Starter – Quiz

• Write a question on the quiz card & possible answers

• Your question should be something we have done in the last two sessions on water stores and flows

• Indicate which answer is correct by writing the letter in the ‘correct answer’ box (to help me when I read them out!!)

Seasonal and temporal variations in water

stores & flows

• Last week we were looking at inputs, outputs and flows between the stores in the water cycle

• These vary from one place to another i.e. spatially

• The flows also vary temporally e.g. seasonally

Task: complete the gaps in the tables to explain how these stores and output might vary

Learning objective To understand how condensation moves water through the water cycle

Where does condensation occur?

• When we talk about where clouds form are we talking about the whole of the earth’s atmosphere?

• No – just the troposphere

Add notes to the boxes on your condensation sheet as we go

through the slides

• The troposphere is the lowest portion of Earth's atmosphere, and is also where all weather takes place. It contains approximately 75% of the atmosphere's mass and 99% of its water vapour and aerosols.

What happens to the temperature of the air as you travel

up through the troposphere?

• It gets colder

• Approximately 6.5OC per km

• Not always this exact though – it varies depending on factors such as height, season, and type of surface the air is above (i.e. ocean or land)

• This is called the environmental lapse rate

• Diagram of the Environmental lapse Rate (ELR) is the decrease in temperature usually expected with an increase in height through the troposphere

This information is often presented on an ELR Graph

But why does it not get hotter as you move

closer to the sun?

• The troposphere is primarily heated from the bottom because the surface is much better at absorbing a wide range of solar radiation as compared to the air.

• The surface is warmed by the sun and then this energy warms the air above the ground through conduction.

• This warm air rises upwards into the troposphere through convection.

How else does our atmosphere change as you move

up?HINT: Think about what they have to do to

aeroplanes!

• Air pressure refers to the weight of the earth's atmosphere pressing down on everything

• On the earth, the average air pressure at sea level is 1.03 kilograms per square centimetre (kg/cm2) this is commonly measured in bars, in which atmospheric pressure is about 1 bar.

• This means that hundreds of pounds of pressure are pressing on everyone from all sides, at all times.

• Air pressure or "atmospheric pressure" decreases as you move up through the atmosphere.

• On the top of Mt. Everest, the tallest mountain on earth, the air pressure is just about a third of what it is at sea level

• At the cruising altitude of a typical jet liner, pressure is only 1/10th what it is at sea level

• Sometimes a small ‘parcel’ of air is heated at the earth’s surface and is different temperature to the surrounding air

• Because this air is warmer than the surrounding air it will begin to rise. Just like a hot air balloon rises when it is filled with hot air

• As it rises it gets colder

• It also expands as it is under less pressure

What happens to air as it rises?

Why does it cool?

• When air expands, there are less interactions between the molecules and the temperature falls.

• Think of compressed air being released out of aerosol can

• This is called adiabatic expansion

At what rate does this parcel of air cool?

• It depends on how saturated the air is.

• The dry adiabatic lapse rate (DALR): is the rate at which a parcel of dry air (i.e less than 100% humidity so that condensation is not taking place) cools. Cooling caused by adiabatic expansion is approximately 100C/km

• The saturated adiabatic lapse rate (SALR): is the rate at which a saturated parcel of air (i.e. one in which condensation is occurring) cools as it rises through the atmosphere. The rate of cooling is slower, about 70C/km because condensation releases latent heat.

Why does saturated air not cool as quickly?

• Saturated air contains condensed water (i.e. water that has turned from a gas to a liquid).

• When condensation occurs latent heat is released. The exact opposite of evaporation!

• The prevents the air from cooling as rapidly

When do clouds form?• Air that is warmed above that which surrounds it

(the environmental air, which cools with height at the ELR of 6.5°C per 1000m) will rise through our atmosphere.

• It will cool at a rapid rate called the DALR (10°C for every 1000m)

• As it does so it will cool, and this means its relative humidity will rise (because warm air can hold more water vapour than cooled air).

• As it cools and relative humidity rises a critical point is reached where condensation happens & clouds form

• This is called dew point or condensation level• This air will now cool at a slower rate, the Saturated

Adiabatic Lapse rate (SALR), which is varies from 4°C to 9°C for every 1000m ascent.

• This situation is known as Instability.• Once the parcel of air reaches a temperature the

same as that surrounding it, it will stop rising. The air is now stable.

Task: Complete the missing words on your sheet on QUESTION 9

So far we have learnt that clouds form because warm air rises. However there are some occasions when air

rises even though it isn’t warm

Topography or orographic uplift

- The topography - or shape and features of the area - can cause clouds to be formed.

- When air is forced to rise over a barrier of mountains or hills it cools as it rises.

