Evaporation
- This is the process whereby liquid water changes into a gas. It requires energy provided by the sun and is aided by the wind. Heat energy from solar radiation is transferred to the surface water, which then encourages the water to change state from a liquid to a gas. Rate of evaporation is affected by various factors:
- Amount and intensity of solar radiation.
- Availability of water.
- Humidity of the air – an increased humidity means that less evaporation will occur.
- Air temperature – warmer air can hold more water vapour than cooler air, so more water is able to evaporate when surrounded by warm air.
- Transpiration links to evaporation. All terrestrial plants lose water via transpiration, the loss of water from vegetation through pores (stomata) on the surfaces.
- Leaves on plants act as interception storage when rain falls, and it may evaporate before it reaches the soil.
- When water evaporates, latent heat is used as energy, meaning that the surroundings are cooled.
Cryospheric Processes
- The cryosphere is all water stored in the Earth’s surface in the form of ice. Cryospheric processes affect the total mass of ice at any scale. The two main processes are accumulation and ablation.
- Accumulation is the build-up of ice mass which generally occurs in winter. Ablation is when ice mass is lost through melting, and usually occurs during summer. Another, more minor process is calving which is when ice masses break away from each other.
- Due to climate change, ablation and calving is becoming more and more common, in contrast the accumulation which is happening less.
- There is a predicted 60m rise in sea level if all the polar ice on Earth was to melt, this is an example of a positive feedback loop.
- Currently, 95% of Earth’s frozen water is in Greenland and Antarctica, however, throughout history, these sorts of things have always varied.
- It is believed that there have been five major glacial periods in Earth’s history, the most recent of them having started around 2.58mya, which is referred to as the Quaternary glaciation.
- It has involved glacial periods, an interruption of the hydrological system, with large volumes of ice meaning the sea level was around 120m lower than what it is now, continental glaciers covering much of Europe, North America and Siberia and interglacial periods of global ablation exceeds accumulation so the hydrological cycle functions properly. This is the stage at which the Quaternary glaciation currently is.
- Mass ablation could have very damaging effects on mankind. There is the obvious risk of a great amount of sea rise, which could lead to floods, but as well as that, in some undeveloped parts of the world, they rely on glacial melt for their water supply, particularly in Asia round the Himalayas.
- In Greenland, some climate models have predicted that in the next 500-1000 years, it will lose half of its current mass. In Bangladesh, land is not very far above sea level, so a rise of just 1.5m could potentially displace 17 million people.
Condensation/Cloud Formation
- Condensation is the process by which water vapour a gas becomes a liquid. Cooler air has a reduced ability to hold water vapour, so if air gets to a certain temperature, it will become saturated by the gas; this temperature is referred to as the dew point.
- Any excess water vapour in the air after it becomes saturated will be condensed so it becomes a liquid again.
- However, for condensation to occur, there must be either condensation nuclei which are tiny particles to condense on, like smoke or dust or a surface cooler than the dew point temperature, such as leaves or windows.
- If the surface is even cooler than the freezing point, the water vapour will sublimate to become hoar frost.
- Condensation can occur when the air volume remains the same, but temperature is reduced to dew point such as when warm moist air passes over a cold surface or during a clear, winter night, heat is radiated to space so the ground cools, which cools the air it is in contact with.
- It can also occur when there is no heat change but the air volume increases. This is called adiabatic cooling. When air rises, it expands due to the reduced pressure of the upper atmosphere:
- Air rises and expands in lower pressures, like having to rise to go over a mountain as it rises, it cools, condenses and forms clouds. This causes relief/orographic rain which is why highlands tend to have wetter conditions with the extent depending on the topography.
- When masses of air at different temperatures/densities meet, the less dense warmer air rises over the denser cooler air.
- Different masses do not mix, they from fronts against each other as the warm air rises, it cools and is also cooled against its front with the cooler air, so condenses and forms clouds, this causes frontal rain.
- Warm surfaces heat cooler air above it, so it expands, lessens in density and rises, as it rises, it eventually cools and condenses, forming clouds.
- This causes convection rain common in tropical areas, also in South East England during warm spells.
- Depressions are weather spells which can come because of activity of and variation between air masses.
- Essentially, a depression is an area of low atmospheric pressure which brings wind, rain and cloud.
- They tend to begin over the Atlantic and move in an eastwards direction towards the UK. The main air masses which travel over the UK are polar maritime, tropical maritime, polar continental mass, tropical continental mass and arctic maritime.
- These air masses are what influences British climate. Polar maritime and tropical maritime affect Britain for the longest period during the year.
- They travel over the Atlantic Ocean, polar from Northern Canada and tropical from more southern areas like the Caribbean, so they can be quite moist.
- The polar maritime is responsible for the cold sectors of depressions, as it begins very cold and dry.
- As it moves over the sea, it warms up so can absorb more moisture through sea evaporation.
- The tropical maritime is much warmer, so can absorb lots more water. It is lighter and warmer, so when it meets the polar maritime, it rises above it, with a front between them.
- As the rising air cools, water condenses along the front, so clouds form, and frontal rain is produced.
- The polar-tropical maritime front brings the changeable weather that we experience in the UK, aka depressions.
- At the centre of the depression/low-pressure system, it brings a short dry spell, but when the heavier cooler air meets the centre, it forces the warm air up quite quickly, which initiates strong winds and colder weather.
- As it quickly rises, it cools and condenses forming large clouds which bring rain and thunderstorms. As the front moves east, it becomes clearer.