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Practise Meteorology
Temperature
Meteorology · Chapter 5

Temperature

Measuring temperature, and what temperature is

6 min read
Written from Joshi 3, Instruments and Heat and Temperature Oxford Vol 9, 5, Measurement and Instruments

Temperature is one of the few atmospheric quantities a pilot reads directly, on the outside air temperature gauge, and one of the most worked in the exam. This page fixes what the number means and how it is taken. The rest of the chapter is about why it changes.

Heat is not temperature

Heat is the total kinetic energy of all the molecules in a body. Temperature is their average kinetic energy. A bathtub of water at 60°C holds far more heat than a cup of boiling water, because it has far more molecules, even though the cup is hotter. This is also why the very high temperatures of the thermosphere carry almost no heat, there are too few molecules to add up to much.

Scales

Only Celsius and Kelvin are used in meteorology.

The conversions °C = 5/9 × (°F − 32)  ·  °F = 9/5 × °C + 32  ·  K = °C + 273

The two scales cross at one point: −40°C is exactly −40°F. Ice melts at 0°C, 273 K, 32°F. Water boils at 100°C, 373 K, 212°F.

Specific heat, and why land and sea behave so differently

Specific heat is the heat needed to raise unit mass of a substance by one degree. For a fixed amount of heat, the temperature rise is inversely proportional to specific heat.

The numbers to keep Water has a specific heat of about 1, the highest of the common substances. Ice is about 0.5, dry soil about 0.2. So land heats and cools roughly five times faster than water. Almost every contrast later in this chapter, day against night, summer against winter, land against sea, comes back to this one line.

Instruments

  • On the ground. A mercury thermometer in a Stevenson screen, a louvred white box 4 ft, about 1.25 m, above the ground, so the reading is the free air temperature in shade and not the temperature of the ground itself. The screen door is opened away from the sun. A thermograph using a bi metallic strip gives a continuous trace. A minimum thermometer uses alcohol, not mercury.
  • Upper air. A radiosonde, carried up on a balloon, transmitting temperature, pressure and humidity as it climbs at about 1200 ft per minute.
  • From aircraft. Often the only source over the oceans, but less accurate, because the sensor is affected by compressibility heating and by lag.
Exam wording Surface temperature is the temperature measured in a Stevenson screen at about 4 ft, 1.25 m, above the ground, in shade. The screen exists to protect the thermometer from wind, weather and direct sunshine.

How the troposphere is heated

7 min read
Written from Joshi 3, Insolation, Laws of Radiation and Radiation Budget Oxford Vol 9, 5, Heating of the Troposphere

The sun is the source of the heat, but it does not heat the air on the way in. The troposphere is heated from the surface upward, which is the single fact that explains the lapse rate, the daily temperature cycle and most of the weather in later chapters.

Why the air warms from below

Solar radiation is short wave, and the troposphere is nearly transparent to it, so it passes through almost without heating the air. It heats the surface instead. The heating of the surface by the sun's radiation is called insolation, which is also the name for the amount of solar radiation received per unit area.

Exam wording The surface of the earth is heated by short wave solar radiation. The troposphere is then heated from the surface by conduction, convection and the release of latent heat, together with the long wave radiation the surface gives back.

The four processes

  • Terrestrial radiation. The warmed surface radiates long wave infra red back upward. This is absorbed by the greenhouse gases, mainly water vapour, carbon dioxide and methane, and it is the main reason the air has a lapse rate at all.
  • Conduction. Air in direct contact with the surface is warmed by contact. Air is a poor conductor, so this only matters in the lowest few feet.
  • Convection. Air warmed by conduction becomes less dense and rises as thermals, carrying heat up through the troposphere. With terrestrial radiation, this is one of the two main processes.
  • Condensation. As rising air cools and its water vapour condenses into cloud, the latent heat stored in the vapour is released into the air. Joshi notes that about 77 per cent of the heat leaving the surface travels as latent heat rather than as sensible heat.

The radiation laws, briefly

  • Stefan Boltzmann. Energy radiated by a body is proportional to the fourth power of its absolute temperature, so the sun radiates enormously more per unit area than the earth.
  • Wien. The hotter the body, the shorter the wavelength of its peak radiation. The hot sun radiates short waves, the cool earth radiates long waves.
Numbers worth carrying The solar beam is roughly 46 per cent infra red, 45 per cent visible, 9 per cent ultra violet. Of the solar energy arriving at the top of the atmosphere, about 30 per cent is reflected, the planet's albedo, a little under 20 per cent is absorbed by the atmosphere, and about 51 per cent is absorbed by the surface. A snow surface reflects about 80 per cent.

