Temperature

Where does the warmth around you come from?

Sun in, heat out

Six real photographs, from the sun to a city at night. Swipe or tap Next.

1. A star at 6000 °C Almost all the Earth's heat comes from the sun, whose surface is about 6000 °C. Hot bodies give out short waves. The sun's radiation is about 46% infra-red, 45% visible light and 9% ultraviolet. Wien's law: the hotter the body, the shorter its waves. Stefan–Boltzmann: energy radiated ∝ T⁴, so very hot bodies radiate intensely.
2. A third bounces straight back Of all the sunlight arriving, about 30% is reflected or scattered back to space, mostly by clouds. That reflecting power is the Earth's albedo. About 19% is absorbed in the air itself (by water vapour, dust, ozone and cloud) and about 51% warms the ground and the sea.
3. Snow throws sunlight away Different surfaces reflect very differently. Fresh snow reflects about 80% of the sunlight falling on it, so snowfields stay cold even in bright sun. On a clear day about 5/6 of the sun's radiation reaches the surface, and the Earth's surface reflects roughly 10% of that.
4. Sand heats fast, the sea barely moves It takes much more heat to warm water than to warm soil or sand. Water's specific heat is 1, ice 0.5 and soil only about 0.2. So deserts like the Thar bake by day and chill by night, while the sea changes by less than 1 °C between day and night. Specific heat: the heat needed to warm 1 unit of mass by 1 °C. Low specific heat = quick to heat, quick to cool.
5. The ground glows invisibly all night The Earth radiates its heat away as long waves, mostly invisible infra-red. At night, with no sunshine coming in, this nocturnal radiation cools the ground steadily. Cloud catches much of that outgoing heat and sends some back down, so cloudy nights stay warmer than clear ones.
6. When is it coldest? The sun has set, the night is clear and calm. The ground keeps losing heat until the incoming sunshine finally balances the outgoing radiation.

Takeaway: Short waves from the sun warm the ground; the ground warms the air from below and sends long waves back to space. Day and night, that balance sets the temperature.

How do °C, °F and K line up?

Three scales

Drag the slider or tap a landmark. All three scales read the same thermometer.

• ISA tropopause The ISA temperature from 11 km to 20 km. It is −69.7 °F and about 217 K.
• Where C meets F The one temperature with the same number on both scales: −40 °C = −40 °F. A favourite exam question.
• Ice melts 0 °C = 32 °F = 273 K. Fahrenheit's fixed points are 32 and 212, Celsius's are 0 and 100.
• ISA sea level The standard atmosphere's sea-level temperature: 59 °F, 288 K.
• 68 °F A worked exam conversion: 68 °F → C = 5/9 × (68 − 32) = 20 °C → K = 20 + 273 = 293 K.
• Water boils At normal pressure: 100 °C = 212 °F = 373 K. Absolute zero, the coldest anything can be, is about −273 °C (0 K).

Memory hook: −40 is −40 on both. Add 273 for Kelvin.

Takeaway: F = 9C/5 + 32 and K = C + 273. The two scales cross at −40. Aviation and science use Celsius.

How cold can anything get?

Absolute zero

Cool a substance and watch its molecules.

1. Heat is molecules moving Heat is a form of energy. In a warm substance the molecules move about fast and at random.
2. Cool it, they slow down Take heat away and the molecules move more slowly and settle into a more orderly pattern.
3. Absolute zero Keep extracting heat and you reach a state where the molecules hardly move at all and no more heat can be taken out. That lowest possible temperature is the same for all substances: absolute zero, 0 K = about −273 °C.
4. Three scales Celsius (used in aviation and science) and Fahrenheit are fixed by melting ice and boiling water: 0 and 100 °C, 32 and 212 °F. Kelvin starts at absolute zero. Ambient temperature: the temperature of the surroundings. Virtual temperature: the temperature dry air would need to have the same density as a moist parcel at the same pressure, so the dry-air equation can be used for moist air. K = C + 273 · F = 9C/5 + 32 · C = 5/9 (F − 32) · −40 °C = −40 °F.

Memory hook: 0 K = −273 °C: nothing colder.

Takeaway: Temperature measures how fast molecules move. Take heat away and they slow down and line up, until at about −273 °C (0 K, absolute zero) there is no heat left to take. It is the same for every substance.

Isn't heat the same as temperature?

Heat or temperature?

Each pair looks alike. The rule underneath tells them apart.

