Explaining Low Air Pressure
- Pressure is a force pushing on an area.
- Air pressure is the force of the weight of air pushing on a surface.
- As we go higher the air pressure will normally go down as there is less air above.
- Normal air pressure at sea level is 1013 mbar.
Plastic bottle sealed at 14000 feet (4,267 metres)
Image: Plastic bottle at 14000 feet, 9000 feet and 1000 feet, sealed at 14000 feet © Wikimedia Commons
Picture 1: 14,000 feet (4,267 metres)
Picture 2: 9,000 feet (2,743 metres)
Picture 3: 1,000 feet (305 metres)
The air sealed in the bottle up a mountain at 4,267 metres was at a lower air pressure than the air lower down.
This graph shows how air pressure decreases with altitude (height)
Image: Atmospheric Pressure vs. Altitude - Geek.not.nerd © Wikimedia Commons under the Creative Commons CC0 1.0 Universal Public Domain Dedication license
Understanding Pressure - Fact Box
Pressure is the force applied perpendicular (think about a right angle) to the surface of an object per unit area.
Pressure = a force pushing on an area.
Image: Pressure force area - Klaus-Dieter Keller © Wikimedia Commons under the Creative Commons Attribution-Share Alike 3.0 Unported license
- Think about a pile of exercise books sitting on a desk. The weight of the books = the pressure on the desk.
- Try putting them on a weighing scale.
- Air pressure is the weight of the atmosphere pushing down on the surface of the Earth.
- The SI (scientific) unit for pressure is the pascal (Pa), which is roughly the weight of a typical apple pressing down on an area of a square metre.
- In normal life the most common unit for measuring pressure is the bar.
- The bar is equal to 100,000 Pa.
- Air pressure is normally measured using millibar (symbol = mbar or mb).
- A millibar is 1 thousandth of a bar just like a millimetre is one thousandth of a metre.
- Normal air pressure is 1013mb.
Think about our bottle of air sealed at 4,267 metres up a mountain.
Lower down the air pressure outside of the bottle is much more than the air pressure inside the bottle so the bottle is crushed.
- As well as altitude air pressure is affected by rising and sinking currents of air.
- Where air at the surface of the Earth is warmed:
- It expands
- Becomes lighter
- So it rises.
- Rising currents of air create areas of low air pressure.
- These can be seen as ‘bands’ on the surface of the Earth.
- The bands change with the seasons.
Image: Mslp-jja-djf - William M. Connolley © Wikimedia Commons under the Creative Commons Attribution-Share Alike 3.0 Unported license (attr: William M. Connolley at the English language Wikipedia)
15 year sea level average air pressure; June, July & August = top map; December, January & February = bottom map.
Global Pressure Belts
- Where the sun is directly overhead of the Equator the air is warmed the most becomes lighter and rises.
- The rising air cools and water vapour condenses into rain.
Image: Sun Rays and the Atmosphere - GitN Edition 6
This creates a band of rain and low air pressure on or around the Equator which moves North and south as the seasons change.
Image: MeanMonthlyP - PZmaps © Wikimedia Commons under the Creative Commons Attribution-Share Alike 3.0 Unported license
The band of clouds can be seen on satellite images.
The band of clouds is caused by air rising which causes low air pressure.
The clouds form rain and this where we get rainforests forming.
Image: MeanMonthlyP © Wikimedia Commons
Convection
Eventually the air will cool and form a convection cell.
In a convection cell warm air rises.
After rising it cools and starts to sink.
The rising and sinking air forms roughly circular currents.
The most heat is received along the Equator; the air rises and splits into two convection cells. One to the North and one to the south.
Image: Convection - User:Oni Lukos © Wikimedia Commons under the Creative Commons Attribution-Share Alike 3.0 Unported license
Where the air sinks it creates a band of high pressure.
Image: Sahara satellite hires © Wikimedia Commons
As air sinks it warms and any clouds/water vapour is evaporated so the skies are clear.
This is where and why we get hot deserts like the Sahara Desert at these latitudes.
Image: Libya 4985 Tadrart Acacus Luca Galuzzi 2007 - Lucag © Wikimedia Commons under the Creative Commons Attribution-Share Alike 2.5 Generic license
Student Activity
Complete the investigation part 1 activity sheet.
