This week, Aidan McGivern takes an in-depth look at temperature scales and how they were invented. Bringing you this deep dive is Met Office meteorologist Aidan McGivern.
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You may also enjoy:
– 10 Day trend forecast https://www.youtube.com/playlist?list=PLGVVqeJodR_ZDSHKqsgszMnk9d5IEF5UH
– Podcasts exploring weather and climate https://www.youtube.com/playlist?list=PLGVVqeJodR_brL5mcfsqI4cu42ueHttv0
– Daily weather forecasts https://www.youtube.com/playlist?list=PLGVVqeJodR_Zew9xGAqYVtGjYHau-E2yL
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NewsTranscript
00:00Hello and welcome to the Met Office Deep Dive, our weekly in-depth look at all matters meteorological
00:06exclusive to our YouTube channel.
00:08So if you are not already subscribed to our YouTube channel, please do hit that bell icon
00:14and the subscribe icon so that you never miss one of these when we upload them.
00:19And feel free to send us your comments, we love reading them.
00:23And feel free to share the love as well with many others who like these more in-depth looks
00:29at the weather and climate.
00:31Now often we talk about current weather topics in this Deep Dive, but I've had a look around
00:37and well, the UK weather at the moment is changeable but not particularly exciting.
00:45I'll tell you what is exciting though, temperature scales.
00:48So in this week's Deep Dive, I'm going to be taking an in-depth look at temperature
00:54scales.
00:55So Celsius, Fahrenheit, Kelvin, how many can you name?
01:00There are others and I know what you're thinking, 20 minutes on temperature scales, it's not
01:06long enough.
01:07Well, let's see how we get on.
01:08And before we cover temperature scales actually, I wanted to do a quick history lesson and
01:12look at, well, how we started measuring temperature in the first place because you have to go
01:18all the way back to ancient Greek.
01:21And the first recorded thermometer-like device was from Philo of Byzantium back in ancient
01:28Greece and it was something like this.
01:31This is a schematic of what he used and it basically shows this bulb of air here and
01:40this tube of air going into this pot of water.
01:46And as the sun heated up the air, of course, the Greeks realised that the air expanded
01:52and what they saw was the expanding air exerting pressure and pushing bubbles out.
01:58So they could tell when it was warming up and equally they could tell when it was cooling
02:02down because then the air would contract in this bulb and you'd get lower pressure and
02:09the air, well, being sucked back into the bulb basically and the water as a result rising
02:15up into this tube.
02:17So this was a very early thermometer-like device.
02:20I say thermometer-like device, it wasn't technically a thermometer, it didn't have a metre, it
02:25didn't have a scale.
02:26So there was no temperature scale associated with this so it's more accurately described
02:30as a thermoscope.
02:33Now one person that people often associate with the early use of thermometers, of course,
02:37was Galileo.
02:39But actually he also didn't really use thermometers.
02:44He was mucking about with thermoscopes again, things that looked like this.
02:49This was a Galilean thermoscope and it's a similar thing to the ancient Greeks actually.
02:55What you've got here is this bulb containing air and then this tube with air all the way
03:00down until you get into a vase of water with water extending up into this tube.
03:07And so just like the ancient Greeks, when the air in that bulb heated up it expanded
03:13and the water actually dropped and came out of the tube and filled up the vase.
03:19And so you actually, unlike what you associate with today's thermometers where you see the
03:25mercury or the alcohol or the ethanol rising in the tube, you saw the water level drop
03:32as it got warmer and then rise again as it got cooler.
03:36A couple of issues with these thermoscopes.
03:40As you can see, it's pretty big, not exactly portable.
03:43The reason it's so big is because the air expands only slightly as the air warms up.
03:51And so it needs to be quite large to detect the movement of that air and water and the
03:57changes as the air warms up or cools down.
04:00So it has to be quite big.
04:02Also it's affected by air pressure so it's not particularly reliable.
04:07It would have a different reading when there's high pressure and low pressure as well as
04:11high and low temperatures.
04:14So it's not ideal, really.
04:16Now, moving on to temperature scales.
04:21The first, or one of the first temperature scales in more modern times was this young
04:28chap on a horse.
04:30This was Ferdinando de Medici.
