Temperature

Question 1

How does temperature ‘drive the weather machine’?

Answer 1

Temperature and heating differences are important in weather processes. Unequal heating within the atmosphere ‘drives the weather machine’ by setting up pressure gradients that give us our winds and frontal systems. The developments of clouds, showers and thunderstorms are largely governed by the vertical temperature-structure of the atmosphere.

Question 2

What is the formula to convert °F to °C.

Answer 2

T(c) = (T(f) -32) x 5/9

Question 3

Describe absolute zero.

Answer 3

The Kelvin scale has its zero at -273.2 °C. At this temp a body is said to contain no heat energy.

Question 4

Describe how observed temperature is recorded.

Answer 4

A temp sensor is placed in a louvered shelter called a Stevenson screen, set at a height of about 1.5 m above a grassed surface. This shelters it from direct radiation and precipitation but exposed to the wind flow. This is the temp reported in the METARs.

Question 5

Is runway temp likely to be the same as observed?

Answer 5

No.

Question 6

Define solar radiation.

Answer 6

Short wavelength radiation from the sun that warms our earth/atmosphere system. The temp of the surface of the sun is very hot. It averages around 6000 °C, about 1 billionth of the sun’s total output reaches the earth’s atmosphere.

Question 7

Define terrestrial radiation.

Answer 7

Radiation emitted by the earth and its atmosphere to space. As the earth’s average temperature is only 15 °C, it emits radiation in long wavelengths which carry much less energy than short wavelengths do.

Question 8

Describe the balance of solar versus outgoing terrestrial radiation.

Answer 8

Incoming solar radiation must balance outgoing terrestrial radiation for the temperature of the earth’s atmosphere to remain at an average temperature of 15 °C at mean sea level.

Question 9

Describe how solar and terrestrial radiation affects the daily minimum temperature.

Answer 9

The rate at which long-wave terrestrial radiation is emitted from the earth depends on the amount of incoming solar radiation heating it. Incoming short-wave solar radiation is weak when the sun first rises, but the earth has been losing heat to space through terrestrial radiation all night, so that the min temp occurs just after dawn.

Question 10

Describe how solar and terrestrial radiation affects the daily maximum temperature.

Answer 10

Solar radiation increases with the sun’s elevation. As the ground warms, the rate of loss by long-wave radiation also increases. After midday, the short-wave intensity starts to decrease whilst long-wave radiation is still increasing. The two rates become equal about three hours after midday on land, so the is the time of max temp.

Question 11

Describe the effect of latitude on daily air temperature.

Answer 11

Near the equator, the midday rays from the sun strike the earth almost perpendicular to its surface. This results in maximum heating per unit area of the surface and consequently the daily maximum temperature is very warm.
Near the poles, the suns angle, even at the height of summer is very low. Thus, heating per unit area is low resulting in much colder maximum temperatures.

Question 12

Describe the effect of the seasons on daily temperature.

Answer 12

No matter the time of year, near the equator the sun beats relentlessly down during the day. Thus only small temp fluctuations occur here.
At the south pole, the sun is below the horizon for all winter months, so the temp falls rapidly due to constant outgoing terrestrial heat and no incoming solar radiation to offset it. Thus the temp gradient between the equator and south pole is more pronounced during the southern hemisphere winter and spring than autumn and summer.

Question 13

Describe the effect of strong winds on daily temperature.

Answer 13

If it it windy, mixing of the air occurs through a deeper layer, and the gain of heat by day and loss by night is then shared by more molecules in the air through greater depth. Thus diurnal variation of surface temp tends to decrease during windy conditions.

Question 14

Describe the effect of wind direction on daily temperature.

Answer 14

Certain winds will decrease or increase temperatures depending on the origin of the wind e.g. a southerly flow into NZ will be colder than a northernly flow and reduce ambient air temp.

Question 15

Describe the effect of cloud cover on daily temperature.

Answer 15

Cloudiness reduces diurnal fluctuation of temp. During the day, thick clouds reflect much of the incoming solar radiation back into space but at night will absorb outgoing terrestrial radiation and then re-radiate much of this heat back to the earth’s surface. Acts like a blanket.

Question 16

Describe the effect of coastal locations on daily temperature.

Answer 16

Sea surface temps are slow to change over a 24 hr period. Thus, temps at coastal locations will be moderated by the effect of the cooler sea by day and the warmer sea by night, reducing diurnal fluctuations.

Question 17

Describe the effect of inland locations on daily temperature.

Answer 17

Inland, temps can be considerably warmer by day and colder by night as the tempering effect of the sea is non-existent (land is quicker to warm up and cool down).

Question 18

Describe the effect of surface type on daily temperatures.

Answer 18

Different land surfaces may heat up at differing rates - mostly based on colour. Darker surfaces absorb incoming solar radiation much quicker than lighter surfaces.

Question 19

List seven factors which can alter daily temperature fluctuations.

Answer 19

Latitude.
Seasons.
Strong winds.
Wind direction.
Cloud cover.
Coastal/inland locations (proximity to the sea).
Surface type (colour).

