Unit 3.1 - W&C - Precipitation and Excess Runoff Flashcards
Describe the formation of precipitation
Air uplift leads to cooling due to falling pressure with altitude, the results in the expansion of air. As a result, there are fewer collisions between air molecules. This reduces the heat energy per unit volume and air temperature falls. Cloud formation occurs when temperature is low enough for vapour to condense into water droplets.
What are the three causes of precipitation?
Orographic rainfall - the result of uplift related to relief features.
Convectional rain - this results from intense daytime heating of the land (air parcels adjacent to the ground heat up by conduction; they rise, cool and form cloud).
Frontal rain - this forms when two surface air streams meet (for instance, when polar and tropical air masses meet in the mid-latitudes, the latter will rise up over the former).
What factors can affect the three types of rainfall?
Orographic (relief) rainfall is concentrated in high relief areas and can be a relatively localised phenomenon.
In contrast, convection rainfall is a feature of equatorial climates on a global scale.
Rates of evaporation vary locally in accordance with differences in land use: dark tarmac surfaces warm up quickly, meaning that summer rainfall will evaporate quickly when it collects on the ground.
What are the two main theories for cloud formation?
The Collision Mechanism
The Bergeron-Findeisen Process.
Describe 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 the water droplets. Eventually, the ice crystals become too large to be held aloft; in falling to the ground, they pass through warmer air and melt to produce rain.
Describe the Collision mechanism.
A second theory explains rainfall in the warm tropics. ‘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.
Define the term ‘Runoff’
Runoff is all of the rainwater that contributes to river discharge following a rainfall event. Three drainage basin water flows contribute to runoff: overland flow, throughflow and groundwater flow.
The balance between these three water pathways determines how flashy the river response is; and thus, if there’s any likelihood of flooding.
Give examples of the effects of excess runoff.
Record-breaking rain fell on the UK in early 2020. It was the wettest February on record and formed part of the fifth wettest winter (December-February) to date overall. Increased water cycle flows included far-higher-than-usual levels of precipitation, runoff and river discharge. The sheer volume of water transported by these cyclonic weather systems very likely owes something to long-term carbon cycle and climate changes - because higher rates of evaporation over an unseasonably warm North Atlantic Ocean leads, in turn, to a greater input of water into the atmospheric store.
How does excess runoff effect an area with a monsoon climate.
A monsoon is an especially wet season which is part of a region’s regular climate. In Mangalore in southwest India, more than two-thirds of the year’s takes place in a single three-month period from June to August each year. During the monsoon, rapid water transfers from the atmosphere to the land can take place in just hours or minutes.
Describe the effects of urbanisation on surface runoff.
Urbanisation leads to an increase in the proportion of impermeable ground surfaces in a drainage basin. Surfaces like concrete and tarmac increase overland flow generation and decrease the effectiveness of infiltration, throughflow and soil storage. The more ground that is covered by impermeable hard surfaces such as concrete or paving slabs, the less rainfall will soak into the ground (instead, more will flow over the surface into drains and sewers). In England, Wales and Scotland, around 80% of the forest which once covered much of the land has been removed over many thousands of years. Flood risk in some parts of the UK is undoubtedly higher as a result of these long-term land use changes.