• Heat: The sum total of the kinetic energy of all the molecules. A warm bath holds more heat than a cup of boiling water. Temperature: The average kinetic energy of the molecules. The boiling cup is hotter than the bath. Heat: sum total. Temperature: average. The thermosphere is very hot but has almost no heat, because its molecules are so few.
• Specific heat: Heat to warm 1 unit of mass by 1 °C. Water 1, ice 0.5, soil 0.2. Latent heat: Heat taken in or given out when a substance changes state (ice ↔ water ↔ vapour) with no change in temperature. Specific heat changes the temperature. Latent heat changes the state.
• Solar radiation: Short waves from the very hot sun. Comes in by day only. Terrestrial radiation: Long waves (infra-red) from the much cooler Earth. Goes out day and night; at night it is called nocturnal radiation. Hot sun, short waves. Cool Earth, long waves.
• Ambient temperature: The temperature of the surroundings. Virtual temperature: The temperature dry air would need to have the same pressure and density as a given parcel of moist air. Virtual temperature lets the dry-air equations work for moist air.

Why does it take so long to boil a pan dry?

Specific and latent heat

Heat ice until it turns to steam.

1. Specific heat Specific heat is the heat needed to warm unit mass of a substance by 1 °C. Water's is the highest (1); ice 0.5; soil about 0.2. So land heats up and cools down much faster than water.
2. Melting: heat goes in, temperature does not move While ice melts, the heat goes into breaking it up, not into warming it. That hidden heat is latent heat.
3. Boiling takes far more To turn boiling water into vapour takes more than five times the heat needed to bring ice-cold water to the boil. All the while the temperature stays at 100 °C.
4. ...and it all comes back Latent heat is absorbed going from solid to liquid to gas and released going the other way. When water vapour condenses in a cloud, that heat warms the air: the engine of thunderstorms and cyclones.

Memory hook: Melting and evaporating take heat in; condensing and freezing give it out.

Takeaway: Specific heat: heat to warm 1 g by 1 °C (water 1, ice 0.5, soil 0.2, so land heats and cools fast). Latent heat: heat taken in when ice melts or water evaporates, with no change of temperature, and given back on condensing or freezing.

Why does the sun send short waves and the Earth long ones?

Short and long waves

Tap Next. In the chart, wavelength runs across and energy up; the two curves are not drawn to the same height.

1. Everything radiates Every object gives out energy as electromagnetic waves (radiation): the sun, a hot stove, the ground, even you. The hotter it is, the more it gives out. An object that is a perfect radiator is called a black body.
2. The sun: short waves The sun's surface is about 6000 °C, so its radiation is strongest at short wavelengths, in visible light.
3. What sunlight is made of Visible light (violet to red, VIBGYOR) is 45% of it. Invisible infra-red, beyond the red end, is 46%. Ultraviolet, beyond the violet end, is 9%. All of it counts as short-wave (solar) radiation.
4. The Earth: long waves The Earth, at about 15 °C, gives out much longer waves: invisible infra-red. This is terrestrial radiation. At night, with no sunshine, it is called nocturnal radiation. The warmer the Earth, the stronger it is.
5. Hotter means much stronger The Stefan-Boltzmann law: the total energy given out goes with the fourth power of the absolute temperature (in kelvin). So doubling the temperature gives 2 × 2 × 2 × 2 = 16 times the energy. Hot bodies radiate very strongly.
6. Hotter means shorter waves Wien's law: the wavelength of the strongest radiation gets shorter as the absolute temperature rises (λmax ∝ 1/T). Hot bodies send short waves, cool ones long waves. Planck's law gives the shape of each curve. E ∝ T⁴. λmax ∝ 1/T. Sun: short wave. Earth: long wave.

Memory hook: Hot = short and strong. Cool = long and weak.

Takeaway: Everything gives out radiation. A hotter body gives out far more (energy goes with T⁴: the Stefan-Boltzmann law) and at shorter wavelengths (Wien's law). So the sun sends short waves (46% infra-red, 45% visible, 9% UV), and the Earth sends long, invisible infra-red waves.

Why is midday sun hotter than evening sun?

Insolation

The same four rays of sunlight, two sun heights.

