Block 4 - Precipitation + Excess Runoff In The Water Cycle Flashcards
(40 cards)
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
