energy for a change of state = mass × specific latent heat
energy transferred = power × time
to a real world situation.
When water evaporates from the leaves of trees, is it called evapotranspiration. The latent heat required for this comes from the thermal store of energy in the surroundings. This question compares the cooling power of evapotranspiration from an oak tree to air conditioning.
An oak tree can lose up to 400 kg of water through evaporation from its leaves in one day (24 hours). Assuming that the energy for evaporation comes from the tree’s surroundings, calculate the average cooling power of the tree. The specific latent heat of vaporisation of water is 2700 kJ/kg.
energy for a change of state = mass × specific latent heat
400 kg * 2700 kJ/kg
power = energy transferred / time
time = 24*3600
power = 10 kW
An air conditioning unit has a power of 2.5 kW. How many air conditioning units would be required to provide the same cooling power as the evapotranspiration of one oak tree?
10 kW / 2.5 kW = 4 units
Describe: Trees as Reflectors
Aim: Students should be able to describe the effects of reflection, transmission and absorption of waves at material interfaces.
This activity would work well as a think/ pair/ share activity, and could also be displayed as a poster or other presentation.
The chlorophyll in green leaves of a tree absorbs the red and blue light in the visible Electromagnetic radiation from the Sun. The green light is reflected.
A green umbrella would do the same.
Both cast a shadow on the ground, implying that less visible light reaches the ground.
The ground also reflects some of the sunlight and absorbs the rest.
Absorbed sunlight increases the thermal store of energy.
The greater the thermal energy stored in an object, the more infrared radiation, heat, it emits.
Would you feel cooler walking along a road that is shaded by trees than you would if you carried a green umbrella?
Both the umbrella and the trees will reduce the amount of sunlight reaching you and warming you up.
They will also reduce the amount of sunlight reaching the ground. However, the trees are permanent and will have been shading the ground all day, so the ground in the shadow of the trees will be cooler than the ground in the shadow of a moving umbrella.
The heat, infrared radiation, reaching you from the ground will be lower in the shadow of the trees.
These could include:
evaporative cooling as described above,
the fact that the trees will also be a barrier to convection so heat could be trapped near the ground,
the fact that trees will be a barrier to the wind and so air will be trapped below them. This will also reduce the conduction of heat from the ground,
in the long term, trees remove carbon from the atmosphere and so reduce the greenhouse effect,
trees can also reduce air pollution,
trees can reduce flooding by slowing the flow of water.
Climate is the average weather over a long time period (30 years) for a particular region or place. The climate affects a number of environmental factors within the region including the type and growth of vegetation and wildlife. The climate is determined by large scale factors such as the Earth’s orbit around the Sun, the position of the continents and the composition of the atmosphere. Weather describes the short-term state of our atmosphere. This may include information about the air temperature, precipitation, air pressure and cloud cover. Our local weather changes daily due to the movement of air in our atmosphere.
Experiences and Outcomes:
I can investigate the relationship between climate and weather.
Difference between weather and climate
You will need:
Tally chart for numbers 2-12 Graph paper
In pairs, throw the two dice about 100 times and record the combined score shown each time.
Draw a bar graph of the results.
The results should show a smoothish distribution, with a score of 7 being most frequent. Ask each group to predict what their score will be if they throw the dice one more time – they can’t. However, with one more throw, the mean of all the scores will stay about the same (about 7). In the same way, the weather may be very different from day to day but the climate, the weather we ‘expect’, stays about the same.
One part of the Earth’s surface is always facing the Sun – it varies between the Tropic of Cancer at the June solstice to the Tropic of Capricorn at the December solstice.
This latitude receives the most energy per unit surface area, and is known as the Inter-Tropical Convergence Zone or ITCZ. Here, the air rises, hits the top of the troposphere and spreads out towards the poles. The Coriolis Effect means that this poleward moving air is deflected ever more to the right, becoming westerly (remember we name winds by the direction they are blowing from) and eventually sinking in the sub-tropics and returning towards the ITCZ – this time becoming easterly and giving us the Trade Winds. The whole circulation is known as the Hadley Cell (Figure 1).