Question 20

Recite how heat enters and leaves the earth’s atmosphere.

Answer 20

Heat enters and leaves the earth/atmosphere system via solar and terrestrial radiation respectively.

Question 21

Recite how heat is transferred throughout the atmosphere.

Answer 21

Heat is transferred within the atmosphere by conduction, convection and advection.

Question 22

Describe the process of radiation in relation to heat transfer in the atmosphere.

Answer 22

Short-wave, high energy electromagnetic solar radiation heats the earth/atmosphere, and low energy long-wave radiation is emitted by the earth (terrestrial radiation) which acts to cool the earth/atmosphere system.

Question 23

Describe the process of conduction in relation to heat transfer in the atmosphere.

Answer 23

Conduction is heat transfer by contact or touch. Air is a very poor conductor of heat, so conduction only occurs within a shallow layer of about 10 cm above the earth’s surface (which includes both heating and cooling of the earth).

Question 24

Describe the process of convection in relation to heat transfer in the atmosphere.

Answer 24

Convection is heat transfer by the vertical movement of mass e.g. the cumuliform cloud formation process. This process relies on the atmosphere changing density as the temp changes and is an important method of heat energy transfer in the atmosphere.

Question 25

What significant heat transfer is convection responsible for? (2)

Answer 25

Convection is responsible for the redistribution of heat from the equator to the poles and for transporting most of the heat away from the earth’s surface.
Convection currents also transport large quantities of water vapour aloft which release latent heat into the atmosphere when condensation occurs.

Question 26

Describe the process of advection in relation to heat transfer in the atmosphere.

Answer 26

Advection is horizontal heat transfer by wind e.g. a cold southerly outbreak or a warm humid air-flow from the subtropics.

Question 27

List the four ways in which heat is transferred in the atmosphere.

Answer 27

Radiation.
Convection.
Advection.
Conduction.

Question 28

Define the albedo of a surface.

Answer 28

The albedo of a surface is a measure of the amount of incident radiation that is reflected e.g. a great deal of solar radiation is reflected back by snow (albedo 80%). The albedo of water changes with the angle of the sun (it is low when the elevation is high and increases as the sun is lower).

Question 29

Define specific heat.

Answer 29

The specific heat of a substance is the quantity of heat energy required to raise the temp of unit mass of the substance by 1 °C. The SH of a substance indicates its resistance to temp change.

Question 30

What are the two factors in seasonal variation which alter air temperature?

Answer 30

Tilt of the earth’s axis and its annual orbit around the sun both affect the amount of solar radiation received at the earth’s surface.

Question 31

What are the division of the seasons based on?

Answer 31

They are based on average temp rather than the position of the earth relative to the sun. The warmest temp generally occur approximately one month to six weeks after the date at which the sun reaches its highest elevation and vice versa. This effect is due to the lag of the balance between incoming and outgoing energy.

Question 32

What is the function of RADIOSONDES?

Answer 32

They transmit measurements of temp, humidity and pressure from heights of approximately 100,00 ft back to the ground receiving station. As well as freezing level data, the vertical temp structure gives vital information on cloud base/tops and atmosphere stability.

Question 33

What are RADIOSONDEs?

Answer 33

Hydrogen balloons carrying electronic instruments which are launched every 12 hrs from launching sites at Whenuapai, Paraparaumu and Invercargill.

Question 34

Where are the warmest air temperatures within the earth’s atmosphere?

Answer 34

As the atmosphere is heated from below , the warmest air temps are nearest the earth’s surface.

Question 35

Is the TLR in the troposphere always correct?

Answer 35

No, the TLR at any point may vary considerably e.g. an inversion or an isothermal layer.

Question 36

What is the TLR critical in forecasting?

Answer 36

Thunderstorms, turbulence, showers and the general stability of the atmosphere. It also indicates the height of the freezing level.

Question 37

How is the local TLR generated?

Answer 37

The vertical temp profile gathered by a RADIOSONDE balloon is plotted onto a graph (along with dew point info) called a tephigram. This gives a profile of the local TLR.

Question 38

Define freezing level.

Answer 38

The freezing level is the height at which the air temp equals 0 °C.

Question 39

What is a tephigram?

Answer 39

A graph which depicts temp versus height. Used to generate data on the TLR for a specific location.

Question 40

Define radiation inversion and identify when this occurs.

Answer 40

Radiation conversion occurs overnight in clear sky conditions. The earth loses long-wave radiation to space at night and thus cools down. The air in immediate contact with the cooling ground is also cooled by conduction and low-level mixing. The cooled air may eventually reach 100 to 300 ft, with the coldest air remaining near the surface.

Question 41

What two weather phenomenon can only form in the presence of a radiation inversion.

Answer 41

Radiation fog and mist.

Question 42

What conditions must occur for a turbulence inversion to form?

Answer 42

Turbulence inversion layers are created at the top of the friction layer. They usually start with a standard ELR. Turbulence is induced by a wind of at least 10 kn blowing over surface obstacles (trees, hills and buildings etc). The rougher the surface and stronger the wind, the greater the turbulence. Occur between 1000 and 5000 ft but most common from 2000 to 3000 ft.

