Pressure, Winds, Storms, and Cyclones · Lesson 5 of 8
Fast Winds and Pressure Differences
“Explore why fast airflow changes pressure around balloons, roofs, paper, and banners.”
• Interpret the two-balloon blowing investigation as a pressure difference. • Explain how different pressures across a roof can exert a lifting force. • Relate paper-strip movement and banner holes to surrounding airflow. • Distinguish the cause of wind from an effect of already fast-moving air.
Blowing between two balloons
Hang two inflated balloons close together and blow through the gap. Many people expect the breath to force them apart. Instead, the balloons move toward each other. The fast-moving air between them is accompanied by lower pressure than the slower air on their outer sides, so the greater outside pressure pushes them inward.
Blow harder and watch again: the balloons can approach faster. This activity links speed, pressure, and a sideways push. We are comparing nearby regions in one arrangement; it does not mean every fast wind has the same pressure or that wind speed alone tells us the complete weather pressure at a place.
Problem
Two hanging balloons are 8 cm apart. What happens when air is blown through the gap?
- 1.The air in the gap speeds up compared with much of the air outside the balloons.
- 2.Pressure in the central flow is lower than the pressure on the outer sides.
- 3.The stronger outside pushes move the balloons toward each other.
Airflow over a roof
During very strong winds, fast-moving air over a roof may be associated with reduced pressure above it. If pressure below the roof is greater, air pushes upward on its underside more strongly than air pushes downward on top. A weak or poorly secured roof can be damaged when the difference is large. The roof diagram in the chapter is a pressure comparison, not a safe procedure to carry out during a storm.
Wind around an actual building is complicated: direction, openings, roof shape, and gusts also matter. For the pressure model, mark the two sides of the roof, compare their pressures, and find the direction of the net force. During severe wind, seek a sturdy shelter and stay away from windows; follow local emergency alerts.
Problem
During a gust the air above a roof is at lower pressure than the air just below. What is the direction of the resulting pressure force?
- 1.Outside air pushes downward on the top and inside air pushes upward on the underside.
- 2.The upward push is greater because pressure below is greater.
- 3.The net pressure force is upward; secure roofs and safe shelter matter in a storm.
A strip of paper and a hoarding
Hold a light strip of paper so it hangs freely and blow across its upper surface. The strip can lift toward the fast-moving air, as lower pressure above and higher pressure below make an upward push. As with the balloons, identify which side has the quicker flow and which side pushes harder. Try with a strip about 18 cm by 2 cm and repeat to see if the observation is consistent.
A large solid banner catches the wind. Air pressing on one side can create a large force because the banner has a large area. Holes allow some air to pass through and reduce the pressure difference and overall push. The holes need not make the air pressure zero; they reduce the load on the banner and its supports.
Problem
A tall hoarding has a large fabric banner. Explain why holes can reduce the chance of tearing in a wind.
- 1.Wind exerts force on the banner over its large area.
- 2.Openings let some air through, helping reduce the difference in pressure across it.
- 3.A smaller net push places less force on the fabric and supports.
Differences in air pressure set air moving from high to low pressure. Within an airflow arrangement, a faster stream can be accompanied by lower pressure than nearby slower air. Use the first idea to explain why a breeze begins and the second for the paired balloons and roof comparison.
Check your thinking
For each scene, label air speed, pressure on each side, and the direction of the larger push. That sequence is more reliable than memorising which way an object moves.
Quiz
What happens when you blow between two nearby hanging balloons?
Which region in the paired-balloon activity has relatively lower pressure?
If pressure below a roof is greater than above it, what is the net pressure push on the roof?
A light hanging paper strip may rise when you blow over its top because...
Why do holes help a banner in strong wind?
Practice Problems
- Describe how to set up the paired-balloon activity and explain the unexpected motion.
- Draw a roof cross-section with relative high and low pressures and show the direction of the net force.
- Predict the movement of a freely hanging paper strip when you blow across its top; explain the pressure comparison.
- Explain why a large solid banner can be strained by wind and how holes help.
- Distinguish the reason a sea breeze begins from the reason paired balloons approach when air passes between them.
- Why is the roof-pressure model insufficient as a household safety instruction during a severe storm? State a safer action.
Key Takeaways
• In the paired-balloon setup, the fast central stream has lower pressure than the air outside. • A pressure difference across a roof can produce an upward lifting force. • A paper strip can lift toward fast-moving air, and holes reduce wind loading on a banner. • During damaging winds, follow alerts and shelter away from windows.