2.1 Atmosphere and weather: diurnal energy budgets

Question 1

Define evaporation

Answer 1

Evaporation is the physical process by which moisture is lost directly into the atmosphere from water surfaces and the soil due to the effects of air movement and sun’s heat. This involves a change of state (liquid to gas). Rates vary depending upon humidity, wind velocity, insolation and nature of ground surface.

Question 2

Define condensation

Answer 2

Condensation is the process by which the state of water vapour (gas) in the atmosphere is changed into a liquid (1), or if temperatures fall below 0 °C, a solid. It results from air being cooled until it is saturated at the dew point. (1)

Question 3

Define sublimation

Answer 3

Sublimation is the change of state from a solid, usually ice or snow, directly into a gas, usually water vapour.

Question 4

Define relative humidity

Answer 4

Relative humidity is the amount (percentage) of water vapour in the air compared to that which can be held at a particular temperature, with 100% relative humidity being saturated.

Question 5

Define absolute humidity

Answer 5

Absolute humidity is the mass of water vapour which is present in the air at any one time.

Question 6

Define humidity

Answer 6

Humidity is the water content of the atmosphere and can either be absolute or relative humidity.

Question 7

Define stability

Answer 7

Stability is when the rising parcel of air is cooling more quickly than the surrounding air (i.e. the parcel of air has a higher lapse rate).

Question 8

Define instability

Answer 8

Instability is when the rising air is cooling more slowly than the surrounding air (i.e. the parcel of air has a slower lapse rate than the surrounding air).

Question 9

Define convection

Answer 9

Convection is when air is warmed during the daytime and rises in pockets as thermals. (1) As air expands, it uses energy and loses heat and the temperature drops. (1) Air is cooled by reduction of pressure with height. (1)

Question 10

Define orographic uplift

Answer 10

Orographic uplift is when warm, moist air is forced to rise as it crosses a mountain barrier (orographic ascent).

Question 11

Define fog

Answer 11

Fog is the suspended water droplets producing Fog of visibility less than 1 km.

Question 12

Define dew

Answer 12

Dew is water in the form of droplets (1), caused by condensation or cooling on exposed surfaces (1).

Question 13

Define latent heat transfer

Answer 13

Latent heat is the amount of heat energy needed (1 mark) to change the substance from say a liquid to a gas (1 mark) or the heat released (1 mark) when a gas condenses into a liquid (1 mark).

Question 14

Define sensible heat transfer

Answer 14

Sensible heat is heat absorbed or given off by a substance, transfer is usually by conduction.

Question 15

Define high pressure

Answer 15

High pressure is an area of descending air (high level convergence) that increases pressure (isobars) at the surface

Question 16

Define low pressure

Answer 16

Low pressure is an area of ascending air (1) (often associated with convectional heating (1)) and hence low pressure (isobars) at the surface.

Question 17

Define terrestrial radiation

Answer 17

Terrestrial radiation is outgoing long wave radiation (1) consequent upon the heating of the earth’s surface by solar radiation. (1)

Question 18

Define incoming solar radiation

Answer 18

The energy emitted from the sun (1 mark) that reaches the earth as short wave radiation (1 mark).

Question 19

Define solar radiation

Answer 19

Solar radiation is incoming short wave radiation from the sun.

Question 20

Define reflected solar radiation

Answer 20

Radiation which is reflected (bounced off) the Earth’s surface (1 mark). Any elaboration i.e. shortwave radiation / albedo etc. (1 mark)

Question 21

Define temperature inversion

Answer 21

Temperature inversion is the state of the atmosphere where temperature increases with altitude rather than decreasing. Opposite state to usual

Question 22

Define albedo

Answer 22

The amount / percentage / fraction of short wave radiation (1) reflected back (1) depending on the surface (1)

Question 23

Define radiation cooling

Answer 23

Loss of heat (1) by thermal radiation (1) results from outgoing radiation being greater than incoming radiation (1)

Question 24

What factors affection incoming solar radiation

Answer 24

1) The solar constant – the energy released by the sun. This does vary in amount as it is linked to the amount of sunspot activity on the Sun.

2) The distance of the Earth from the Sun – our distance is not constant due to orbital rotation; this can cause a 6% variance in the amount of solar energy being received.

3) The altitude of the Sun in the sky – as the Earth is a sphere, the amount of incoming solar radiation being received varies greatly depending on the angle of the Earth’s surface that is falling on. The same amount of solar radiation above 60° north and south of the Equator has to cover twice the land surface compared to the Equator.

4) The length of night and day experienced – as the Earth is tilted at an angle of 23.5° there is a long period of time during the year when areas north of the Artic Circle (66.5° north of the Equator) and south of the Antarctic Circle (66.5° south of the Equator) do not receive any incoming solar radiation; the areas between the two Tropics of Cancer and Capricorn (at 23.5° north and south of the Equator, respectively) receive high amounts all year round.

