1. AIR PRESSURE

2. Keeping an Atmosphere Atmosphere is kept by the world’s gravity
Low mass (small) worlds= low gravity
=almost no atm.
High mass (large) worlds = high gravity
= thick atm.
Gravity and pressure
Air pressure depends on how much gas there is i.e. The atmospheric thickness.

3. Gravity and Atmospheric Pressure
The stronger the gravity, the more gas is held by the world and the greater the weight of atm. on a point

4. Earth’s Atmosphere About 10 km thick
Consists mostly of molecular nitrogen (N2) and oxygen (O2)

5. The air is made up of molecules.

6. Gravity pulls the air molecules toward the earth, giving them weight
Gravity pulls the air molecules toward the earth, giving them weight. The weight of the air molecules all around us is called the air pressure.
Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.

7. High altitudes = lower pressure
Air pressure can be thought of as the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up.
Low altitudes = higher pressure

8. Atmospheric Pressure
Gas pressure depends on both density and temperature.
Adding air molecules increases the pressure in a balloon.
Heating the air also increases the pressure.

9. Air pressure is equal in all directions.
Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.
Pressure = force per unit area

10. Barometric pressure goes down.
As elevation goes up
Barometric pressure goes down.
This is an inverse relationship.

11. A Barometer
is used
to measure air pressure.

12. In 1643, Evangelista Torricelli invented the barometer
Torricelli didn’t actually build a barometer, but he gave detailed instructions on how to build one, so he is given credit for the invention. He was actually trying to prove the existence of a vacuum. Many scientists in his day didn’t believe that a vacuum could exist, hence the phrase, “nature abhors a vacuum.”
In 1643, Evangelista Torricelli invented the barometer

13. Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube.
The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.
The abbreviation “Hg” is the chemical symbol for mercury. Some kinds of pressure reading instruments, including some barometers, use the abbreviation “mmHg,” meaning “millimeters of mercury.” 760 mmHg is considered the standard “normal” atmospheric pressure at sea level. This unit is called a “torr,” after Torricelli.
To construct a mercury barometer, fill a tube with a liquid. Invert then tube in a dish of liquid holding your thumb over the top of the tube until the the tube is immersed in the bowl of liquid, the atmospheric pressure will keep the liquid in the tube from emptying such that the weight of the liquid in the tube equalize with the atmospheric pressures. (Do not do this with mercury because of its toxicity)
Mercury was used because it is a very heavy liquid, so the tube could be relatively short. The tube in a mercury barometer still has to be over a meter long. Students may want to try building a barometer using colored water. How high would the tube need to be? Merucy is about 11 times more dense than water. What if they used milk or some other liquid, would the height be the same?

14. As atmospheric pressure increases…
The mercury in the tube rises.

15. The Mercury Barometer Good: Bad: Simple to construct Highly accurate
Glass tube is fragile
Mercury is very toxic!
Although mercury has been used for hundreds of years, its toxic effects have only been fully realized in the last few decades. Students should NEVER handle mercury or broken mercury thermometers or barometers. Mercury should also never be thrown in the trash or washed down the drain, since it moves easily up the food chain from fish to humans. A local health department or environmental professional can assist with disposal of old or broken mercury instruments. GLOBE instrument specifications call for organic-fluid (non-mercury) or digital thermometers, except for the analog min/max thermometer, which is mounted in a shelter and is not handled by the students.

16. The Aneroid Barometer No fragile tubes! No toxic chemicals!
No batteries!
Never needs winding!

17. An aneroid barometer uses a cell which has had most of the air removed.
MILLIBARS
As the air pressure around the cell increases, it presses on the cell, which causes the needle to move.
The word “aneroid” means “no air,” and refers to the partial vacuum inside the cell. The aneroid cell is shaped like a bellows, so that it can flex as air pressure changes. Increasing air pressure compresses the cell, causing the needle to register a change. Decreasing pressure allows the cell to expand, causing the needle to move in the opposite direction.
The use of inches of mercury is a hold-over from the days of mercury barometers. It refers to the actual height of mercury in the glass tube. Millibars are metric system units, and as such are readily understood by scientists around the world.
Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists measure atmospheric pressure in millibars.