- Layered clouds are often produced this way e.g. stratus clouds and lenticular clouds.

Convergence

Streams of air flowing from different directions are forced to rise where they flow together, or converge. This can cause cumulus cloud and showery conditions – we know this as depressionsor frontal rainfall.

Conditional Instability• If air is forced to rise by convergence or topography

we can get a situation called CONDITIONAL INSTABILITY

• Here, the air rises and cools at the DALR, 10°C for every 1000m

• Since this cools at a more rapid rate than the ELR of 6.5°C, the rising air will get continually colder than that which surrounds but is still FORCED to rise.

• Once dew point and saturation is reached, the rising air will cool at a slower rate, the SALR, and clouds will form.

• If the air continues to rise, it will eventually be warmer than the environmental lapse rate as the SALR is a less rapid rate of cooling than the ELR.

• This situation is called conditional instability because it is conditional upon the air mass being forced to rise from the Earth’s surface

Stability

Air that is cooler than the atmospheric air or isn’t given any reason to move will sink to the Earth’s surface, as it does so it warms and does not form clouds, which leads to atmospheric stability and high pressure.

• Complete the last part of the worksheet

• ELR

• Troposphere

• DALR

• SALR

• Latent heat

• Condensation

• Stable atmosphere

• Instability

• Air pressure

• 6.5oC

• 10oC

• 7oC

• Adiabatic expansion

• Dew point

Mini Plenary

Choose 5 words from this list Listen out for the definition of the following words & cross them off if you have them

Precipitation

• Raindrops begin forming when water vapour condenses on micrometer-sized particles of dust floating in the atmosphere

• Precipitation occurs when particles of liquid water or ice formed within a cloud fall towards the ground under the influence of gravity

• It does not always reach the ground – it can evaporate on the way down especially if the air through which it falls is warm and dry

There are many more than you might think!

Task: Match these types of precipitation to the definitions on your sheet

Hail drizzle snow grains rain

Freezing rain ice prisms snowflakes

Sleet snow pellets ice pellets

Watch this video from the planet estream: Weatherybytes; precipitation

http://estream.ttsonline.net/View.aspx?id=627~3C~Mcv4Sl

Formation of rain• Tiny water droplets that form clouds are

pulled by gravity towards the earth’s surface

• However they fall so slowly that they are kept aloft by the convection currents and turbulence of rising air (terminal velocity of 0.01ms -1)

• Rain droplets need to become large enough and heavy enough that they fall quickly enough to fall out of clouds (terminal velocity of 9ms -1)

• There are two theories about how rain drops become large enough to fall• Collision theory states that some rain drops are larger than others and fall at a

greater speed • Therefore they overtake and collide with the smaller ones and become larger• As a result of this process rain droplets can reach 0.2mmm in diameter in

around 50 minutes• Eventually water droplets become too heavy to remain in suspension and fall

from the base of the cloud• This explains precipitation from clouds below the freezing level & is therefore

most relevant in the tropics

• In middle and high latitudes, precipitation often falls from clouds above the freezing level

• Clouds contain a mixture of tiny ice particles and super-cooled water droplets that remain liquid

• Water vapour is evaporated from water droplets and deposited in adjacent ice particles

• Ice particles grow at the expense of water droplets until they reach a critical mass and fall from the cloud

• As the descend and warm they turn to rain• If sub-zero conditions continue to ground level they may fall as snow or

sleet

Task: Complete the activities on the precipitation worksheet

• There are three processes that cause uplift and cooling of air , that will lead to the formation of precipitation:

• Convection cooling

• Frontal cooling

• Orographic cooling

• Cooling that occurs when a parcel of air is heated from below, through convectionbecomes warmer and less dense than its surroundings, and rises

• It produces cumulus clouds, and leads to showers of rain, often high intensity, and possibly with thunder and lightning

• Frontal systems (depressions/low pressure systems) are boundaries between two different densities of air, with different levels of humidity

• They are common in the mid-latitude regions such as the UK

• Cold polar air, meets warm tropical and causes uplift

• Cumulus clouds are produced at the cold front as uplift is strong

• At the warm front , a range of different clouds are produced due to weaker convectional uplift, and rain is lighter

• As air masses approach the UK mountains physically obstruct air movement, forcing it to rise

• This results in cloud formation, as air becomes unstable, that was stable at ground level i.e. air that was the same temperature as surrounding air and wouldn’t have risen

Read pages 219-22 David Waugh Integrated Approach (fourth edition)

Explain the three types of uplift:

- Convectional

- Orographic

- Frontal

World precipitation distribution

a) Describe the distribution of world precipitation shown on the map b) Explain the pattern using p223 D. Waughc) Describe & explain the relationship between rainfall totals and reliability