ATOC 181 Lecture Notes - Lecture 3: Thermosphere, Mean Free Path, Ionosphere
7 Jun 2018
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LESSON 3
THE EARTH AND ITS ATMOSPHERE PART 2
Earth Gravity
• Refers to acceleration of a falling object, due to the attraction by the Earth
• Newton found that the resulting force was equal to the mass of the object multiplied by
the gravity (about 9/8 metres per second)
• Acceleration by 9.8m/s2
• In 1686, found this
• The value of gravity varies with altitude, but we will consider it as a constant
Force Exerted on/by an Object Due to Gravity
• Any object on Earth is subject to the force of gravity
• Force exerted on object due to gravity
• If take object of 1kg, subjected to gravity.
o Force due to gravity: 1 x 9.8m/s2 = 9.8 Newtons (about 10N)
• Use 10m/s2
• If 2kg, 20N
• If put 1 kg on table, not fall, not because of gravity that disappears, but because table
exert force of 10N too, so not fall
Pressure
• Force exerted by an object on a unit of area
• 1 m2 = unit of area
• Unit: Pascal, equal to a force of 1N over a surface area of 1 square metre
• Pascal invented 1st calculator
• If put 1kg on surface of 1m2, force exerted by 1kg=10N.
o pressure is the force on 1m2
o P=10N/1m2 = 10Pa
• If surface area is 4m2, P= 10N/4m2 = 2.5 Pa
• Pressure decreases when surface area increases
• If surface area = 0.5 m2, P=20 Pa
• The smaller the surface for the same mass, the more pressure we have
• Principle of nail is that have smallest surface area, the pressure is large enough so that
can break or get through sometimes
• To find the pressure in Pascal (Pa) exerted by an object over a give surface, just divide the
mass in kg by the area in square metre and multiply the result by 10.
o Pressure (Pa) = Mass (kg)/Area (m2) x 10 m/s2
Air Mass and Pressure
• Total mass of air in a column of 1m2 is about 10 000kg
• Pressure (Pa) = 10 (is the 9.8 of gravity that is rounded to 10) X 10 000kg/1m2 = 100 000
Pa
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• Pressure of atmosphere on 1m2 = 100 000 Pa. This is a big nb
• 1000 Pa = 1 kPa, so 100 000 Pa = 100 kPa
• 100Pa = 1hPa, so 100 000 Pa = 1000hpa
• hPa is the unit used for the atmospheric pressure
• Standard surface pressure at sea level is about 1000 hPa (more exactly 1013.25 hPa)
• Before was using millibar, which is the same thing as 1 hectoPa
• Scale: measure weight not mass, weight is in N, is a force. Machine changes it to kg,
measures surface area and pressure
Air Density
• Density is mass of 1 cubic metre of an object
• Density = 1kg/1m3= 1kg/m3
• If 2m3, D = 0.5kg/m3
• Near the Earth surface, air density is about 1.2 kg/m3
• In comparison, liquid water, 1 cubic litre of water weighs 1 ton
• 1L of water weighs 1kg
Vertical Structure (Pressure-Density)
• Both density and pressure decrease with increasing altitude (decrease with height)
• Density decreases because have less and less air molecules as go up
• Why does pressure decrease with height?
o 10 000 kg with pressure of 1000hpa above the surface
o about half of atmosphere above 5km, so half of atmosphere above 5000kg
o 5000kg/1m2, so pressure = 500 hPa
o Every time as go up, have less and less atmosphere above, so less and less pressure
Air pressure
• Surface = 1000hpa
• 1km = about 900hpa
• 2km = about 800hpa
• 3km = about 700hpa
• 5.5km = about 500hPa
• 10.5km about 250hPa
• Half below 5.5km
• 90% under 17km
• 99% under ~30km. why not go above this, as not enough air
• Curve of pressure vs altitude: from to and to …, loss a hundred each time.
Exponential curve, in first 4 km, loss 10% of mass of atmosphere per km (100hPa)
Air Density Vs Altitude
• Near the Earth surface, air density is about 1.2kg/m2
• Top of Everest: 0.42kg/m2 or about one third than at surface
find more resources at oneclass.com
find more resources at oneclass.com