04:34His father died in 1621 and he ended up being the Grand Duke of Tuscany at that young age.
04:41And he had a miserable time.
04:43He suffered from, well, a poor marriage to his first cousin, for starters, economic bleak
04:50times and also the plague back in the 17th century.
04:55So he had a pretty rough time.
04:57And it appears he found solace in making weather instruments, barometers, hygrometers and thermometers,
05:05which they could be called thermometers because he actually added a scale.
05:10And he invented a thermometer that had alcohol and also apparently urine in it, which was
05:17more reliable than air and water.
05:21So it could be a bit smaller, smaller increments.
05:24And also it was properly sealed.
05:26He melted the glass at the bottom of the tube with the alcohol or urine in it.
05:31And as a result, it wasn't affected by changes in air pressure.
05:34So he could properly standardize it and he could put a scale on this device.
05:39So it was technically a thermometer.
05:41It wasn't the first scale ever invented for temperatures.
05:46Back in the ancient Greek period, this man, Claudius Galenus, he created a temperature
05:53scale using various bowls of water.
05:57Now the middle one had 50% boiling water and 50% ice.
06:02And then either side of that, he put four different bowls with different increments
06:09of or different proportions of boiling water and ice.
06:13So you've got basically, I don't know, 25% boiling water in this one, 75% ice and then
06:19100% ice and so on.
06:21So you've got these different increments, which I guess counted as some kind of scale.
06:29But another scale was also devised.
06:34And this one I love.
06:35This one I've only just discovered, Newton's scale.
06:38Isaac Newton, more famous for apples falling on his head, came up with a temperature scale.
06:44And basically, it's quite subjective, admittedly, he put numbers, and I know it's difficult
06:49to read this on the screen, but he put numbers against just different phenomena.
06:54So we start off at the top, zero, the melting point of snow.
07:00And then zero, one, two, that's the temperature of the air in winter.
07:05Two, three, and four is the temperature of the air in spring.
07:09Up to six, you've got the heat of the air in the summer.
07:12Six is the heat at midday about the month of July.
07:15So I don't know whether you're familiar with British summers or not, but it does tend to
07:21vary a lot at midday in July in this country.
07:25Twelve was the greatest heat which the thermometer takes up in contact with the human body.
07:31More on that in a moment, actually.
07:33Fourteen, or is it seventeen, that's my favourite, the heat of a bath which one can endure for
07:39some time when the hand is dipped in and kept still.
07:43Who doesn't love a bath as hot as that?
07:46And then twenty, the heat of a bath in which liquid wax slowly becomes solid and assumes
07:53transparency.
07:54And then you go all the way up and you've got eighteen, eighty-one, melting point of
07:58bismuth, ninety-six, melting point of lead.
08:01So it's quite subjective, some of that, like the temperature of the air in the summer is,
08:07you know, four, five, and six, but there's a theme emerging which is measuring a scale
08:13against the freezing points and the melting points of various things.
08:18And actually someone who continued that theme was a Danish astronomer called Rømer.
08:25And he, as we said, by this stage, this is the seventeenth century or the early part
08:31of the eighteenth century, and there were lots of people inventing thermometers and
08:37there were lots of people putting scales on these thermometers, but they were all different
08:41scales.
08:42And this chap, he was the first to kind of standardise it and come up with a scale that
08:48is, well, the sort of beginnings of what we use in some places around the world today.
08:56His scale took the freezing point or the melting point of a mixture of salt, water, and ice,
09:03so kind of like a briny, slushy mixture, and he set that to zero on his scale.
09:11And then he set, because 60 was a really important number in astronomy, and of course
09:18clocks and so on, so he set the boiling point of water at 60, and then that made seven and
09:25a half the melting point of normal water, non-salty water.
09:29So that was his scale.
09:31He devised it while he was stuck at home with a broken leg, by the way, he had nothing better
09:34to do.
09:35But that scale set the foundations for a slightly more familiar scale, which is the Fahrenheit
09:42scale.
09:43And this is Daniel Gabriel Fahrenheit, crazy life story.
09:47He was born in 1686 in Gdansk, and his parents died when he was 15 from eating poisonous
09:54mushrooms.
