I typically write about contemporary weather and climate science from many perspectives. But at times, I just like to use my Forbes outlet to provide basic “weather 101″ on topics that may be of interest to the reader. In this installment, I explore the concept of air temperature. You probably check the air temperature reading routinely to plan your day. What does an air temperature of 70 (or whatever) degrees F actually mean?
This essay was inspired by a discussion that I had with students a few months ago. We were discussing the thermosphere, an upper layer of the Earth’s atmosphere between the mesosphere and the exosphere. In the thermosphere, temperatures can reach up to 4500 degrees F, according to NASA. I explained to the students that it would feel very cold if they stepped out of a spacecraft into the thermosphere without special clothing. They looked stunned. So what gives?
To answer that question, I have to define what temperature is. According to the American Meteorological Society’s Glossary of Meteorology, “In gaseous fluid dynamics, temperature represents molecular kinetic energy….” You might be thinking to yourself, “that sounds nice Dr. Shepherd, but what is “molecular kinetic energy?” Molecular kinetic energy is simply the energy associated with molecules.
The physics of temperature and heat flow is called thermodynamics, and to measure temperature, we must delve into that world. Thermometers measure the change in property of a fluid or object such as electrical resistance, gas pressure in closed volume, or length. To do this, it is important to understand two important principles of physics:
- two things are in thermodynamic equilibrium when they have the same temperature.
- If two objects come in contact with each other, they will eventually reach thermodynamic equilibrium.
According an excellent website at NASA’s Glenn Research Center,
The number assigned to the temperature depends on what we pick for the reference condition….The Celsius scale, designated with a C, uses the freezing point of pure water as the zero point and the boiling point as 100 degrees with a linear scale in between these extremes. The Farenheit scale, designated with an F, is a lot more confusing. It originally used the freezing point of sea water as the zero point and the freezing point of pure water as 30 degrees, which made the temperature of a healthy person equal to 96 degrees. On this scale, the boiling point of pure water was 212 degrees. So he adjusted the scale to make the boiling point of pure water 212 and the freezing point of pure water 32.
Absolute zero represents the minimum molecular kinetic energy possible. This value, according to studies, is -273.16 degrees C. This is the lowest number on the absolute temperature scale called the Kelvin scale. Kelvin temperature is found by adding 273.15 to the Celsius value. A similar absolute scale called the Rankine scale uses the Fahrenheit scale.
I have provided you with enough background to finally answer the question about the thermosphere. In the thermosphere, the average kinetic energy of the molecules is very high, which means temperatures are too. The molecules are zipping around with a great deal of vigor. However, there are relatively few molecules to transfer heat to your body. Ok, what’s the difference between temperature and heat? The website EnergyEducationCa.com defines heat as:
the transfer of thermal energy between molecules within a system and is measured in Joules.[2] Heat measures how energy moves or flows. An object can gain heat or lose heat, but it cannot have heat.
The gain or loss of heat is called the sensible heat. In other words, the heat that causes a temperature change is sensible heat. If the temperature is 7o degrees F today, it is a measure of the molecular kinetic energy of the air. Temperature is a measurable value of a fluid or object. Heat is not.
