Lesson 20 — Activity 2:

Calculating Changes in Temperature

You may have noticed that most thermometers have two sets of numbers — one set measures the temperature in Celsius, and the other set measures the temperature in Fahrenheit.

Think back to the activities that you completed earlier in this theme on metric and imperial measurements. We can measure temperature in both metric and imperial units. In Canada, since we use the metric system, we measure temperature in degrees Celsius. In the United States, however, they use the imperial system and measure their temperature in degrees Fahrenheit.

It is important to be aware of the two systems, however, for this lesson, we will focus only on measuring temperature in the metric units of degrees Celsius.

Let's take a look at different temperatures on the Celsius scale.

For example, the weather forecast says that the temperature will change from 18°C to 13°C. What does this mean? Will it get warmer or colder? Since 0°C is the freezing point, the closer you get to 0°C, the colder it will be. So in this example, if the temperature changes from 18°C to 13°C, it will get colder! Let's try another one.

You change the central heating thermostat in your house from from 19°C to 21°C. Will this make the house warmer or cooler?

Let's try one more. The weather forecast says that the temperature will rise by 4°C. What does this mean? Will it get warmer or colder?

You can usually calculate the change in temperature by doing a simple subtraction problem — just subtract the original temperature from the new temperature to see how much it changed.

For example, you are watching the morning news and the weatherman says it is currently 15°C outside. By the end of the day, the weatherman says that it will be quite warm at 29°C. You want to know how much the temperature will change by the end of the day.

All you need to do is subtract 15° from 29°.

29 – 15 = 14

The temperature will go up by 14°C by the end of the day!

The only time it might be a bit tricky to calculate changes in temperature is when the temperature changes from a positive number (e.g., any number that is above 0°C, like +15°C) to a temperature that is below zero (0°C). As we live in Canada, most of us know what it feels like when it gets to minus temperatures in the winter — –30°C is really cold!!

So, how can we calculate temperatures that change from a minus to a plus?

When calculating changes in temperature like the example above, it may be helpful to look at a thermometer. 

Take a look at the thermometer to the right. Notice that the zero (0°C) is in the middle, and then there are numbers above and below the zero that go up by 5 degrees. The numbers above the zero are on the plus side (warmer temperatures), and the numbers below the zero (colder temperatures) have a minus (–) sign in front of them.

Suppose it was –20 degrees an hour ago, and now it has gone down 10 degrees. As it goes down, it will get further away from 0 degrees (since it is below zero to start with), so it will decrease from –20 degrees to –30 degrees! The number part (the 30) increases because we are going away from zero, while the temperature itself decreases (getting colder). If the temperature changed to –10, it would be warmer, not colder; that would be an increase in temperature. 

So what happens if the temperature change is from a plus temperature (e.g., +2°C to –2°C)? How do we find the difference in this case? We can again look at two thermometers to show us how to find the temperature change.

Image Source: Pixabay

Look at the two thermometers above. The one on the left shows a temperature of –2°C, and the one on the right shows a temperature of +2°C. So what is the difference in temperature? All you need to do is count the notches from the –2°C to the +2°C. In this example, you can see that there are 4 degrees difference!

Let's try one more!

In this example, you just count the notches from –4°C to +8°C. In this example, you can see that there is a 12 degree difference!

  Self-check!