18. As we have noted earlier, higher elevations have fewer air molecules pressing downward, and so atmospheric pressure is lower. This means a barometer will read lower as it is carried to a higher elevation. Airplanes use a special type of barometer, called an altimeter, to measure altitude.

19. Two types of barometric pressure measurements:
Station pressure is the actual pressure at the recording location. It is affected by the local altitude.
Station pressure on a mountain top will be lower than station pressure in a valley. Scientists need a fixed point of reference in order to compare barometer readings in different locations. That is why barometer readings are sometimes adjusted for elevation above sea level at the station location.
Sea level pressure is referenced to sea level, so it has the same altitude anywhere in the world.
Station pressure on a mountain top will be lower than station pressure in a valley. Scientists need a fixed point of reference in order to compare barometer readings in different locations. That is why barometer readings are sometimes adjusted for elevation above sea level at the station location.

20. Most aneroid barometers have a needle which can be set to remember the previous reading.
Knowing how the air pressure is changing is as important as knowing the actual barometric reading. The set needle allows students to compare the current reading to the previous one. If the current reading is less than the previous one, the barometric pressure is falling. If the current reading is more than the previous one, the pressure is rising. If it is the same as the previous reading, the pressure is said to be steady. Weather forecasters often use the phrases “falling barometer,” “rising barometer,” or “steady barometer” as a way of referring to the change in atmospheric pressure.

21. Changing Pressure A rising barometer = increasing air pressure.
This usually means:
Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.
Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

22. Changing Pressure A falling barometer = decreasing air pressure.
This usually means:
Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.
Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

23. Air Movement and Flow

24. Fluids (air and water) flow from areas of high pressure to areas of low pressure.
Change in pressure across a horizontal distance is a pressure gradient.
Greater the difference in pressure and the shorter the distance between them, the steeper the pressure gradient and the stronger the wind.
Movement of air across a pressure gradient parallel to Earth’s surface is called a wind and winds are named for the direction from which they come.
6-1

25. Isobars in millibars, the closer the isobar the stronger the winds
Rain
Low Pressure
High Pressure

26. The Atmosphere in Motion
Atmospheric pressure is a measure of the force pressing down on the Earth’s surface from the overlying air.
Pressure is often measured in different units including:
atmospheres (1 atmosphere is the average atmospheric pressure at sea level),
millibars (1 atmosphere = millibars),
pounds per square inch or psi (1 atmosphere = 14.7 pounds per square inch),
mm or inches of mercury (1 atmosphere = 760 mm or inches of mercury)
torrs (1 torr = the pressure exerted by 1 cm of mercury).
Low air density results in rising air and low surface pressure.
High air density results in descending air and high surface pressure.

27. Heating and Cooling of Air

28. The Gas Law
Ideal Gas follows kinetic molecular theory, made up of large number of molecules that are in rapid random motion following perfect elastic collitions losing no momentum
How the Kinetic Molecular Theory Explains the Gas Laws
The pressure of a gas results from collisions between the gas particles and the walls of the container.
Each time a gas particle hits the wall, it exerts a force on the wall.
An increase in the number of gas particles in the container increases the frequency of collisions with the walls and therefore the pressure of the gas.
Avogadro's Hypothesis
As the number of gas particles increases, the frequency of collisions with the walls of the container must increase.
This, in turn, leads to an increase in the pressure of the gas.
Flexible containers, such as a balloon, will expand until the pressure of the gas inside the balloon once again balances the pressure of the gas outside.
Thus, the volume of the gas is proportional to the number of gas particles.

29. The Gas Laws Charles Law Boyle’s Law
The volume of a gas increased with the temperature
The volume of a given amount of dry ideal gas is directly proportional to the Kelvin Temperature provided the amount of gas and the pressure remain fixed.
When we plot the Volume of a gas against the Kelvin temperature it forms a straight line.
V1 / T1 = V2 / T2
Boyle’s Law
the product of the pressure and volume are observed to be nearly constant.
The product of pressure and volume is exactly a constant for an ideal gas.
p * V = constant

32. WATER VAPOR

35. (9.8oC)