09:55He then got sent to Amsterdam to work as a bookkeeper, but he hated it, so he ran away
10:00and became a glassblower instead.
10:04Much more fun, right?
10:06And he spent his time making thermometers and barometers and so on.
10:10Now, he was the first to make a mercury thermometer, which can measure the temperature much more
10:19accurately than the previous alcohol and air and water thermometers and so on.
10:26And he liked Roma's scale, but he didn't like the fractions, so he took 7.5 and changed
10:31it to 8 for the melting point of water.
10:34And then, because his thermometers were more accurate, they could measure in smaller increments,
10:40he decided to multiply the whole thing by 4, so 8 times 4 is 32.
10:45That's where the Fahrenheit scale gets its 32 for the freezing point, melting point of
10:49water from.
10:50And then he added 180 to that to get the boiling point of water, so 212.
10:56And he also decided to set the temperature of the human body to 96.
11:03And he was able to calibrate all his thermometers by sticking them under his wife's armpits
11:08to make sure that they're all set to 96.
11:11So a very important job for Fahrenheit's wife there.
11:14And his scale, obviously, is still used today in some parts of the world, most famously
11:22the US and US territories, also a number of Caribbean countries.
11:28And some countries, the countries here, by the way, in yellow are the countries that
11:32use Fahrenheit.
11:33There's a few spots here that are blue, they're the countries that use Celsius and Fahrenheit.
11:37All the other countries, green, they're the countries that use Celsius.
11:42So still used today, probably not still calibrated under his wife's armpits.
11:47Now, this is Anders Celsius.
11:50So we're getting on to the beginnings of what most of us use today.
11:57Swedish astronomer.
11:58A lot of astronomers actually involved in devising these temperature scales.
12:03He set the boiling point of water to zero on his scale and the freezing point of water
12:12or the melting point of water to 100.
12:16I've not made a mistake there.
12:17You've not heard me incorrectly.
12:19The boiling point of water was zero and the freezing or melting point of water was set
12:26to 100.
12:27The reason he did that was because he didn't want negative numbers during the Swedish winter.
12:33So if you had, say, a very cold morning in Sweden, it would be 110 and not minus 10.
12:42But after he died in 1744, his scale was reversed and it was called the Centigrade scale.
12:50And it was called the Centigrade scale for many, many years, almost 200 years, until
12:54in 1948, it was internationally adopted as a standard for most countries around the world
13:01and renamed to honour Celsius.
13:05So renamed to the Celsius scale and that's what we've got today.
13:08The Celsius scale, zero degrees is the melting point of water, 100 degrees is the boiling
13:13point of water.
13:14The reverse in terms of what he originally came up with.
13:18Here's a scale or a diagram showing the comparison between Fahrenheit on the right, Celsius
13:26in the middle.
13:27You've got the different boiling points and room temperatures and so on, the different
13:32numbers.
13:33So 80 degrees, approximate room temperature there for the Fahrenheit scale, 27 for the
13:36Celsius scale, the boiling point of mercury and so on.
13:40So it's pretty high for the boiling point of mercury.
13:41That's a reason why the thermometers often used mercury, not so much these days because
13:47it is a toxic metal.
13:50But we've also got Kelvin here, of course.
13:54Kelvin is a relatively, when you look back at history, a relatively new scale, but of
13:58course this is the scientific unit, the international scientific unit in terms of measuring temperature
14:08in a scientific way.
14:09So the Kelvin is set to zero degrees for absolute zero.
14:17Basically that's the temperature at which no particles move.
14:23So any temperature above that, as you add more heat, particles move faster and faster
14:29and that's what temperature is.
14:30It's all about the kinetic energy of particles and so on, atoms and so on.
14:35So as the temperature increases, they move faster.
14:38But at absolute zero, they all come to a halt and don't exert any pressure.
14:44That's what absolute zero is.
14:46It doesn't really occur on the earth unless, of course, you create it in a scientific lab.
14:53But it's very cold.
14:54It's minus 273.15 degrees Celsius.
14:58That's what absolute zero is.
15:00Lord Kelvin, who devised the scale, British Lord, he set it so that it was easy to convert
15:07to Celsius.
