Sometimes it is hot outside. Sometimes it is cold outside and sometimes it is ‘just right’. It isn’t hard to tell if it is cold or hot when you walk out the door, but if we want to know what the air temperature is, we have to use a thermometer.

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### What is a thermometer?

A thermometer is a glass tube filled with alcohol or mercury that is sealed on both ends. When the air around it gets hotter, the liquid in the tube starts moving upward. When it’s cold, it falls down.

The tube is usually attached to a flat piece of metal or wood with marks and numbers on it to measure how hot or cold it is—inside or outside.

### More than just hot or cold

If you look at a thermometer you are going to see numbers that start at the bottom and go all the way to the top of the thermometer. The smallest number is always at the bottom of the thermometer. But the lowest number on the thermometer isn’t zero. A thermometer has negative numbers (numbers lower than zero), too.

Most thermometers also have two different sets of numbers:

- One set that has a C by it.
- One set that has an F by it.

The C stands for Celsius, which is a method for measuring the temperature developed in the mid-1700s by Anders Celsius. The system is based on a 0 to 100. This means that if you are measuring the temperature in Celsius, water freezes at 0 and boils at 100 degrees.

The F stands for Fahrenheit. Daniel Fahrenheit developed this system for measuring the temperature in the early 1700s. Fahrenheit also invented the thermometer using mercury or alcohol. The Fahrenheit system says that water freezes at 32 degrees and boils at 212 degrees.

By looking at the thermometer below, you can see the difference in the two methods used to measure temperature.

### How to convert Celsius and Fahrenheit

You might think can change Fahrenheit readings into Celsius readings by subtracting 32 from the Fahrenheit temperature since there is a 32-degree difference between the two when it comes to freezing water. But think about it…is there a 32-degree difference between the two when it comes to the temperature for boiling water? No, there is a difference of 112 degrees. So what’s going on?

A Celsius unit of measure is larger than a Fahrenheit unit of measure. This means it takes more Fahrenheit units to change the temperature than it does Celsius units.

This means you have to have a formula (scientific recipe) for changing Celsius to Fahrenheit or Fahrenheit to Celsius.

### Here’s how you do it:

**FAHRENHEIT TO CELSIUS:** Subtract 32 from the Fahrenheit temperature, multiply by 5 and divide by 9.

88 F – 32 = 56 x 5 = 280 / 9 = 31.1C

**CELSIUS TO FAHRENHEIT:** Multiply the Celsius temperature by 9, divide by 5 and subtract 32.

50 C x 9 = 450 / 5 = 90 – 32 = 58F

Now you try it! Check the temperature in your house and outside in your front yard. Now convert them to using the formulas above to see if you can get it right.