FIGURE 1: A CROSS SECTION THROUGH THE ATMOSPHERE, SHOWING AIR RISING AND FORMING DEEP CLOUDS, AT THE INTERTROPICAL CONVERGENCE ZONE AND SPREADING POLEWARDS, AND THE HADLEY, FERREL AND POLAR CELLS.
Similarly, at the poles where the ground surface is coldest, the air sinks, spreads out towards the Tropics, is deflected to the right and eventually rises and completes a circulation – the Polar Cell.
In between, lies the ‘Ferrel cell’, characterised by surface westerlies and rising motion around 60°. This cell is actually the net product of all the mid-latitude weather systems. Figure 2 shows the 3-Dimensional circulation of the atmosphere.
FIGURE 2: IDEALISED REPRESENTATION OF THE GENERAL CIRCULATION OF THE ATMOSPHERE SHOWING THE POSITIONS OF POLAR FRONT; ITCZ (INTER TROPICAL CONVERGENCE ZONE); SUBTROPICAL JETS (STJ) POLAR FRONT JETS (PFJ)
It’s worth noting that, if the Earth wasn’t rotating, we’d have just one ‘thermally direct’ cell with air rising at the ITCZ and sinking at the poles where the ground is coldest.
The polar and sub-tropical High pressure areas are the source regions for air masses.
An air mass is a large body of air with relatively uniform characteristics (temperature and humidity). Air masses are classified according to their source region and track.
There are six air masses (Figure 1) which can affect the weather in the UK – Polar Maritime is the most common, but we can also experience Polar Continental, Tropical Maritime, Tropical Continental, Arctic Maritime and Returning Polar Maritime air.
The source regions tend to be semi-permanent anticyclones (associated with the sinking regions of the global atmospheric circulation) in the sub-tropics and polar regions (‘tropical’ or ‘polar’ air). Air masses acquire their characteristics by contact with the underlying surface in the source region.
The UK sometimes also get Arctic air, which has travelled straight south from the Arctic. Returning polar air is polar air which changed direction over the Atlantic, hitting the UK from the west or even south of west, but still polar in nature.
FIGURE 1: THE 6 AIR MASSES WHICH CAN AFFECT THE WEATHER IN THE UK
Southward moving air is warmed from below as it passes over warmer land and water and becomes more unstable, eventually rising and producing convective cloud – eg puffy cumulus clouds. When you look at these clouds you can sometimes watch the air rising and the cloud bubbling up. In contrast, northward flowing air is cooled from below and becomes more stable.
Air travelling over the sea is moistened and we refer to this as ‘maritime’ air, whereas the moisture in air with a continental track hardly changes and so this is known as ‘continental’ air.
Looking at Figure 1, it’s easy to think that the North East of the UK always experiences Polar Continental air, whilst the South West always experiences Tropical Maritime air etc, but this is not the case. Usually, the whole country experiences the same air mass at the same time. A front is where two air masses meet.
The table below which summarises what is happening to the air from the four major air masses as they approach the UK.
The satellite image in Figure 1, shows Tropical Continental air over much of continental Europe and the UK. Although there is a front coming in from the west, before it arrives much of the UK is cloud free and sunny. However, it’s worth noting those small, puffy blobs of cloud over the centre of Spain and France. In small areas, the sun has warmed the ground enough to make the air there rise and form localised summer thunderstorms.
In Figure 2 you can see a typical satellite image showing Polar Continental air. Air blowing off Scandinavia is initially very cold and dry, giving a clear band of sky in the east North Sea and Baltic. However, as it travels over the water it picks up moisture and eventually cloud forms – over the western North Sea and the first bit of the UK it reaches – the east coast.
