Rainfall

Question 1

How is rain formed (in summary)?

Answer 1

Water vapour in the atmosphere rises due to different uplift mechanisms. As it rises, it cools to dew point temperature and condenses into tiny water droplets to form clouds. As the water droplets grow, they would become to heavy to be suspended aloft in the air and hence fall back down to Earth’s surface as rain.

Question 2

What are the processes in rainfall formation?

Answer 2

Evapotranspiration, uplift (orographic, convectional, convergence) and condensation.

Question 3

How does latent heat energy play a part in rainfall formation?

Answer 3

During evaporation, water absorbs latent heat energy as it converts from liquid to gaseous state and this energy gets released during condensation when water vapour condenses into tiny water droplets.

Question 4

What is uplift?

Answer 4

Uplift is the rising of air parcels that contain water vapour.

Question 5

What happens to air parcel as it rises?

Answer 5

It begins to cool adiabatically according to the DALR (dry adiabatic lapse rate). Its relative humidity also increases.

Question 6

What is adiabatic cooling?

Answer 6

The process of reducing heat through a change in air pressure due to a volume expansion of the rising air parcel.

Question 7

What is the DALR (dry adiabatic lapse rate)?

Answer 7

The rate at which unsaturated air parcel cools as it rises. 9.8°C per 1000m.

Question 8

What is the ELR (environmental lapse rate)?

Answer 8

The rate at which surrounding air cools with increasing altitude. 6.5°C per 1000m.

Question 9

What is relative humidity?

Answer 9

Relative humidity is the amount of water vapour in the air at a given temperature expressed as a percentage of the maximum amount of water vapour the air can hold at that temperature.

Question 10

When is the air saturated?

Answer 10

RH=100%.

Question 11

When is the air unsaturated?

Answer 11

RH<100%.

Question 12

What is the relationship between relative humidity and temperature?

Answer 12

If the amount of water vapour in a given amount of air is fixed, there is an inverse relationship between relative humidity and temperature.

Question 13

What is condensation?

Answer 13

Condensation refers to the change in state of gaseous water vapour to liquid water.

Question 14

When does condensation take place?

Answer 14

Only when the air parcel cools to or below dew point temperature, causing the air parcel to become saturated as humidity becomes 100%.

Question 15

What is the dew point temperature?

Answer 15

The temperature to which the air parcel must be cooled to before condensation can take place of water vapour into tiny water droplets.

Question 16

What is condensation nuclei?

Answer 16

Ordinary dust, soot and ash from volcanoes and forest fires and particles from burned fuel. In maritime air parcels (those that are over seas and oceans), a high concentration of sea salts derived from ocean sprays supply the needed nuclei. They are required for condensation.

Question 17

What causes convectional uplift?

Answer 17

Uneven heating of the earth’s surface. When warm ground heats the parcel of air above it, it expands, becomes less dense than the surrounding air and rises.

Question 18

How long does air parcel uplift continue?

Answer 18

As long as the rising air parcel remains warmer than the surrounding air uplift continues because the air parcel is still less dense than the surrounding air and is hence unstable.

Question 19

What is the key to the convectional precipitation process?

Answer 19

The release of latent heat during condensation because it encourages the parcel of air to remain warmer than the surrounding air, driving the parcel even higher. Therefore, the release of heat drives the convective process as it encourages the air parcel to remain unstable.

Question 20

When will air parcel stop rising?

Answer 20

When its temperature is equal to that of the surrounding air temperature.

Question 21

What kind of clouds are associated with convectional uplift?

Answer 21

Tall, cumulonimbus clouds that bring localised heavy rainfall.

Question 22

When does convergence uplift occur?

Answer 22

When air enters a center of low pressure.

Question 23

How does convergence uplift occur?

Answer 23

Under the intense sun, surface heating causes moist tropical air to rise, forming a center of low pressure at the surface. As surrounding air converges into the centre of low-pressure area, it is forced to rise off the surface. As the air rises, it expands, adiabatically cools, and water vapour condenses when the dew point temperature is reached.

Question 24

What else contributes to convergence uplift?

Answer 24

Convergence of trade winds in the ITCZ also creates large amounts of rainfall in the wet tropics.

Question 25

When does orographic uplift occur?

Answer 25

When winds move moist air up and over a mountain barrier.

Question 26

What's the difference between orographic uplift from the two other types of uplift?

Answer 26

In convectional and convergence uplift, air parcel continues to rise until it is the same temperature as the surrounding air. In orographic uplift, moist air continues to rise as long as it is blown upwards against the mountain barrier.

Question 27

How does orographic uplift begin?

Answer 27

Moist air arrives at the coast after being passed over a large ocean surface. As it rises on the windward side of the mountain range, it is adiabatically cooled by expansion as it is forced up the mountainside.

Question 28

How does orographic uplift progress?

Answer 28

When cooling is sufficient and dew point temperature is reached, condensation sets in, and clouds begin to form. Eventually, precipitation begin and continues to fall as the air continues up a slope.

Question 29

How does orographic uplift end?

Answer 29

After passing over the mountain summit, the air begins to descend down the leeward slopes of the range. This descent causes air to compress and warm adiabatically. Since cooling is no longer occurring, cloud droplets evaporate and the air clears rapidly.

Question 30

How is a rainshadow created?

Answer 30

The rainshadow is created over the leeward side of the mountain range which is a belt of dry climate that extends down the leeward slope and beyond. This process causes several of the world's great deserts.

Question 31

How does climate affect rainfall?

Answer 31

Different climates experience different amounts of rainfall. For instance, places experiencing the tropical rainforest climate experiences high total annual precipitation all year round due to high insolation received throughout the year that promotes convectional uplift. The atmospheric instability is further reinforced by convergence of tradewinds.

Question 32

How does migration of the ITCZ affect rainfall?

Answer 32

Seasonal migration of ITCZ has high influence on precipitation patterns especially that of tropical savanna climate. For instance, between two places in the tropics - Kano and Lagos - Lagos is closer to the equator and spends more time under the ITCZ during its migration between seasons, hence experiencing more rainfall than Kano. Lagos experiences rainfall every month whereas Kano only experiences seasonal rainfall from March to October.

Question 33

How does topography affect rainfall?

Answer 33

Places with large mountain ranges experience orographic rainfall. For instance, in July, Bombay and Madras — while part of the same climate — receive about 650mm and 100mm of rainfall respectively. The Western Ghats are the reason for this discrepancy. Bombay is located in the windward side of the Western Ghats and Madras is located at the leeward side where the rainshadow occurs.

Question 34

How does continentality affect rainfall?

Answer 34

Winds and air masses of moderate temperature that originate over oceans move onshore to diminish differences in winter and summer temperatures in coastal areas of continents. Interiors of continents are too distant to experience the moderating effect, and as a result they have greater seasonal differences of temperatures. Continental interior climates also tend to be subhumid to arid, as oceans are primary forms of atmospheric moisture. Maritime locations often have more precipitation because of their proximity to a large moisture source. Brisbane's total annual rainfall is 1168mm (maritime) but Oodnadatta's is 185mm (interior).

Question 35

How does ocean currents affect rainfall?

Answer 35

Warming temperatures transported by warm ocean currents can promote atmospheric instability and the potential for precipitation and storms. Conversely, cold ocean currents would promote atmospheric stability and suppress the formation of clouds, resulting in little to no rain. For instance, Rio de Janerio of 1278mm total annual rainfall is near warm Brazil Current. Walvis Bay has a total annual rainfall of 11mm due to Cold Benguela current. Both along South Atlantic Gyre.