Q1.
If you have to paint the roof of your school or home white, what would happen?
Answer
The rooms below would become cooler. On a hot afternoon the difference can be 2 to 5 degrees — enough to feel clearly.
What would change, one thing at a time:
- The white roof reflects most of the Sun's heat back into the sky instead of soaking it in.
- So the roof itself stays much cooler than a grey or black roof.
- Less heat passes down through the roof into the room below.
- The room stays cooler, especially the top-floor rooms, which are always the hottest.
- The fans and coolers work less, so less electricity is used and the bill goes down.
- Children can sit and study in the afternoon without feeling drowsy from the heat.
| Ordinary grey roof | White-painted roof | |
|---|---|---|
| What it does with sunlight | Absorbs most of it | Reflects most of it |
| Roof surface at 2 p.m. in May | Very hot to touch | Noticeably cooler |
| Room below | Stuffy and hot | 2–5 °C cooler |
| Electricity used | More — fans on full | Less |
| Cost | — | One tin of lime wash or white reflective paint, once a year |
Why it works: it is exactly Activity 3, made big. The black tile got hot; the white tile stayed cool. A roof is only a very large tile with a room underneath it. Choose its colour and you choose how hot that room will be.
This is really done: Many houses in Indian cities are now given a coat of white lime or reflective paint before summer — it is called a “cool roof”. It is one of the cheapest ways to cool a building, and it needs no electricity at all. In Gujarat and Telangana whole neighbourhoods have been painted this way.