15:08So each degree of Kelvin goes up by the same degree as Celsius.
15:13All you have to do to convert Celsius to Kelvin is subtract or add 273.15 degrees.
15:24So that's Kelvin and it's useful for scientific measurements because, of course, it starts
15:29at zero, for absolute zero, and then goes up in increments of degrees.
15:36And I like this.
15:37This is good.
15:38I don't know the original source for this, but it shows how these different scales affect
15:46us from zero to 100.
15:47So Fahrenheit is actually a very useful scale for measuring the weather, admittedly, even
15:53though most countries don't use it, because you've got zero, it's really cold.
15:59100, it's really hot.
16:01And the way I've heard some people describe it is that in between, it's kind of like a
16:06percentage of warmth.
16:09So 50, that's 50% hot, 50% cold.
16:12It's about right, whereas 70, you're getting towards more warm than cold.
16:18So it's warming up.
16:1990, it's really quite hot.
16:22Likewise, 10 would be mostly cold.
16:26Celsius, though, is, if Fahrenheit is how humans feel the weather, Celsius is how water
16:32feels the weather.
16:33So zero is fairly cold outside, 100 degrees for humans, dead, but for water it's the boiling
16:40point, of course.
16:41And then for Kelvin, zero and 100, dead, dead, because, of course, both those temperatures
16:46are very, very, very cold.
16:48You're going from minus 273.15 to minus 173.15.
16:55That's how atoms feel the weather.
16:57That's how I've heard it explained, how humans feel the weather, how water feels the weather,
17:01how atoms feel the weather.
17:03So a little scientific joke there.
17:07Moving on to how we actually measure the temperature these days, of course, we keep thermometers
17:13in boxes like this, Stevenson screens, named after Thomas Stevenson, who invented them.
17:21He was, I can't remember if it was the son or the father, one or the other, of Robert
17:25Louis Stevenson, of course, famous author.
17:28And he created these Stevenson screens, which basically shield thermometers from direct
17:34sunlight but allow free-flowing air through.
17:38And thermometers, it's important that we measure the air temperature.
17:41So we're sampling the temperature of the air, but without the thermometer being heated
17:45up by direct sunlight.
17:48Because if they are being heated up by direct sunlight, they're measuring how hot the thermometer
17:53can become when it's sitting in direct sunlight.
17:57It's similar to how we experience temperatures and the weather outside.
18:03It's very unreliable, very subjective when we go outside, because our bodies are being
18:08heated up by direct sunlight if the sun's out.
18:12And our bodies then heat up and give off all this warmth, and we get this layer of warmth
18:17around us.
18:19And we end up feeling a lot hotter than the air temperature would make us feel in the
18:27absence of the sun.
18:28So you know how much cooler it is if you sit in the shade versus the sun.
18:32The actual air temperature doesn't vary that much because the air's flowing through the
18:36shade, it's flowing through the sun.
18:38It's ourselves that are feeling different if we sit in the shade versus sitting in the
18:42sun.
18:43Sitting in the sun, we've got this layer of warmth around us that is making us feel a
18:47lot hotter than the air temperature would manage on its own.
18:51That's why we are not a very good thermometer ourselves.
18:55And of course, you've heard about the feels-like temperature as well.
18:58It's another measure of temperature, or at least a way of expressing how the weather
19:04affects we humans.
19:06And a feels-like temperature takes into account the speed of the wind as well as the temperature.
19:12And it's based on the fact that we're giving off heat, but the loss of heat from our bodies
19:18accelerates when it's windier because the wind then takes that layer of heat away from
19:23our bodies, that skin surrounding us, and we feel colder as a result.
19:28And the feels-like temperature is based on experiments that show the rate of heat loss
19:33for people in a wind tunnel, basically when they are standing still or at least with a
19:41three mile per hour wind, I think, a walking speed kind of wind, and as the temperature
19:46lowers versus the same temperature but with an increasing wind.
19:50So it is based on real experiments that they did on people in wind tunnels based on varying
19:59strengths of wind and also varying temperatures.
20:02We've come up with this scale that we use, that we can use in the winter, it's useful.
20:07Another way of thinking about how the temperature feels on human beings is of course heat stress
20:13and also you might have heard of the wet bulb globe temperature.
