Block 4 - Precipitation + Excess Runoff In The Water Cycle Flashcards
What is a thermal?
Rising current of warm air
Outline the process of cloud formation?
- Air uplifts (in one of three ways)
- As air uplifts it expands
- Air expansion decreases air pressure, so air molecules collide less + air temp falls
- Water within the uplifted air condenses around CNNs when the air cools below its dew point or becomes water saturated
- Water droplets grow, as more water vapour condenses onto the cNN or multiple droplets coalesce
What are CCNs?
Cloud Condensation Nuclei
Small particles, e.g. pollutants, around which water vapour condenses
Define condensation
State change from gaseous water vapour to liquid water
What must happen before precipitation occurs?
Clouds must form
What are the three types of air uplift?
- Convectional
- Frontal
- Orographic
What happens in convectional air uplift?
Intense daytime heating of the land causes air adjacent to ground to be heated and rise
What happens in frontal air uplift?
Air masses of different temperatures (and densities) meet and the warmer, less dense air mass rises over the cooler denser air mass
What happens in orographic air uplift?
Air forced to rise over a barrier of higher land (e.g. mountain range)
What is rain shadow?
Dry area where little rain falls
Found after high land
How can humans intervene in the formation of clouds + rain?
Cloud seeding
Define cloud seeding
Aiming to change the type/amount of precipitation that falls in an area by dispersing substances that act as CCNs
Outline the process of cloud seeding
- Planes/rockets release particles that act as CCNs
- This encourages more cloud formation + therefore more rain in this area
Define anti-cloud seeding
Particles released that act as the opposite of CCNs (prevent condensation, cloud formation + rainfall)
Give an example of humans changing the water cycle through cloud seeding
Beijing Olympics (2008)
- Cloud seeding used to ensure good weather over Beijing
- Cloud seeding North + South of city to draw away rain
- Anti-cloud seeding over Beijing to prevent rain
Does precipitation occur from all clouds?
No
What are the two main theories of precipitation formation?
- Bergeron-Findeison Process
- Collision Process
Briefly summarise what the Bergeron-Findeison Process involves
Precipitation formation theory that involves ice crystal growth
Briefly summarise what the Collision Process involves
Precipitation formation theory that involves condensation around ‘super sized’ CCNs that then collide + join together
Outline the Bergeron-Findeison Process
- Clouds (in high alt, below freezing conditions) contain mix of ice crystals + super cooled water droplets
- Rapid flux movement of water droplets -> ice crystals
- Ice crystals grow + are fractured into hexagonal snowflakes by fast air currents
- Water continues to condense around large snowflakes
- Eventually snowflakes become too large + dense so fall under gravity
- As they fall, snowflakes pass through warmer air layers, become rain
Outline the Collision Process
- Large, ‘super sized’ CCNs provide ‘seeds’ around which water condenses into droplets
- Water droplets formed are larger + heavier than normal droplets
- Large droplets fall due to gravity
- As they fall, these droplets overtake + absorb smaller droplets, as they have a larger terminal velocity
- Heavy raindrops fall
Where does the Bergeron-Findeison Process usually occur?
- Cool, high altitude, high latitude areas
- UK in Winter
Where does the Collision Process usually occur?
- Warm, low altitude, low latitude areas
- UK in Summer
What is excess runoff also known as?
Flooding
What are the 4 main physical causes of flooding?
- Prolonged precipitation
- High intensity precipitation
- Snowmelt
- Monsoon rainfall
What is prolonged precipitation and how is it a physical cause of flooding?
Precipitation that falls for a long time period
- Causes saturation-excess overland flow (soil saturated, no more water can infiltrate, causes excess runoff/flooding)
What is high intensity precipitation and how is it a physical cause of flooding?
Precipitation that falls fast + heavy
- Causes infiltration-excess overland flow (intensity of rain so great, not all water can infiltrate, causes excess runoff/flooding)
When do prolonged precipitation + high intensity precipitation usually occur?
During storms
What is snowmelt and how is it a physical cause of flooding?
Accumulations of snow and ice suddenly melt
- Causes saturation-excess overland flow (soil saturated, no more water can infiltrate, excess runoff/flooding)
What is monsoon rainfall and how is it a physical cause of flooding?
Season of high precipitation, driven by a reverse of the prevailing wind direction
- Prolonged precipitation effects (saturation-excess overland flow)
- High intensity precipitation effects (infiltration-excess overland flow)
Where does monsoon rainfall occur?
Southeast Asia
When is the monsoon season?
June -> Sept
What are the 3 human causes of flooding?
- Urbanisation (impermeable surfaces, drain/gutter/sewage networks, bridges)
- River mismanagement
- Deforestation
What is urbanisation - impermeable surfaces - and how does it cause flooding?
Natural, permeable surfaces replaced with impermeable surfaces in urban areas (e.g. tarmac, concrete)
- Reduces infiltration, increasing surface runoff + chance of flooding
What is urbanisation - network of drains, gutters, sewers - and how does it cause flooding?
Increase in drainage density through human networks
- More water reaches river faster (lower lag time)
- River is quickly overwhelmed, increasing surface runoff + chance of flooding
What is urbanisation - bridges - and how does it cause flooding?
River channels are constricted when bridges are built
- River capacity reduced, bursting banks more quickly, increasing chance of flooding
What is river mismanagement and how does it cause flooding?
Rivers managed poorly
- E.g. Channelisation (river channel straightened) speeds up river flow, overwhelming downstream + increasing chance of surface runoff + flooding there
- E.g. Deforestation (trees/plants removed) reduces interception, vegetation store + evapotranspiration, decreasing lag time + increasing surface runoff. Also less protected soil, more into river + this sedimentation reduces river capacity
Do urban or non-urban areas have flashier hydrographs
Urban
What are the three examples of physical causes of excess runoff?
Prolonged, high intensity precipitation (storms): UK 2007
Snowmelt: Kazakhstan 2018
Monsoon rainfall: Pakistan 2010
What are the three examples of human causes of excess runoff?
Urbanisation: China 2020
River mismanagement: Romania 2006
Deforestation: Haiti 2004