Question 43

Describe the changes to the air temperatures within a turbulence inversion.

Answer 43

The tumbling motion in a turbulence inversion means that some air is rising and some falling. The rising air is subject to less pressure and so it expends and cools adiabatically, and the descending air is compressed and warmed adiabatically. A steeper ELR is created (approximately 3 °C per 1000 ft).

Question 44

What clouds may form above a turbulence inversion if there is enough moisture present?

Answer 44

Stratocumulus (tops are capped by the top of the inversion).

Question 45

Describe the process by which a subsidence inversion is formed?

Answer 45

A subsidence inversion is first created in the mid-troposphere. High level air just beneath the tropopause converges and begins to sink. The sinking air is subjected to increasing pressure, is thus compressed and warmed as a result (beginning the formation of a surface anticyclone). As the air continues to descend, the inversion develops and becomes stronger. The descent usually stops between 3000 and 8000 ft where it meets weak convective currents rising off the surface.

Question 46

Describe the features of a subsidence inversion.

Answer 46

Characterised by dry air (marked reduction in humidity and dew point) and the absence of cloud above the inversion.

Question 47

Describe the formation of a frontal inversion.

Answer 47

A frontal inversion occurs at any frontal surface when warm air is forced to rise over the top of a layer of colder air.

Question 48

At what height would an aircraft intercept a frontal inversion?

Answer 48

The height at which the inversion is intercepted depends on your position relative to the surface position of the front. Closer to this position and the inversion will be lower and vice versa.

Question 49

Why is a frontal inversion different to a subsidence, radiation or turbulence inversion?

Answer 49

In other inversions, the atmosphere tends to dry out above the inversion. In a frontal inversion the air temp and dew point temp will remain close together through the inversion as the cloud layer will often be continuous through this area, with heavy rain falling through it.

Question 50

List the four types of inversion.

Answer 50

Subsidence.
Frontal.
Turbulence.
Radiation.

Question 51

List the primary effects inversions have on aviation. (6)

Answer 51

Formation of cloud. 
Visibility.
Turbulence.
Dew point.
Presence of wind shear.
Decreased aircraft performance.

Question 52

How do inversions influence the formation of stratocumulus?

Answer 52

Stratocumulus formed by turbulence is formed if there is sufficient moisture present beneath a turbulence inversion.
Stratocumulus can also form when a developing cumuliform cloud that, on reaching a strong inversion stops growing, or on reaching a weak inversion, punches through it and continues to grow vertically. In either case Sc cloud will spread out horizontally beneath the inversion.

Question 53

How do inversion influence the formation of cloud (other than Sc)?

Answer 53

Very low level cloud sometimes requires the development of a radiation inversion. Such inversions develop off the surface and so the moisture is trapped at very low levels (beneath the inversion).
Layered cloud found at higher levels in the troposphere can also be associated with weak inversions.

Question 54

What effect do inversions have on visibility?

Answer 54

Inversions act like a blanket to trap low-level pollutants, especially around population-dense settlements, industrial areas and near the coast on windy days. Consequently, visibility beneath an inversion can be substantially reduced. It is not uncommon to have 100 km plus above the inversion, but only 10 - 20 km below.

Question 55

What effect do inversions have on turbulence?

Answer 55

Beneath an inversion, light, mechanical turbulence is common. Just below the top of the inversion the turbulence may become more moderate as the fast-moving laminar air above the inversion interacts with the slower moving air beneath, creating wind shear.

Question 56

What effect do inversions have on dew point?

Answer 56

All inversions (other than frontal inversions) tend to prevent or limit the vertical transportation of moisture through the inversion surface. Thus the water vapour content will tend to decrease over time and so will the dew point temperature.

Question 57

What effect do inversions have on wind shear?

Answer 57

Inversions enhance the effect of wind shear by decoupling the faster moving air above the inversion from the slow moving air beneath. When inversions occur close to the ground, the risk posed by this wind shear increases.

Question 58

What effect do inversions have on aircraft performance?

Answer 58

Notwithstanding wind shear and turbulence, most a/c will experience a decrease in performance. This is because the air within the inversion is considerably less dense dense than the air at take-off due to the combined effects of increasing temp and lowering pressure.

Question 59

Why is the temp given in the METARs measured in the shade?

Answer 59

All meteorological temps are measured in the shade to minimise the direct warming of the sensor by solar radiation.

Question 60

Which gases are the primary heat absorbers in the lower atmosphere?

Answer 60

Cardon dioxide, ozone and water vapour.

Question 61

Why does the temperature usually decrease with height in the troposphere?

Answer 61

Since the atmosphere is heated from below, the warmest air temps are nearest to the earth’s surface and there is a decrease of temp with height up to the tropopause. Temp is also a function of pressure and density, both of which decrease as we ascend through the troposphere. The effects of decreasing pressure on temp however, are greater than those caused by decreasing density, therefore as pressure decreases with increasing height, so does temp.