The problem with "feels-like" temperatures
Each day we see a figure quoted in just about every weather forecast — today’s temperature. As I’ve mentioned before, it’s one of the first things people look for in weather forecasts. We think we know what temperature is and what is meant by the figure we see. High figures are warm and low figures are cold. This idea is not entirely true. Warm and cold are what we feel, temperature is a number you can measure and even changes by how you measure it. Let me explain.
Fundamentally, temperature is a measure of the kinetic energy of atoms and molecules within a substance i.e. how fast they are moving. In solids this is a vibration which is absorbed by bonds holding the solid together. As energy/temperature increases the bonds loosen and the solid melts but is still held in a specific volume. If the rise continues, then atoms/molecules can escape out of this volume and become independent, i.e. a gas. This true of all substances but the amount of energy required varies widely. For us, a substance is solid if it remains solid at ‘normal’ temperatures. Iron melts at a very high temperature, for us, so we think of it as a solid. Similarly, nitrogen becomes a liquid at a very low temperature and we think of it as a gas.
Temperature is measured in many ways. The method depends upon the range of temperature to be covered, the accuracy required, the time required to produce a reading and several other criteria. In science, the SI unit of temperature is the KELVIN. The Kelvin (K) scale was developed from work done on thermodynamics in the 19th and early 20th centuries. 0K is absolute zero, the temperature when an atom has no kinetic energy. This cannot be achieved in the real world but we have got close. 3.8x10 -11 K is the current record. So why is the Kelvin not used in everyday life? Well, the Kelvin scale has a problem. We do not live at absolute zero…. not even close! At sea level, ice melts at 273.13 K. Water boils at 373.13K. These numbers are large, difficult to remember and not easy to use day-to-day.
Before the rigour of thermodynamics, many temperature scales were developed. They relied on parameters which changed at a, more or less, constant rate in temperature ranges required. These are empirical temperature scales. Expansion of a thin mercury column gives us the typical thermometer. Changes in electrical resistance is another commonly used parameter. The Celsius and centigrade scales are names for the same thing. A centigrade scale is any scale with 100 values of any parameter. Celsius refers only to a temperature scale. Celsius has two set points. These are approximately the melting point of pure ice and the boiling point of pure water — both measured at sea level. We call the ‘colder’ one, 0°C (or 273.13K) and the ‘warmer’ one, 100°C (or 373.13K). A change of 1°C is the equivalent to a change of 1K. We now have a scale, directly related to Kelvin, with values that are easier to handle. These values are useful for specific tasks mainly in medicine, industry and public services.
Nowadays, the Celsius scale is used by the vast majority of countries, with the USA the main exception. American scientists use Kelvin, but the public use the Fahrenheit scale. The Fahrenheit scale pre-dates the Kelvin scale but is not based on thermodynamic principles. It is purely empirical, ice melts at 32°F and water boils at 212°F. It is a 180-degree scale. I do not know why the USA still use Fahrenheit other than they like to be different and do not like all things European. Beware if you go to the US in winter. You hear a forecast of 0° in the media but 0°F has a very different effect on your extremities to 0°C!
In the UK, I have found that the general public’s relationship with the Fahrenheit scale is seasonal. In winter, Celsius ‘sounds’ colder, 0°C = 32°F, 10°F = -12°C, so the media ‘like’ °C. In summer, Fahrenheit ‘sounds’ warmer, 21 °C =70°F, 32°C = 90°F, so the media ‘like’ °F. I always found quoting the ‘wrong’ one to a journalist amusing, they were completely bemused. Muggles eh!
In truth the numbers are of very little use to the general public. What matters to them is how the air feels. A good way of looking at it is to say temperatures are high or low, but air is warm or cool. How the air ‘feels’ does not just depend on temperature — humidity and windspeed both make major contributions. Enter ‘feels-like’ temperatures. These are scales that have been developed for very specific uses. The likely impact of cold on the efficiency of soldiers in the field is one I know. ‘Feels-like’ temperatures are beloved of the media. In this country, they are more often used in winter, but heat indices also exist. They provide nice headlines when the actual temperature is just not cold or warm enough. They confidently say, ‘The temperature will be …something not too bad but it will feel like something a bit more scary’.
There is, however, a problem. I have hung around with a lot of weather nerds in my time and no one has ever gone out and said ‘that feels like 12°C’ or any other number for that matter. Warm and cool are not digital concepts. They are emotional, human concepts. The same temperature can feel very different to different human beings, at different times and under different circumstances. For example, 12°C in January ‘feels’ very different to 12°C in July. In January, a day maximum of 12°C is probably surrounded by days with lower maxima so it feels mild. In July, a day maximum of 12°C is probably surrounded by days with higher maxima so it feels cool. Feelings of warm and cool are more about context than numbers.
I appreciate most of the media would not let that get in the way of a good headline but a ‘feels-like’ number is meaningless. It is like saying, ’This joke is 5 on the funny scale but you will ‘laugh-like’ it is a 10’. I do not anticipate a massive change in weather presentation in the media as a result of this piece, but giving a veneer of scientific authenticity to something that produces little more than exciting headlines, is something that annoys this weather nerd.
About the author
Frank Barrow retired from the Met Office in 2020, after a career of 39 years during which he worked as an observer in the 1980s, a forecaster in the 90s and, since 1996, as a trainer at the Met Office College. He describes the latter role as “fitting him like a glove”, so he stayed... for almost 25 years! During that time, he was involved in the training of the vast majority of current Met Office forecasters.