1. Sun overhead When the sun is high, a beam of sunlight lands on a small patch: lots of energy per square metre.
2. Sun low in the sky The same beam arriving at a low angle is spread over a much larger area, so each square metre gets less.
3. So where and when? Insolation is greatest when the sun is overhead and least at sunrise and sunset. Over the year the tropics (between the Tropic of Cancer and the Tropic of Capricorn) get the most, the poles the least.

Takeaway: Insolation is the total solar radiation received over an area. The more slanting the rays, the more ground the same energy is spread over: most when the sun is overhead (and in the tropics), least at sunrise, sunset and the poles.

Where on Earth is the sun overhead today?

Where the sun is overhead

A real NASA picture of the Earth, lit by the sun as it is at noon over India. Drag to turn it.

• 21 June At noon over India in June the sun is overhead near the Tropic of Cancer, which crosses India (Gujarat, Madhya Pradesh, Jharkhand, West Bengal). North India gets the most sunshine of the year.
• March and September At the equinoxes the sun is overhead at the equator, and day and night are equal everywhere.
• 22 December In December the sun is overhead at the Tropic of Capricorn. The northern hemisphere gets slanting rays, short days and its winter.
• Night side Turn the globe: the side away from the sun is dark. With no sunshine there, the ground only loses heat (nocturnal radiation).

Takeaway: The sun is overhead on the equator in March and September, over the Tropic of Cancer (23½° N) in June and over the Tropic of Capricorn in December. Where it is overhead, insolation is greatest.

What happens to 100 units of sunlight?

The heat budget

Follow 100 units of sunshine. The bar at the bottom keeps count. Tap Next.

1. 100 units arrive Picture all the sunlight reaching the top of the atmosphere as 100 units. Counting in units makes the sharing easy to follow. Every unit ends up either bounced back to space or soaked up by the air or the ground.
2. 30 go straight back Air scatters 6 units back to space, clouds reflect 20 and the ground reflects 4. These 30 units warm nothing. This share is the Earth's albedo, its reflecting power. Albedo = reflected radiation ÷ incoming radiation.
3. 19 warm the atmosphere On the way down, water vapour, dust and ozone absorb 16 units, and clouds absorb 3 more. So the air takes in only about a fifth of the sunshine directly. Most of its heat comes later, from the warm ground below.
4. 51 reach and warm the ground The rest, 51 units, is soaked up by the Earth's surface. On a clear day about 5/6 of the sunshine reaches the ground, which reflects about 10% of it. Fresh snow reflects about 80%.
5. In balance The Earth's average temperature hardly changes from year to year. So all the energy it takes in must go back out to space, as long-wave radiation. What comes in equals what goes out: the budget balances. Albedo = reflecting power ≈ 30%. Snow reflects about 80%.

Memory hook: 30 back, 19 in the air, 51 in the ground.

Takeaway: Of every 100 units of sunlight, 30 bounce straight back to space (the albedo): 6 off the air, 20 off clouds and 4 off the ground. 19 are absorbed in the atmosphere (16 by water vapour, dust and ozone, 3 by clouds), and 51 by the surface.

How does heat get from the ground into the air?

How heat moves

Six ways, all at work in the troposphere.

1. Conduction Molecules pass heat on by touch. Only important in the air right against the ground.
2. Convection: free and forced Hot air rises and carries its heat up. More than 70% of the Earth is water, so convection matters everywhere. Free convection is started by strong sunshine heating the ground; forced convection happens when the wind is pushed up over hills.
3. Radiation Every body radiates at its temperature. Radiation needs no medium and does not heat the air it passes through: sunlight heats the ground directly. It is the biggest player in heat transfer.
4. Advection and turbulence Advection moves heat sideways with the wind (a cold wave reaching Delhi on north-westerlies). Turbulence, the irregular eddies of the air, mixes heat around.
5. Latent heat Heat taken in by evaporation (and melting) at the surface is released higher up when the vapour condenses or freezes. In the stratosphere none of these work: there, only ozone absorbing short-wave sunlight heats the air.

Takeaway: Conduction (very near the ground), convection (free, from heating; forced, by hills), radiation, advection (sideways by wind), turbulence, and latent heat. In the stratosphere only the short-wave radiation absorbed by ozone heats the air.

Which way is the heat travelling?

How heat moves

Tap a case, then the way heat moves in it.

