Concept 9 of 9
The Big Freeze: Temperature Above and Below Zero
Today's big idea
Zero is not the bottom, so temperatures below it carry a minus sign.
Below zero, -8°C is colder than -2°C because it sits further down the scale.
How it works
👀 What's going on?
On a summer afternoon a city can read 40°C, while a winter night in the hills falls below 0°C. The same thermometer shows both, because the scale simply keeps going under zero.
💡 Here's the trick
Look at the thermometer above. The numbers climb as it warms and fall as it cools, and zero is not the bottom. Below zero the numbers carry a minus sign, so one degree under zero is written as -1°C. Reading it is just finding where the red stops.
🪄 How to do it
- 1.Read the start temperature on the thermometer above.
- 2.Count the degrees up for a rise, or down for a fall.
- 3.A rise adds and a fall subtracts, and you may cross zero on the way.
✍️ Try it
At dawn a hill reads -3°C. By noon it rises 8 degrees. What does it read? Then the evening drops it 6 degrees from the noon reading.
From -3°C a rise of 8 crosses zero, so -3 + 8 = 5°C at noon. The evening fall of 6 gives 5 - 6 = -1°C. Crossing zero is the only tricky part.
⚠️ Watch out
You might read -8°C as warmer than -2°C because 8 is the bigger digit. On the thermometer above, -8°C sits lower down than -2°C, so it is actually colder. The lower the mark, the colder it is.
🌟 Why this matters
Weather reports, freezer settings and hill-station trips all use temperatures below zero, so reading the minus sign correctly is an everyday skill. This is your first look at numbers below zero, and in Class 5 you will meet them properly as integers on a number line.
Exam tip: When a change crosses zero, count up to zero on the thermometer first and then carry on, so you never lose a degree.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“At dawn a hill reads -3°C. By noon it rises 8 degrees. What does it read?”
- 1
Start below zero
The reading begins at -3°C, three degrees under zero.
- 2
Climb to zero first
Of the 8 degree rise, 3 are used just getting up to 0°C.
- 3
Carry on above zero
8 − 3 = 5 degrees are left, so it keeps climbing to 5°C.
- 4
Answer ✓
Noon reads 5°C
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Dawn: it starts at -3°C and rises 8 degrees. Tap Warmer one degree at a time and watch it cross zero.
moved +0° of +8°
Quick check
✅ VERIFICATION
A freezer is at -6°C. It warms up by 4 degrees. What does it read now?
⚡ Finish the activity above first to answer.
Keep going!