Taking Temperature

This is a sample lesson page from the course METEO 101 which is part of the online Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: David Babb

 

Prioritize...

This page contains some important concepts about temperature.  Make sure that you understand temperature scales as well as how to read the temperature from a station model.

  Read...

Check out the list of world records for highest and lowest temperatures compiled by the National Climatic Data Center (NCDC). The North American all-time marks for highest and lowest temperatures are, respectively, 134 degrees Fahrenheit in California's Death Valley (see photograph below and behold this stunning panorama of Zabriskie's Point) and minus 81.4 degrees Fahrenheit at the village of Snag (near Beaver Creek) in the Yukon Territory of Canada). If you ever want to keep pace with current global extremes in temperature, check out this website which keeps track of 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. Credit: Zabriskie's Point - Death Valley / April Westervelt / CC BY-NC-SA 2.0

For the record, 134 degrees Fahrenheit and minus 81.4 degrees Fahrenheit convert to 56.7 degrees Celsius and minus 63 degrees Celsius, respectively (read more about the Fahrenheit scale and the Celsius scale). To easily convert from Fahrenheit to Celsius (or vice versa), I recommend the National Weather Service's weather calculator.

Some common markers:

  • 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
     
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 water 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" never capitalized except where any word would be capitalized, such as the beginning of a sentence. 

So what exactly is temperature? To answer this existential question, let's get down to brass tacks. Air molecules are restless little lumps of matter, continually vibrating, wriggling and bumping into their many neighbors (think of a very crowded, jam-packed dance floor). 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. Scientifically, such a lively "jitterbug" performed by air molecules translates into ample kinetic energy, which is the energy of motion. Thus, air temperature is a measure of the average kinetic energy of air molecules (oxygen and nitrogen are the most abundant gases in the atmosphere). Please file this relationship between temperature and kinetic energy away for future reference.

To explore the relationship between absolute temperature (in kelvins) and the mean kinetic energy of air molecules, experiment with the interactive flash animation below. As it turns out, kinetic energy is proportional to the square of the velocity, so the flash animation allows you to determine the mean molecular velocity (in miles an hour) for each absolute temperature (in kelvins) that you enter by turning the dial (click, hold and drag).

An interactive flash animation that allows you to determine the mean molecular velocity (in miles an hour) of air molecules as a function of absolute temperature (in Kelvins). Credit: David Babb

Key Skill...

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 station. Figuring out the temperature from a station model is pretty straightforward.  Check out the interactive station model tool below.  The number located in the upper-left corner of the model is the station temperature expressed in degrees Fahrenheit (or Celsius, depending on the country of origin).  In this case, the station temperature is 72 degrees F.  Feel free to alter the temperature (using the input field on the right) and see how the station model changes.

To see station-model temperature in action, check out the most current surface observations. In Lesson 2, you'll learn to use surface station models to analyze temperatures (check out the most recent temperature analysis).

  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, when the official air temperature is 32 degrees, any precipitation will fall as snow. We'll, 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.).  We'll explore the connection between temperature and the type of precipitation that reaches the ground in Lesson 12, so stay tuned for more pieces to the puzzle.

There are other misconceptions about 32 degrees Fahrenheit (0 degrees Celsius). Indeed, one of my pet peeves is the popular notion 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 you die by turning into an ice cube!  Folks, people don't freeze to death. They are dead long before they are frozen stiff. People die of exposure or hypothermia, and this affliction can occur when air temperatures are in the 40s, even the 50s. Yes, you can die when your core body temperature is far above 32 degrees Fahrenheit.

Many on-air weathercasters like Hale Stone continually refer to 32 degrees Fahrenheit as "freezing." Technically speaking, 32 degrees Fahrenheit (0 degrees Celsius) is the melting point of ice (not the freezing point of water...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%. At this salinity, the freezing point of ocean water is about 28.5 degrees Fahrenheit. So 32 degrees Fahrenheit is not the freezing point of water, no matter what you hear on TV.

Yes, 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 froze and four did not. Lesson learned: Water can exist as a liquid at temperatures below 32 degrees Fahrenheit (0 degrees Celsius). Credit: David Babb