

Question 18

What is the P-E balanced by?

Answer 18

P-E is balanced by convergence or divergence of water vapour by the atmospheric circulation

• If assuming the amount of water vapour in the atmosphere doesnt change ‘steady state’

Question 19

Why can only have P-E < 0 over oceans over long time scale?

Answer 19

Over oceans, the amount of precipitation (P) that falls into the ocean is generally less than the amount that evaporates (E) from the ocean. (This is because some falls over land)

• Over a long time scale, this process tends to reach a steady state where E is about equal to P. However, in most cases the amount of water that evaporates in slightly greater than P. This leads to a negative water balance (P-E<0)

(This can be balanced by inflow of water from rivers and other sources which replenish water lost from E. But if it stays negative for long time scale then ocean level can drop which can effect climate, Human populations and transportation etc)

Question 20

How is ocean salinity closely tied to P-E?

Answer 20

When more water falls into the ocean as precipitation than is lost through E (when E is positive), the excess freshwater reduces the salinity of the ocean. = dilutes it