Lapse rate, inversions and isothermal layers

7 min read
Written from Joshi 3, Effect of Clouds on Surface Temperature Oxford Vol 9, 5, Lapse Rate and Inversions

Because the air is heated from below, temperature normally falls with height. The rate at which it falls, and the places where the pattern breaks, are named terms that come up in almost every later chapter.

Three profiles, three names

NameWhat the temperature does with height
Lapse rateFalls. The ISA rate is 1.98°C, about 2°C, per 1000 ft, or 0.65°C per 100 m
Isothermal layerStays constant with height
InversionRises with height, the reverse of normal

How inversions form

  • Radiation, or nocturnal, inversion. On a clear, calm night the surface radiates its heat away and chills the air touching it, so the coldest air is at the bottom. This is the most common inversion.
  • Turbulence inversion. Mixing in the friction layer can leave an inversion at the top of the mixed layer, a few thousand feet up.
  • Subsidence inversion. Air descending in an anticyclone is compressed and warms, forming a warm layer aloft. This is the one from chapter 4.
  • The tropopause itself often shows a slight temperature rise, and in the stratosphere temperature increases with height.
Interactive Temperature against height, three conditions
Surface layerInversion
Compared with ISAWarmer aloft than the surface
Inversion aloftNo
The dashed line is the ISA lapse of about 2°C per 1000 ft from the surface value. On a clear calm night the lowest air is coldest, a surface inversion. On a sunny afternoon the ground superheats the air just above it, a steep lapse. Subsidence puts a warm lid a few thousand feet up.

What cloud and wind do to temperature

  • Cloud by day reflects solar radiation back to space, so it lowers the maximum temperature.
  • Cloud by night absorbs the earth's long wave radiation and re radiates much of it back down, so it raises the minimum temperature. This is why cloudy nights are warmer than clear ones.
  • Wind mixes surface air with the air above, which lowers the maximum by day and raises the minimum by night.
The one line to keep Cloud and wind both pull the maximum down and the minimum up, so both reduce the diurnal variation. A clear, calm day has the widest temperature swing.

Surface temperature and what controls it

7 min read
Written from Joshi 3, Diurnal Variation of Surface Temperature Oxford Vol 9, 5, Surface Temperature

The screen temperature at any place is set by four things working together: the time of day, the time of year, the latitude, and the nature of the surface underneath.

Diurnal variation

The daily cycle does not track the sun exactly, because the ground keeps gaining heat for a while after noon and keeps losing it for a while after dawn.

Exam wording Maximum temperature occurs about 1500 local time, two to three hours after local noon, when incoming radiation finally falls below outgoing. Minimum temperature occurs about half an hour to an hour after sunrise, when the two balance again. In temperate latitudes the swing is about ±6°C about the mean.
Interactive The daily temperature curve
Maximum near1500 local
Minimum nearsunrise plus 30 min
Diurnal rangeabout 12 °C
Widest swingClear and calm
Set the cloud and the wind. Each one pulls the afternoon peak down and lifts the pre dawn minimum, so the range closes up. Over the sea, where the specific heat of water is high, the same curve would be almost flat, under 1°C.

Latitude and season

  • Latitude. Near the equator the sun is high, so its energy falls on a small area and heating per unit area is greatest. Towards the poles the same beam is spread over a larger area, so heating per unit area is least.
  • Season. The equinoxes fall about 21 March and 21 September, the solstices about 21 June and 21 December. Because land and sea keep heating after midsummer and cooling after midwinter, temperate latitudes are warmest in late July or early August and coldest in late January or early February.

Nature of the surface

  • Sea. High specific heat, slow to heat and cool, so its diurnal variation is less than 1°C. This is why radiation fog never forms over the sea, and why the land and sea temperature difference drives sea breezes.
  • Land. Bare rock, sand, tarmac and concrete heat up far more than woods, grass and wet soil. The air over a concrete runway can be 4°C warmer than the air over adjacent grass. A desert surface gives the largest diurnal range of all.
  • Continents against oceans. In winter the land is colder than the sea, in summer the land is warmer, because the sea changes temperature so slowly.
Where students lose the mark Maximum temperature is not at local noon and minimum is not at midnight. The surface keeps gaining heat until mid afternoon and keeps losing it until just after dawn, so the peak is around 1500 and the trough is around sunrise plus 30 minutes.