Rainfall Flashcards
(35 cards)
How is rain formed (in summary)?
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.
What are the processes in rainfall formation?
Evapotranspiration, uplift (orographic, convectional, convergence) and condensation.
How does latent heat energy play a part in rainfall formation?
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.
What is uplift?
Uplift is the rising of air parcels that contain water vapour.
What happens to air parcel as it rises?
It begins to cool adiabatically according to the DALR (dry adiabatic lapse rate). Its relative humidity also increases.
What is adiabatic cooling?
The process of reducing heat through a change in air pressure due to a volume expansion of the rising air parcel.
What is the DALR (dry adiabatic lapse rate)?
The rate at which unsaturated air parcel cools as it rises. 9.8°C per 1000m.
What is the ELR (environmental lapse rate)?
The rate at which surrounding air cools with increasing altitude. 6.5°C per 1000m.
What is relative humidity?
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.
When is the air saturated?
RH=100%.
When is the air unsaturated?
RH<100%.
What is the relationship between relative humidity and temperature?
If the amount of water vapour in a given amount of air is fixed, there is an inverse relationship between relative humidity and temperature.
What is condensation?
Condensation refers to the change in state of gaseous water vapour to liquid water.
When does condensation take place?
Only when the air parcel cools to or below dew point temperature, causing the air parcel to become saturated as humidity becomes 100%.
What is the dew point temperature?
The temperature to which the air parcel must be cooled to before condensation can take place of water vapour into tiny water droplets.
What is condensation nuclei?
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.
What causes convectional uplift?
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.
How long does air parcel uplift continue?
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.
What is the key to the convectional precipitation process?
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.
When will air parcel stop rising?
When its temperature is equal to that of the surrounding air temperature.
What kind of clouds are associated with convectional uplift?
Tall, cumulonimbus clouds that bring localised heavy rainfall.
When does convergence uplift occur?
When air enters a center of low pressure.
How does convergence uplift occur?
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.
What else contributes to convergence uplift?
Convergence of trade winds in the ITCZ also creates large amounts of rainfall in the wet tropics.