Pressure Definition

>> Pressure is force per unit area
>> Pressure is exerted by molecular collisions

Gases, unlike solids and liquids, do not have their molecules tightly bound together.  Instead, they move about freely and frequently collide with one another and with adjacent surfaces.  When any molecule collides with another object, a force is exerted by the molecule on that object.  When all of these individual forces are added together in a given area, it results in a pressure.

>> Standardized measurements

There are many different units in which pressure can be measured - from the atmosphere to the pound per square inch.  However, meteorologists typically only use three of these pressure measurement units.  The two most commonly used pressure units are the millibar and the inch of mercury - both of which are regularly used in broadcast meteorology.  The third unit, the Pascal, is the preferred standard measurement of pressure.  Figure 1, below, summarizes the various units used to measure pressure and their relations to one another.

>> Also acts in the horizontal (doesn't just "push down")

Pressure doesn't just act as a force in the vertical, it also works to exert horizontal forces as well.  In fact, the horizontal component of pressure is a key variable in many atmospheric processes.  The gory details of many of these processes will be discussed later this semester.

Atmospheric Constituents

>> Constant quantities: Nitrogen and Oxygen account for 99% of dry air

The composition of dry air in the atmosphere can be seen in Figure 2 below.

>> Variable quantities: Water vapor most important

While the composition of atmospheric gases is fairly constant, there are some quantities that vary.  The most important of these quantities is water vapor.  The amount of water vapor in the atmosphere at any time can range from 0% to 4% by volume.  The amount of water vapor in the air at any time plays a large part in determining various meteorological quantities from the relative humidity and the dewpoint to when and where clouds will form.

Gas Law

>> Pressure depends on temperature and density

The ideal gas law relates air pressure, density and temperature.  This law states that the air pressure is proportional to the air density times the air temperature.  This statement becomes an equality if the density and temperature are multiplied by a constant called the Universal Gas Constant.

>> Increase temperature

If there is an increase in air temperature and the pressure remains constant, the air density must decrease.  This is because in warmer air the molecules are moving faster and exerting a greater force on the surroundings, so in order to keep the pressure constant, less molecules are needed to exert the same force.  This relationship can be seen in Figure 3 to the left.

>> Increase density

If, instead, the density of the gas increases and the pressure is to remain constant, there must be a decrease in temperature.  In this situation, an increase in the density of a gas will cause more molecules to be contained in a certain volume.  Since there are more molecules in any given volume of air, the molecules don't need to be moving as fast to exert the same force on the surroundings.  This leads directly to a decrease in the temperature of the gas.  Figure 4, to the right, illustrates this relationship.

Gravity and the Compressibility of Air

>> Weight of air above compresses air below
>> Pressure increases as altitude decreases

Atmospheric pressure can be defined as the weight of a column of air above any given point in the atmosphere.  Therefore, it makes sense that the atmospheric pressure would increase as altitude decreases.  As the altitude decreases, the amount of air molecules found above a given point will be greater and, therefore, the weight of the air in that column will be greater.  This concept is illustrated in Figure 5 to the left.

>> The atmosphere is very thin: compare it to an onion skin

Earth's atmosphere is incredibly thin in comparison with the radius of the planet.  This property of the atmosphere can be attributed to gravity.  The force of gravity works to hold the air molecules found throughout the atmosphere as close to the surface of the planet as possible.  This basic force causes more than half of the air molecules in the Earth's atmosphere to reside below 5.5 kilometers.  As a result, air pressure does not decrease in a linear fashion.  Instead, atmospheric pressure decreases more rapidly near the surface than it does at higher altitudes.  Figure 6, found to the right, illustrates this principle.

Constant Height Maps and the Correction of Station Pressure to Sea-Level Pressure

>> Removes effect of vertical variations in pressure

The correction of station recorded atmospheric pressure to sea-level pressure allows for vertical variations of pressure to be disregarded.  This process is completed by adding in the air pressure of an imaginary column of air that would stretch down to sea-level.  Figure 7, below, illustrates the correction of station pressure to sea-level pressure.

>> Horizontal pressure gradients are much smaller than vertical changes, but they are critical in describing atmospheric motions

One way that meteorologists look at variations in pressure is through the use of a constant height map.  This type of map allows the horizontal variations of pressure to be seen at any given height.  This can be helpful to meteorologists by allowing them to see horizontal pressure gradients at a given height, which is crucial to understanding and describing various atmospheric motions.  The only constant height map we will routinely use is the "sea-level" (height equals zero), or "surface" weather map.

Constant Pressure Maps: Analogous to Constant Height Maps

>> Show lines of constant height along a single pressure surface

A constant pressure map is analogous to a constant height map in that it displays contours at a given level.  In the case of a constant pressure map, however, contours of constant height are plotted along a single pressure surface, such as 500 millibars.  This type of map essentially serves the same purpose as a constant height map.  An example of this type of weather map can be seen in Figure 8 to the left.

>> Warm column of air: Higher heights at a given pressure surface or higher pressure at a given height
>> Cold column of air: Lower heights at a given pressure surface or lower pressure at a given height

The two notes presented directly above work to link the constant pressure and constant height maps together through one concept.  This concept is illustrated in the image below.