20:19In those Stevenson screens, the thing I just showed you, we've got what's known as dry
20:24bulb thermometers.
20:25So they're just thermometers that sample dry air.
20:29But often we also have wet bulb thermometers.
20:32Wet bulb thermometers have a wet cloth on the end of them.
20:35And the wet bulb temperature that those wet bulb thermometers measure is the temperature
20:40of the air after you've evaporated water through it.
20:44And the temperature of the air when you evaporate water through it will often go down, will
20:48normally go down, unless the relative humidity is at 100%.
20:52So on a foggy day or whatever it might be 100%.
20:55But normally the temperature of the air goes down when you evaporate water through it because
21:01the energy needed to evaporate water takes heat from the atmosphere, that's called latent
21:06heat, and reduces the temperature of the air surrounding that thermometer as a result.
21:11So if you've got this wet cloth, it's evaporating water, the air surrounding that wet bulb thermometer
21:16is going to reduce and you get this wet bulb temperature.
21:21And that's useful for determining how much moisture is in the atmosphere because the
21:27more moisture in the atmosphere, the less that thermometer will go down when you evaporate
21:32water through it.
21:33The reason for that is because evaporation will be slower because you've already got
21:39plenty of moisture in the atmosphere.
21:42So we want to know what the humidity is because that's another factor when you go outside,
21:48as well as the wind and the strength of the sun, on how the temperature feels, how the
21:52weather feels outside.
21:54And something that is really important when it's hot, particularly in countries in the
21:59tropics and so on, or for people that do jobs that are very strenuous, for example if you're
22:06in the military or something, then you need to know something called the wet bulb globe
22:11thermometer.
22:13And that's a measure of the humidity, the strength of the solar radiation, and the temperature
22:20outside.
22:21So it takes all those factors and comes up with a wet bulb globe thermometer.
22:25And what you've got on the screen there is a device that's measuring the temperature,
22:29the wet bulb temperature, so the water content of the atmosphere, and the black globe temperature.
22:36That's what this is and that's measuring the strength of the radiation.
22:40So the wet bulb globe temperature is a formula, taking from several readings, temperature,
22:46wet bulb temperature, and the black globe temperature, that take into account the humidity
22:50of the atmosphere and the strength of the solar radiation, so that it's possible to
22:56see how the heat outside is going to affect humans.
22:59But of course, going back to the original point, thermometers in a Stevenson screen
23:03aren't affected by humidity or by solar radiation, because they're not humans, they don't sweat,
23:11and they don't absorb heat from the sun, or they shouldn't do, they should just sample
23:15the air temperatures.
23:17Thermometers are not the best way of measuring temperature, of course.
23:22It varies as well on our own physical activity, what we're wearing, what we're used to.
23:28Is it colder today than it was yesterday?
23:31All of those factors matter when it comes to how it feels outside.
23:35And so the only way to get a consistent and reliable temperature reading is by taking
23:40a temperature reading from a Stevenson screen from a properly calibrated thermometer.
23:46Or another way of doing it is by listening to the chirps of a cricket.
23:53Because this fellow, Amos Emerson Dolbier, American physicist and inventor, he discovered
24:02that chippets, crickets, I beg your pardon, chirp, fast or slow, depending on the air
24:08temperature outside.
24:09And he even came up with a formula, the number of cricket chirps in a minute, minus 40, all
24:15divided by 4, plus 50 is the temperature in Fahrenheit.
24:20So that, supposedly, is a relatively reliable way of determining the temperature outside
24:24if you listen to the crickets, although apparently it's only relevant to a certain species of
24:29cricket in North America, I think the snowy tree cricket, or something like that.
24:33But crickets, if you don't have a thermometer, but if you do have a thermometer that's properly
24:37calibrated and a Stevenson screen, that's a much more reliable source for measuring
24:41the temperature and just guessing based on your own experiences outside.
24:47I hope that's been an interesting tour of temperature and scales and how we measure
24:54it all.
24:55But do let us know if you have any comments or questions or feedback and we'll be back
25:00again with another Deep Dive next Tuesday.
25:03Thank you for watching.
25:04Bye bye.