• Sunshine warming the ground through empty space
• The lowest few centimetres of air warmed by hot tarmac
• Hot afternoon bubbles rising over Rajasthan
• Air forced up a mountain slope
• A cold wave reaching Delhi on north-westerly winds
• Water evaporating from the Bay of Bengal, then condensing in a cloud
• The Earth cooling on a clear night
• The stratosphere warming up

Takeaway: Radiation needs no medium. Conduction works by contact, so only right at the ground. Convection lifts warm air; advection carries it sideways; latent heat moves with water vapour.

Why is the coldest moment just after sunrise?

A day's temperature

Play the day, then change the surface, the sky and the wind. Watch the range shrink or grow.

• Inland plains (Delhi)
• Land has a low specific heat, so it heats fast by day and cools fast at night. The diurnal range can average up to about 20 °C.
• Thar desert
• Dry sand and clear skies give the biggest range of all: scorching afternoons and chilly nights.
• Open sea
• Water's high specific heat, mixing and evaporation keep the sea's day–night change under 1 °C.
• Coast, sea breeze
• At a coastal station the range depends on the wind direction. A wind off the sea keeps it small; a wind off the land makes it almost as large as inland.
• Clear sky
• Sunshine reaches the ground by day and heat escapes freely at night: the largest range.
• Thin high cloud
• Even a thin sheet of cirrostratus cuts off a fair part of the sunshine.
• Low overcast
• Low cloud blocks the sun by day and sends heat back down by night. The lower the cloud, the less the night cooling: the smallest range.
• Calm
• With no wind the day's heat and the night's chill stay in the lowest air: the biggest range.
• Light wind
• A little mixing spreads the heat through a deeper layer.
• Strong wind
• Strong wind mixes the surface air through the friction layer (up to about 1 km), so the range at the ground is small.

Memory hook: Min ½–1 h after dawn. Max 2–3 h after noon.

Takeaway: The ground turns first and swings widest; the screen air follows later. Calm, clear and dry land gives the largest day–night range; sea, low cloud or strong wind the smallest.

Why does the desert swing 20 °C and the sea less than 1 °C?

What sets the daily range

Four places, one day each.

1. Land Land has a low specific heat and heats only a thin top layer: in clear, calm weather the daily range can average 20 °C.
2. Sea The sea mixes its heat through a deep layer and has a high specific heat: its surface temperature changes less than 1 °C from day to night. At the coast it depends on the wind: off the land the range is almost as large as inland; off the sea it is small, and the sea breeze cools strongly.
3. Wind Calm air gives the largest range. In strong wind the surface air mixes with the air above, spreading the day's heat and the night's cold through the friction layer (about 1 km), so the range is small.
4. Cloud Under low cloud the range at the ground is small: less sunshine gets in by day, less heat escapes by night. The lower the cloud, the more it reduces the night cooling. Least daily range: an overcast day. Most: a dry desert.

Takeaway: Biggest daily range: over land in clear, calm weather. Smallest: over the sea, in strong winds (heat mixed through the friction layer, about 1 km) and under low cloud. At the coast it depends on the wind direction.

Why is a cloudy night warmer than a clear one?

Cloudy nights are warmer

Two nights side by side.

1. A clear night The ground radiates its heat as long waves, and on a clear night most of it escapes to space. The ground and the air on it cool strongly: frost and fog form.
2. A cloudy night Cloud absorbs most of the outgoing radiation, and its underside radiates heat back towards the ground. So the night stays warmer. By day the same cloud cuts out sunshine: a thin sheet of cirrostratus removes a fair part of it; thick low cloud or deep fog can block it almost completely. Clear, calm night: coldest. Low cloud: smallest daily range.

Takeaway: By day cloud reflects sunshine and keeps the ground cooler; by night it absorbs the Earth's outgoing radiation and sends part of it back down. So cloudy nights are warmer, and the lower and thicker the cloud, the stronger the effect.

Where exactly is "surface temperature" measured?

The Stevenson screen

Open the box.

1. The screen A white, louvred wooden box on legs keeps the thermometers in the shade while letting air flow through. Its thermometers sit 1.25 m (4 ft) above the ground.
2. The door The door opens away from the sun, so sunshine never falls on the thermometers when the box is opened.
3. Four thermometers Dry bulb: the air temperature. Wet bulb: its bulb wrapped in muslin kept moist; evaporation cools it, giving the humidity. Maximum: like a doctor's thermometer, it keeps the highest reading. Minimum: filled with alcohol (not mercury). A small dumb-bell index is dragged down by the alcohol's surface when it cools and is left behind when it warms, marking the lowest reading. Minimum thermometer: alcohol. Thermographs and upper-air sondes use bimetallic strips.
4. A continuous record A thermograph draws the temperature on a turning drum, using a bimetallic strip (two metals that bend as they warm). Radiosondes use the same principle aloft.

