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Investigation 2 – Concept Day








Atmosphere: Investigation 2

Concept Day


  • In Investigation 1, you performed experiments that established a relationship between air temperature and air density. You also learned that the sinking of cool and rising of warm air created convection currents that have an impact on global weather.
  • In this Investigation, you will explore the involvement of air temperature and density on the pressure air masses exert on the Earth’s surface. You will also explore how high and low air pressure is involved in wind and weather activity.



  • This slide introduces the concept of atmospheric pressure.
  • Notice that, in the definition of atmospheric pressure, it is the mass of molecules in the air pressing down on the surface of the Earth that causes the pressure. What causes the molecules in the air to “press” down on the Earth’s surface? Gravity, of course. Thus, if there were no molecules in the air, there could be no air pressure. On the other hand, if there was no gravity, there could be no air pressure. Thus, there is no atmospheric pressure in outer space, and can it can only occur on planets that have an atmosphere (i.e. have molecules in the space that immediately surrounds them).
  • Atmospheric pressure is represented by the equation:


  • Atmospheric pressure is measured and reported in units called millibars, abbreviated as mb.  We will see later that millibars are plotted on the familiar weather maps that you routinely see on the Internet and television weather reports.



  • This slide demonstrates the relation between rising and sinking air masses (parcels) and atmospheric pressure at the Earth’s surface.
  • When air warmed at the Earth’s surface rises into the troposphere, its upward movement creates a low-pressure zone beneath it. This makes sense since rising air molecules are no longer pressing down on the Earth’s surface due to gravity. Rising warm air therefore acts directly against the force of gravity. It can do this due to its low density caused by its increased kinetic energy. We will see in the Lab how this leads to wind formation as surrounding air rushes to the site of rising air and prevents a vacuum from forming.
  • Once the air parcel rises high enough into the troposphere, its temperature rapidly cools due to the reduced temperature of the troposphere at increasing altitude (see insert in slide). This will have an immediate effect on the air parcel’s density because of Charles’ Law (cooler gas takes up less volume than warmer gas). The cooler, denser, heavier air then begins to sink. As it does so, it exerts greater atmospheric pressure on the surface of the Earth beneath it. We will see later how the sinking of cool, dense air can generate wind activity at the Earth’s surface as it acts to displace warmer, less dense air as it crashes to the surface.



  • We have seen this slide before, in Investigation 1, where differential heating of the Earth’s surface by the sun and the Coriolis Effect sets up major global convection systems.



  • This slide is an extension of the previous slide. It shows how, on a global level, the major convection systems act to establish large-scale high and low atmospheric pressure cells. Smaller, local convection systems can occur within these major pressure systems.



  • This slide depicts a view of the typical weather maps used in predicting weather.
  • An area of high atmospheric pressure is indicated by a large capital “H”, often indicated with blue text. An area of low atmospheric pressure is indicated by a large capital “L”, often indicated with red text. The millibar lines are derived from reports from weather stations located across the country.
  • The map on the right shows not only the magnitude of the millibar lines but the distance between them as well. The Pressure Gradient Force (PGF), which takes into account both the difference in pressure between higher and lower areas of atmospheric pressure (millibar lines) and the distance between them, determines the strength of the winds likely to be associated with various locations on the map. PGF is calculated according to the formula:


  • Thus, the difference in pressure between locations and the distance between the locations both have an impact on the force of winds likely to occur at various geographic locations. A simple look at this formula tells us two things.
    • The greater the difference between pressures, the more forceful the wind is likely to be, and
    • The closer the high and low-pressure zones are located to each other, the more forceful the wind is likely to be.
  • You will perform experiments in Investigation 2 Lab to demonstrate both of these important factors.



  • From the above discussions of weather prediction, it is clear that we must be able to determine the atmospheric pressure at precise and multiple geographic locations. Also, because pressure changes rapidly as warmer air rises and cooler air sinks at any given location, we must be able to obtain atmospheric pressure measurements frequently at many different locations simultaneously. Prior to rapid communications and atmospheric pressure readings, accurate weather forecasting was difficult.
  • We determine atmospheric pressure by using an important apparatus called a barometer. On the left of this slide, we see a typical barometer. Forms of the instrument were in use as early as the mid-1600s. Modern barometers contain a long glass column filled with mercury. When the atmospheric pressure rises, pressure is exerted on the reservoir that holds the liquid metal, and the level of mercury in the column rises. By convention, millibars of atmospheric pressure are calculated by dividing the height to which the mercury rises by the constant 0.750062.


  • The figure on the right side of this slide illustrates the barometer simulation that you will construct in the second part of Investigation 2 Lab. It is a good illustration of how a barometer works.
  • The 500ml Erlenmeyer flask containing colored water represents the atmosphere and the reservoir of the barometer. The inverted test tube with its calibrations represents the mercury-filled glass tube of the barometer. When it is lowered, open-end down, into the water, it traps a certain volume of air. The pressure of this trapped air is approximately the same as the surrounding air in the Erlenmeyer flask before the experiment begins. The syringe simulates changing atmospheric pressure.
  • When the stopper is placed onto the flask and pressure is applied to the plunger of the syringe, this forces more air into the flask and, since the flask is sealed, increases the internal air pressure inside the flask. This, in turn, increases the pressure by which the internal air “presses down” on the surface of the liquid. Since the air in the inverted test tube is now lower than in the rest of the flask (remember, it was captured before the top of the flask was inserted), the water will begin to move up into the test tube toward the lower air pressure. This will increase the air pressure inside the test tube by decreasing its volume. Theoretically, the water will rise in the test tube until the trapped air is at the same pressure as the air inside the flask.
  • When reverse pressure is applied to the syringe plunger, air pressure inside the flask will decrease. If the pressure of the air trapped in the inverted test tube is greater than that in the flask, it will begin to “Push down” on the surface of the water in the test tube until the test tube air pressure is the same as the flask air pressure.
  • If the syringe were removed, the remaining apparatus would be much like a real barometer. An increase in real atmospheric pressure would push down on the water surface outside the inverted test tube. This would cause the water inside the test tube to rise since the external air pressure would be higher than that of the air trapped inside the inverted test tube. A drop in atmospheric pressure would cause the reverse to occur and the liquid in the inverted test tube would drop.
  • Real barometers use a much thinner and longer tube than the test tube we use in this model so that slight changes in atmospheric pressure are detectable.