Q1.
I have read that high-speed winds can blow off roofs. I wonder how?
Answer
Because high-speed winds are accompanied by reduced air pressure. When a fast wind sweeps over a roof, the pressure above the roof falls below the pressure of the still air inside the house, and the roof is pushed upwards from below.
Think of the roof as a lid with air on both sides. Under it, inside the house, the air is calm and at ordinary atmospheric pressure. Above it, the wind is racing past, and where air moves fast its pressure is lower. The air below therefore pushes up harder than the air above pushes down. If this difference is large and the roof is weak, the roof is lifted and carried away (Fig. 6.14 a).
Wind speeds up over the roof → pressure above the roof falls
Pressure inside the house stays high
Upward push from below > downward push from above
→ weak roof is blown off
Pressure inside the house stays high
Upward push from below > downward push from above
→ weak roof is blown off
What to do about it: the book's advice is to keep the doors and windows of houses open during storms with high-speed winds. The wind can then move over the roof and through the house, so the pressure inside falls too. The difference between inside and above is greatly reduced, and the roof stays in place (Fig. 6.14 b).
Check it yourself: Activity 6.6 shows the same effect on a small scale — blow between two hanging balloons and they swing towards each other instead of apart.