Takeaway: Surface temperature is the air temperature 1.25 m (4 ft) above the ground in the shade of a white, louvred Stevenson screen whose door faces away from the sun. Inside: dry and wet bulb, maximum and minimum thermometers.

How is the temperature actually measured?

Measuring it

The instruments behind every METAR temperature. Swipe or tap Next.

1. In the shade, 1.25 m up Surface temperature means the air temperature 1.25 m (4 ft) above the ground, in shade, inside a louvred white box called a Stevenson screen. The louvres let air flow through but keep the sun and rain out. Its door opens on the side away from the sun, so sunshine never falls on the thermometers when the observer reads them. Door of the Stevenson screen: opposite to (away from) the sun.
2. Five instruments in one box Dry bulb: the plain air temperature. Wet bulb: its bulb wrapped in moist muslin; evaporation cools it, which gives the humidity. Maximum thermometer: works like a doctor's thermometer. Minimum thermometer: uses alcohol, not mercury, with a small dumbbell-shaped index that the falling liquid drags down. Thermograph: a bimetallic strip drawing a continuous record. Minimum thermometer = alcohol. Thermograph and upper-air sensors = bimetallic strip.

Takeaway: Screen at 1.25 m, in shade, door away from the sun. Dry and wet bulb, maximum (like a doctor's thermometer), minimum (alcohol, with an index) and the thermograph.

Numbers to know

The figures the exam keeps asking for.

Quick test

10 questions to check this chapter. Each answer explains why.

1. A clear, calm winter night in Lucknow. When is the temperature usually lowest?

   • At sunset
   • Around midnight
   • Just after sunrise

   Answer: Just after sunrise. The ground keeps losing heat until the morning sun outweighs it, so the minimum comes shortly after dawn.

2. Why is the daily temperature range small on a cloudy day?

   • Cloud stops the wind
   • Cloud blocks sunshine by day and holds heat in at night
   • Cloud makes the air drier

   Answer: Cloud blocks sunshine by day and holds heat in at night. Cloud cuts the day's heating and sends long-wave heat back down at night, so both extremes are trimmed.

3. Which station would show the biggest gap between the afternoon high and the dawn low in May?

   • Port Blair
   • Jaisalmer
   • Kochi

   Answer: Jaisalmer. Dry desert air and sand heat fast by day and lose heat fast at night; the sea and moist air damp the swing.

4. Surface air temperature is measured

   • on the runway surface
   • at 10 m on the wind mast
   • in a ventilated screen about 1.25-2 m above grass

   Answer: in a ventilated screen about 1.25-2 m above grass. A white louvred screen in the shade gives the true air temperature, not the temperature of sunlit surfaces.

5. −40 °C on a thermometer is the same as

   • −40 °F
   • 233 °F
   • −72 °F

   Answer: −40 °F. The Celsius and Fahrenheit scales cross at −40.

6. How does the Earth's surface mostly send heat back out?

   • By conduction into space
   • As long-wave (infra-red) radiation
   • As short-wave light

   Answer: As long-wave (infra-red) radiation. The cool Earth radiates in the infra-red; the hot sun radiates mostly visible short waves.

7. Heat carried up by rising thermals is moved by

   • conduction
   • convection
   • radiation

   Answer: convection. Convection moves heat with the moving air; conduction only works where things touch.

8. About what share of incoming sunlight is reflected straight back to space (the albedo)?

   • About 70%
   • About 30%
   • About 5%

   Answer: About 30%. Clouds, air and the surface together reflect roughly 30 of every 100 units.

9. Why does land heat up faster than the sea in the afternoon?

   • The sea reflects all sunlight
   • Water mixes and stores heat; land heats only a thin layer
   • Land is closer to the sun

   Answer: Water mixes and stores heat; land heats only a thin layer. Sunlight penetrates water and is spread through a deep, mixing layer with a high specific heat; land heats only its top few centimetres.

10. When water vapour condenses into cloud, it

    • absorbs heat from the air
    • releases latent heat to the air
    • neither takes nor gives heat

    Answer: releases latent heat to the air. The heat used to evaporate the water is given back on condensation: the fuel of thunderstorms and cyclones.