5.1 - Water In The Environment

Question 1

Why is the water cycle so important?

Answer 1

It has societal and economic importance
- Agriculture
- Drinking water
- Sanitation
- Power generation

It can impact natural hazards
- Flooding
- Tropical cyclones
- Drought
- Wildfires

Question 2

Why is the water cycle important for climate?

Answer 2

Water is important for maintaining and changing earth’s climate

Question 3

How does water vapour in the atmosphere impact climate?

Answer 3

Water vapour in the atmosphere is good at trapping energy in the form of radiation and it has a very strong greenhouse effect

Question 4

How do clouds impact climate?

Answer 4

• Clouds cool the earth by reflecting sunlight. They can also warm the earth by trapping radiation emitted from the surface. (Don’t really know how clouds will change in future and is main source of uncertainty for future climate change)

Question 5

How does ice impact climate?

Answer 5

Ice reflects lots of sunlight which cools the earth. Ice on land is critical for sea level = melts, sea level rises

Question 6

How do oceans impact climate?

Answer 6

Oceans set the pace for climate change because of their huge heat capacity. They store lots of heat and carbon dioxide.

Question 7

What does water vapour in the air drive?

Answer 7

The amount of water in the air (known as the water vapour feedback) also drive changes in monsoons, storms, hurricanes in a warming climate.

Question 8

Where is earth’s water stored?

Answer 8

Oceans = 96.5%

Land (lakes, rivers, groundwater) = 1.8%

Ice and snow = 1.7%

Atmosphere (clouds, invisible water vapour) = 0.001%
- tiny proportion but still important

Question 9

How much water is rained out and evaporated each year?

Answer 9

0.5 million cubic km of water is rained out and evaporated each year (about 100cm spread over the earths surface)

Question 10

How does the residence timescale vary between reservoirs?

Answer 10

Atmosphere = days (dynamic reservoir)

Land = hundred years

Oceans = thousand years

Ice and snow = thousand years

Question 11

What does the TRMM GPCP do?

Answer 11

It combines satellite and rain gauge observations

Question 12

What does precipitation do?

Answer 12

• It controls the input of water to earth’s surface
• It is a major factor in controlling soil moisture, vegetation and salinity
• It provides knowledge of rainfall patterns in space and time which is essential to understanding water flows on land

Question 13

Where is there heaviest rain?

Answer 13

At the ITCZ = inter tropical convergence zone
(Mm/day)

Question 14

How does precipitation vary with latitude?

Answer 14

In general, tropical regions are very wet, subtropics are dry and mid-latitudes are wet.

Question 15

Why are different regions wet?

Answer 15

Because of the atmospheric circulation

• Earth receives more solar energy at the equator than at the poles. This drives a global circulation of air in the atmosphere
• There is ascending air in the tropics = rainforests, wet
• There is descending air in the subtropics = deserts, dry

At Mid latitudes there is rainfall caused by cyclones driven by:
- uneven distribution of solar energy
- earth’s rotation

Question 16

Does precipitation vary over small regions?

Answer 16

Yes

• West of Scotland is wetter than east because air rises over topography at west and falls as rain
• air is dry by time in reaches east

Question 17

Precipitation formation mechanism

Answer 17

• Air parcels near earth’s surface containing lots of water vapour are moved upwards
• It gets colder the higher in atmosphere. The air becomes saturated and cannot hold onto its water vapour
• Water vapour condenses to form clouds and rain

(Thermodynamically, cold air can’t hold as much water vapour as warmer air)

Question 18

What causes precipitation?

Answer 18

For precipitation to from, air needs to be pushed upwards. This can happen in 3 main ways:

1) Fronts in high/low pressure systems

2) Convention

3) Orography

Question 19

How do fronts in high/low pressure systems cause precipitation?

Answer 19

Frontal precipitation forms when cold and warm air masses meet.

• At the cold front, advancing cold air forces warm air to rise along a steep slope, causing intense rain for a short time.
• At the warm front, there is a less sharp accent and light or moderate steady rainfall.

Frontal precipitation is very common in the uk

Question 20

What causes convection precipitation?

Answer 20

Convective precipitation occurs when there is strong, quick heating of the Earth’s surface by intense sunlight. This makes air parcels unstable and they ascend rapidly. This is important in the tropics and in mid-latitudes during summertime

It is more common over land because land heats up quicker than oceans. Land has a smaller heat capacity and so there are more storms over land.

Question 21

What causes orography precipitation?

Answer 21

Orographic precipitation forms when moist air is forced upwards over barriers such as mountains. There is rain on the windward side and it is much drier on the leeward side where air is descending.

Wind and orogony forces air to move up

• St. Andrews is dry because west is wet due to orogony and the rain is rained out

Question 22

Why must precipitation be balanced by evaporation?

Answer 22

Otherwise all water vapour would be rained out in 10 days

• need evaporation to sustain precipitation

Question 23

Where and why are the largest values of evaporation?

Answer 23

Evaporation is highest at the sub tropics (20 N +S of equator).

• The Gulf Stream has the largest evaporation rates on earth due to the heat from the warm ocean currents and dry winds from the North Atlantic. The contrast of dry air and the warm moist air leads to evaporation.
• The equator is rainy and cloudy and this reduces the energy coming in from the sun which means the equator has less evaporation.
• need sunlight energy for evaporation

Question 24

What physical factors affect evaporation?

Answer 24

For a surface to evaporate 2 things are required:

1) Available water (unlimited for oceans but not for land)
2) Available energy to convert water from a liquid to a gas (latent heat)

Question 25

What factors affect the rate of evaporation?

Answer 25

• Incoming solar energy (controlled by latitude, clouds) and long wave radiation (from clouds, water vapour and C02 in the atmosphere)
• Soil moisture and vegetation
• Wind speed (faster winds = more evaporation)
• How moist the air above the surface is (drier air = more evaporation)
• Surface temperature (all else equal, warmer surface = more evaporation)

Question 26

How do we know that water is transported to land from oceans

Answer 26

There is 85% evaporation over oceans but only 77% precipitation back down.

This implies that the atmosphere moves some of the water over to land. 23% is precipitated over land and 15% is evaporated back up.