Question 25

What percentage of incoming solar radiation is: scattered, reflected, and absorbed

Answer 25

Scattered – 5% - by dust and smoke straight back to space

Reflected – 24% - water droplets in clouds and Earth’s surface

Absorbed – 23% - atmospheric gases such as CO2 and water vapour

Question 26

How does reflected solar radiation affect the diurnal energy budget

Answer 26

As the radiation from the Sun passes through the atmosphere, some is absorbed by liquids, gases, and solids. Some is reflected and scattered, especially by the tops of clouds. The amount of energy that is reflected by a surface is determined by the reflectivity of that surface, called the albedo. Albedo is expressed as a percentage. A high albedo means the surface reflects the majority of the radiation that hits it and absorbs the rest. A low albedo means a surface reflects a small amount tof the incoming radiation and absorbs the rest. For instance, fresh snow reflects up to 95% of the incoming radiation. Generally, dark surface have a low albedo and light surfaces have a high albedo.

Thin clouds reflect 30-40%, thicker clouds 50-70%, while towering cumulonimbus clouds can reflect up to 90%.

Question 27

How does energy absorbed into the surface and subsurface affect the diurnal energy budget?

Answer 27

This incoming short wave solar radiation is converted into heat energy when it reaches the surface of the Earth. Incoming solar radiation exceeds outgoing heat energy for many hours after noon and equilibrium is usually reached in mid-afternoon, from 3-5pm.

The amount of energy absorbed by the surface and sub-surface during daylight hours can be affected by a variety of factors, such as the presence of large bodies of water and snow cover. These can have a high albedo and reflect as much as 80-90% of the incoming radiation.

Some of the incoming energy will be transferred from the surface into the sub-surface soil and rocks by conduction. A light-coloured soil or rock, like chalk, is a poor conductor, so heating will mainly be confined to the surface; this explains the high temperatures of 50-60° recorded in hot deserts in daytime. In contrast, a dark volcanic soil or dark rocks like basalt and slate, with low albedos of 5-10%, will absorb heat well.

The moisture content of sub surface soil will also affect its ability to conduct heat. A coarse sandy soil that has large pore spaces will be a poor conductor of heat, so the heat will concentrate on the surface, whereas a soil with a high water content will conduct heat down into the sub-surface and so the soil surface will be cooler.

Wind can remove heat quickly from a land surface, while the amount of cloud cover and the amount of water vapour in the atmosphere will affect the amount of reflection of incoming radiation and therefore the amount of incoming radiation that will reach the surface and sub surface.

Question 28

How does sensible heat transfer affect the diurnal energy budget

Answer 28

During the day, when incoming short-wave solar radiation enters the atmosphere it is being absorbed by the land surface before being re-radiated as long-wave Earth radiation which then heats the air above it. This is an example of a sensible heat transfer. Sensible heat is the energy required to change the temperature of a substance with no phase change. The temperature change can come from the absorption of sunlight by the soil or the air itself. Or it can come from contact with the warmer air caused by release of latent heat (by direct conduction).

Question 29

How does long-wave radiation affect the diurnal energy budget

Answer 29

As the Earth’s surface warms up, it then radiates energy, at a longer wavelength, back to the atmosphere as Earth or terrestrial long-wave radiation. Of this, 94% is absorbed by the greenhouse gases in the atmosphere such as carbon dioxide, water vapour, methane, warming the atmosphere and producing the natural greenhouse effect. The remaining 6% is lost to space.

During the day, the outgoing long-wave radiation transfer is greater than the incoming long-wave transfer, so there is a net loss of energy from the surface.

During a cloudless night, there is a large loss of long-wave radiation from the Earth. There is very little return of long-wave radiation from the atmosphere, due to the lack of clouds. Hence there is a net loss of energy from the surface. In contrast, on a cloudy night the clouds return some long-wave radiation to the surface, hence the overall loss of energy is reduced. Thus in hot desert areas, where there is a lack of cover, the loss of energy at night is maximised. In contrast, in cloudy areas the loss of energy (and change in daytime and night-time temperatures) is less noticeable.

Question 30

How does latent heat transfer - evaporation, dew and absorbed energy returned to Earth - affect the diurnal energy budget.

Answer 30

Latent heat is the energy absorbed by or released form a substance during a phase change from a gas to a liquid or a solid or vice versa, for example, when water changes to water vapour by evaporation. When heat is taken from the atmosphere to help with this process it will result in the atmosphere being cooled.

When this process of evaporation is reversed, for example when water vapour is changed to water by condensation, heat energy is released into the atmosphere which will heat up as a result. The main processes that do this type of transfer is conduction and convection.

During the night, there is no incoming solar radiation, which means that the only source of energy is the radiation that is being held/retained within the atmosphere. The main energy flow is therefore a net loss of heat from the land, which cools the air from the surface upwards.