Taking Temperature

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


This page contains some important concepts about temperature. Make sure that you can discuss different temperature scales and identify / interpret the temperature on a station model.


While you probably think of temperature as "how hot or cold something is," that's a pretty ambiguous definition (since "hot" and "cold" are somewhat subjective). More precisely, temperature is a measure of energy. You see, air molecules are restless little lumps of matter, continually vibrating, wriggling and bumping into their many neighbors. As air temperature increases, the molecular dance becomes increasingly frenetic. At a temperature of 72 degrees Fahrenheit, the average speed of air molecules is about 1,000 miles an hour, which translates into ample kinetic energy (energy of motion). Thus, air temperature is a measure of the average kinetic energy of air molecules (air consists mostly of nitrogen and oxygen molecules).

In the United States, we typically express temperature using the Fahrenheit temperature scale, but most countries in the world use the Celsius temperature scale (undoubtedly, you've heard temperature expressed in "degrees Fahrenheit" or "degrees Celsius" before). Undoubtedly, you'll encounter instances when you need to convert between the two scales. In those circumstances, the National Weather Service temperature conversion calculator is great!

To give you some weather context, the North American all-time marks for highest and lowest temperatures are, respectively, 134 degrees Fahrenheit (56.7 degrees Celsius) in California's Death Valley (see the photograph below), and -81.4 degrees Fahrenheit (-63 degrees Celsius) at the village of Snag (near Beaver Creek) in the Yukon Territory of Canada. If you're interested in current global temperature extremes, this website summarizes the extremes from all the hourly weather observations around the world.

Zabriskie's Point, Death Valley, California.
The stark but beautiful landscape of Death Valley, California, from Zabriskie's Point.

You may also be familiar with some other common temperature markers:

  • 100 degrees Celsius (212 degrees Fahrenheit) is the boiling point of water
  • 37 degrees Celsius (98.6 degrees Fahrenheit) corresponds to normal body temperature
  • 22.2 degrees Celsius (72 degrees Fahrenheit) represents the "ideal" room temperature
  • 0 degrees Celsius (32 degrees Fahrenheit) is the melting point of ice

Note that I referred to 0 degrees Celsius (32 degrees Fahrenheit) as the melting point of ice, and not the freezing point of water. That phrasing was chosen deliberately! Indeed, ice melts at 0 degrees Celsius, but not all water freezes at 32 degrees Fahrenheit! For more details, check out the Explore Further section below, but this fact has some important consequences for how precipitation forms in clouds that we'll get into later in the course. So, when you hear that 0 degrees Celsius or 32 degrees Fahrenheit is the freezing point of water, keep in mind that it's not usually true.

By the way, there are other temperature scales besides Celsius and Fahrenheit. For example, there's the Kelvin scale (sometimes called the absolute temperature scale). Please note that the number of kelvins = the number of degrees Celsius + 273.15. So, the melting point of ice is 273.15 kelvins and the boiling point of water, at standard pressure, is 373.15 kelvins (100 degrees Celsius or 212 degrees Fahrenheit). For the record, it's bad form to say "degrees kelvin." Indeed, the proper way to express the units of absolute temperature is simply "kelvins." Also note that the word "kelvins" is never capitalized except where any word would be capitalized, such as at the beginning of a sentence. The Kelvin scale is used commonly in the physical sciences, and in fact it's the most direct way to describe the relationship between the average speed of air molecules and their temperature (higher temperatures = faster average molecule speeds).

Now that you know what temperature is, the next step is to be able to identify and interpret temperatures from a station model, which is covered in the Key Skill section below.

Key Skill...

See caption.
A sample of a station model with temperature (52 degrees Fahrenheit) annotated.
Credit: David Babb

In this lesson, you will be learning not only about some of the basic observed atmospheric variables, but you will learn how these variables are represented on a station model. As mentioned previously, station models are a graphical way of displaying the different types of data collected at each observing site. Figuring out the temperature from a station model is pretty straightforward. As shown in the sample station model on the right, the number located in the upper-left corner of the model is the station temperature expressed in degrees Fahrenheit (degrees Fahrenheit is the standard used on surface station models in the United States, but many other countries use degrees Celsius). In this case, the station temperature is 52 degrees Fahrenheit.

I also strongly recommend practicing with the interactive station model tool below (which we'll be coming back to throughout the lesson). You can alter the temperature (using the input field on the right) to see how the station model changes. The default setting is 72 degrees Fahrenheit, but if you change that number, you will see the number located in the upper-left corner of the station model change accordingly. Don't worry about the other numbers and symbols on the station model quite yet. We'll be covering those throughout the remainder of the lesson.

Finally, check out the most current surface observations, and pick out three or four station models. You should be able to identify and interpret the temperature at each.

Explore Further...

In my opinion, the temperature that frequently causes the most confusion is 32 degrees Fahrenheit (0 degrees Celsius). For example, many people automatically assume that the air temperature has to be 32 degrees Fahrenheit or lower for precipitation to fall as snow. But, I've seen it snow at 44 degrees Fahrenheit in early spring! On the flip side, I've seen it rain when the air temperature was 11 degrees Fahrenheit in winter. Granted, the rain froze after it hit the ground, trees, and power lines, etc. (photo credit: Steve Seman). We'll explore these mysteries regarding precipitation later in the course, so stay tuned!

Precipitation type isn't the only misconception surrounding 32 degrees Fahrenheit (0 degrees Celsius). Another is the idea that people "freeze to death." The dangerous implication of this myth is that you can't die unless the temperature is below 32 degrees Fahrenheit and that you die by turning into an ice cube! But, people don't freeze to death. People die of exposure or hypothermia, and this affliction can occur when air temperatures are in the 40s or even the 50s, and death occurs when your core body temperature is far above 32 degrees Fahrenheit.

Finally, you may read articles or hear weather broadcasters refer to 32 degrees Fahrenheit as "freezing." Technically speaking, only pure water freezes at 32 degrees Fahrenheit. As it turns out, most ordinary water is "filthy" (dissolved impurities) and freezes at temperatures lower than 32 degrees Fahrenheit! For example, the average concentration of salt in seawater is about 3.5 percent. At this salinity, the freezing point of ocean water is about 28.5 degrees Fahrenheit. So, it's accurate to say that 32 degrees Fahrenheit is the melting point of ice, but it's not really the freezing point of water in most practical situations.

As a consequence, water can exist as a liquid at temperatures well below 32 degrees Fahrenheit. Check out the pair of photographs (below) documenting a home experiment. I placed water drops onto the bottom of an empty tin can and then shoved the can in a freezer for several minutes (the photograph on the left is the "before" picture and the photograph on the right is the "after" picture). Please note that some drops froze while others did not. I'll explain this discrepancy in a later lesson, but I just wanted you to see with your own eyes that water and ice can simultaneously exist at (and below) 32 degrees Fahrenheit.

Experiment with water drops placed on a tin can set in a freezer.
I carefully placed nine drops of water on a can (left) and put the experiment in my kitchen freezer. After several minutes, five drops had frozen and four had not. Lesson learned: Water can exist as a liquid at temperatures below 32 degrees Fahrenheit (0 degrees Celsius).
Credit: David Babb