Water Cycle 4: precipitation and runoff Flashcards
(27 cards)
What leads to cooling and condensation because of the fall in pressure with altitude?
Air uplift
Air uplift results in what?
Expansion of the air
Because of the expansion of the air from air uplift, there are fewer what which reduces what?
There are fewer collisions between air molecules: this reduces the amount of heat energy per unit volume and air temperature falls
When does cloud formation occur?
When temperature is low enough for vapour to condense into water droplets
What are the three air lift mechanisms?
Orographic rainfall
Convectional rain
Frontal rain (cyclonic rainfall)
Explain orographic rainfall
When air is forced to rise over a barrier (e.g. a mountain), it cools and condenses, forming rain. The leeward (downwind) slope receives relatively little rain, which is known as the rain shadow effect.
What is orographic rainfall related to?
Related to relief features, augmented by the feeder-seeder mechanism
Explain the feeder-seeder mechanism
Involved water droplets from high altitude ‘seeder’ clouds falling through a lower-level orographic stratus cloud (the ‘feeder cloud’), collecting more cloud water as they do so, which results in heavier rainfall
Explain convectional rain
Common in tropical areas, & in UK during summer
When land becomes hot, air above it expands and rises
As air rises, it cools and condenses, if it continues to rise, rain will fall
Convectional rain results from what?
From intense daytime heating of the land
Explain frontal/cyclonic rainfall
Happens when warm air, which is lighter and less dense, is forced to rise over cold denser air. It cools and condenses as it rises, forming rain
When does frontal rain form?
When two surface air streams meet, e.g. when polar and tropical air masses meet over the North Atlantic ocean
Air uplift and condensation does not always lead to what?
Precipitation
What evidence should you look for to tell which clouds will produce rain or snow?
Thickness of clouds, the thicker the higher chance of precipitation
Looks gray - blocking more sunlight so looks more gray
Temperatures e.g. temps below 4 degrees has a greater chance of snow
Explain the Bergeron-Findeisen process
Clouds at high altitude contain a mixture of water droplets and ice crystals; the ice crystals grow rapidly by attracting vapour from water droplets
Eventually, the ice crystals become too large to be held aloft; falling to the ground, they pass through warmer air and melt to produce rain
What second theory explains rainfall in the warm tropics?
Collision process
What is the process of ice crystal growth that occurs in mixed phase clouds?
Bergeron-Findeisen process
Explain the collision process
Super-sized condensation nuclei, e.g. large sea salt particles, provide ‘seeds’ around which very large water droplets form
The larger ‘super’ droplets fall and collide with smaller droplets, absorbing them
In support of this theory, unexpected downpours and flash flooding in arid areas demonstrate how high numbers of large raindrops can be generated quickly
What is runoff?
All of the rainwater that contributes to river discharge following a rainfall event
What three drainage basin water flows contribute to runoff?
Overland flow
Throughflow (through soil)
Groundwater flow (through rocks)
The balance between the three water pathways determines what?
How flashy the river response is and thus any likelihood of flooding
What does urbanisation do to previously permeable ground?
Renders previously permeable ground surfaces impermeable
Surfaces like concrete and tarmac increase what and decrease what?
Increase overland flow generation and decrease the effectiveness of infiltration, through flow and soil storage
The more ground that is covered by impermeable hard surfaces, the less what?
The less rainfall will soak into the ground and the more will flow over the surface into drains and sewers