Figure 3, shows a very characteristic winter satellite image, as Polar Maritime air dominates UK weather. In the winter, the ocean is warmer than the land as well as being the moisture source – most of the convection (warm air rising) and rainfall occurs there. You can see the small blobs of convective cloud – puffy, cumulus clouds. The first bit of land the air reaches will be the west coast of Ireland, Wales, Scotland and England. As the air rises over the land, it cools further and more cloud, and rain, form.
In Tropical Maritime air (Figure 4), the air is cooling as it travels North, so the cumulus clouds associated with convection don’t form. However, the air is cooling without rising, so cloud can still form – this time in large horizontal sheets of stratus cloud. Again, the water source is the ocean, so the cloud mainly forms there. This cloud won’t produce rainfall as heavy as that associated with polar air, but might give a steady drizzle.
Precipitation has been recorded in the British Isles for over 200 years. Through the dedication and enthusiasm of W R Symons, the mid-19th century saw the formation of the British Rainfall Organisation whose main objective was to establish a countrywide network of rain gauges where daily measurements were made. This network has grown to over 7,000 gauges today, including many that are automatic.
As most people in the British Isles know, precipitation can be extremely variable, both in intensity and duration. The spatial distribution of precipitation during an individual month is very uneven, just as it is on an individual day. Rainfall in Britain is associated with several distinct synoptic situations; all places may get rain from most such types, but some areas get more from some types than others. Therefore, it is not surprising that patterns of temporal variation of rainfall are complex.
Precipitation over the British Isles is the result of one or more, of three basic mechanisms.
1. Cyclonic, or frontal, rain associated with the passage of low-pressure systems. Bands of rain are associated with the passage of warm and cold fronts across the UK. These rain events are caused by the uplift and cooling of moist air parcels.
2. Convectional, with local showers and thunderstorms, caused by the localised thermal heating and overheating of the ground surface. Large towering cumulonimbus clouds may be generated, producing heavy rain.
3. Orographic, or relief rain, with precipitation increasing with altitude over upland areas. The mechanism for relief rain is the uplift and cooling of moist air over upland areas. The normal rate of cooling (environmental lapse rate) is 6.5 Â°C per 1,000 metres. Therefore, near the summit on the windward side of the hill or mountain, the air will have cooled sufficiently for thick cloud, rain and possibly snow to fall.
Air will descend and warm on the leeward side, so there is little or no rain on this leeward side of the hill or mountain. This is called a rain shadow, and sometimes there are warm winds in these sheltered areas as the air, now much drier than during its ascent, descends quickly and warms up. This is called a föhn effect, and the warm winds are called föhn winds.
Rain forms when air cools, the rate of condensation becomes faster than the rate at which water is evaporating and cloud droplets form. If these get big enough, they form as rain – or snow, sleet or hail.
There are various ways in which the air can cool to form rain – three common types which are often talked about are frontal, orographic or relief and convective rain. Frontal Rain This is found where warm air meets cold at the cold and warm fronts in a depression.
Convection is the term given to warm air rising. Convection is normally marked by cumulus clouds which billow upwards as the air rises. The base of such clouds is usually flat, marking the level where temperatures are cold enough for more condensation to be going on than evaporation.
Image copyright RMetS
Extreme convection can be found in thunder clouds – towering cumulonimbus, which can reach all the way up to the top of the troposphere – the lowest 10km or so of the atmosphere in which our weather is found. As air can’t rise into the stratosphere above, the top of the cumulonimbus cloud spreads out, giving it a characteristic ‘anvil’ shaped top. Such convection can occur where the ground has become particularly warm, heating the air above it. It is particularly associated with the Tropics, characteristically giving heavy rain in the late afternoon.
Cumulonimbus clouds can give heavy rain, hail, thunder lightning and sometimes tornadoes.
Cumulonimbus cloud over Kettering, UK, August 2014 Image copyright Sylvia Knight
Orographic or Relief Rain When air is forced to rise over land, particularly higher ground such as hills and mountains, it cools as the pressure falls. Dry air cools at 9.8°C per 1000m it rises. Eventually, the air can cool enough for cloud to form.
Orographic cloud forming upstream of the Matterhorn Image copyright RMetS
The cloud droplets may get big enough to fall as rain on the upstream side of the mountain. If that happens, then, when the air has passed over the top of the mountain and starts to descend, and warm, on the far side, there will be less water to evaporate back into the air. The air will end up drier than it was on the upstream side of the mountain.
This can produce ‘rain shadow’ – an area of land downstream from some mountains (for the prevailing wind direction, in the UK, this would be to the east) where there is noticeably less rainfall. The Gobi desert in Mongolia and China is so dry because it is in the rain shadow of the Himalayas.
Orography can enhance frontal or convective rain; for example, we have explored how polar maritime air, our prevailing air mass, brings convective rain to the Atlantic. As the air reaches the UK and rises over the land, the precipitation is increased.
A precipitation map showing the rainfall ‘climate’ (averaged over 30 years) of the UK. With prevailing westerly winds, there is clearly more rain on the western side of the country, enhanced by the mountains of Wales and Scotland and the English Pennines. As well as being drier on the downwind side of the mountains, it can also be warmer. Remember that water releases heat into the atmosphere as it condenses and takes it up as it evaporates. If there is less water in the air on the downwind side, then there is less to evaporate and not all the heat that was released on the upwind side will be taken up again. This is known as the Föhn Effect.
The onset of a Föhn is generally sudden. For example, the temperature may rise more than 10°C in five minutes and the wind strength increase from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name originated) and in the Rockies (where the name chinook is used). They also occur in the Moray Firth and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.
Where there are steep snow-covered slopes, a Föhn may cause avalanches from the sudden warming and blustery conditions. In Föhn conditions, the relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fires have occurred during Föhn conditions that fire-watching is obligatory when a Föhn is blowing.
El Nino/ La Nina Every few years a very noticeable change comes about in the temperature of the equatorial Pacific Ocean. The eastern side, which is usually the coolest part, warms up considerably, particularly in the ‘tongue’ of cold water in the equatorial east Pacific seen in Figure 1, whilst temperatures in the west decrease a little.
The result of this is that the gradient of temperature from east to west decreases. The atmosphere responds to this change with the heaviest rainfall moving out into the centre of the equatorial Pacific and the eastern side of the ocean also becomes much wetter. This shift has a big impact on land regions bordering the equatorial Pacific. Northeast Australia and Indonesia/Papua new Guinea become much drier whilst coastal parts of Peru and northern Chile, which are usually rather dry, experience much more rain.
The increase in ocean temperatures is known as an ‘El Niño’ event and has been known about for well over 100 years. The name, which means ‘the boy child’ in Spanish, derives from the fact that the warming tends to be strongest around Christmas time and was named by the fishermen of Peru. They noticed that, around Christmas every 3-7 years or so the fish stocks in the equatorial Pacific reduced as the water warmed. The opposite phenomenon often occurs the following year when the eastern equatorial Pacific becomes even colder than normal and the west becomes even warmer. This leads to flooding in Indonesia and eastern Australia and drought conditions in Peru and Northern Chile. This state of the ocean is known as ‘La Niña’, or the ‘girl child’.
Both El Niño and La Niña affect weather patterns far beyond the equatorial Pacific as the whole global pattern of winds and precipitation in the atmosphere adjusts to the changes in the Pacific. The southern USA tends to experience wetter winters during El Niño episodes whereas north-eastern Brazil and south-east Africa become drier than normal. Western Canada, south-east Asia and Japan all tend to be warmer than normal during an El Niño event and in fact the average temperature of the atmosphere averaged around the whole globe tends to be higher than normal in the months during and immediately following an El Niño event, as vast amounts of heat are transferred from the ocean into the atmosphere.