More on air pressure More on the Ideal Gas Expression
Our discussion of air pressure thus far in the semester has primarily been from a microscopic point of view. We have been considering air pressure by considering molecules in a container. We will now expand our interpretation of pressure and apply our argument to the open atmosphere.
Atmospheric Pressure
In meteorology, we constantly hear about high pressure systems and low pressure systems. What is meant by these weather systems and why do they play such a large role in determining the weather.
Actually, it is pressure gradients, or, changes in pressure over horizontal distances, that helps to drive the weather on Earth.
It is very important to note that pressure decreases with elevation at all points on Earth. It is also important to realize that pressure changes horizontally as well as vertically in the atmosphere.
Atmospheric pressure is defined as the weight of the column of air above an area on the Earth's surface. Air moves from regions of higher atmospheric pressure toward lower atmospheric pressure. This movement of air is called wind.
Here's an interesting question. Since we know pressure decreases in the atmosphere as we go up in elevation. Why then does air not blow from the surface upward if air moves from regions of higher pressure toward lower pressure? Why is there not perpetual vertical motion? The answer will come next week when we talk about a subject called hydrostatic balance. The answer has to do with the fact that while there is a pressure gradient force upward as air wants to move from higher pressure regions at the surface toward lower pressure regions aloft, there is also the force of gravity downwards. Meteorologists often consider these forces to be equal and opposite to each other when making simple meteorological inferences. However, when describing and predicting verticfal motion, these assumptions can no longer be made.
But it is horizontal pressure gradients that drive the wind we are accustomed too. The stronger the pressure gradient, the higher the wind speed.
Next week and in coming weeks, we will be greatly enhancing our understanding of the wind.
Meteorologists typically measure pressure in terms of millibars (mb). Typical values of atmospheric pressure are around 1000 mb and pressure will rarely rise above 1040 mb and rarely fall below 950 mb on the Earth's surface.
Using pressure as a vertical coordinate
We have discussed numerous examples where height in units of length is chosen as the vertical coordinate. But meteorologists commonly use pressure to define the vertical coordinate. Since pressure diminishes at nearly the same rate around the planet, the height of pressure levels is fairly similar. For instance, there is a level in the atmosphere where the pressure is 500 mb. A meteorologist may refer to an event occurring at this level by making reference to the 500 mb level.
Pressure and weather
Even from watching the TV weather reports a few times, many people can see the relationship between high pressure and fair weather and low pressure and stormy weather. But why?
To answer that question, we need to reflect upon some lessons we have already learned. For instance, moist air is lighter than dry air. So a given volume of moist air at temperature T compared with the same volume of dry air at temperature T will be under less pressure. The density of the gas is the mass divided by the volume. Since the gases occupy the same volumes, the lighter moist air will be at a lower density. Density is directly related to pressure, so with temperature being equal in both situations, the moist air would be at lower pressure.
Moist air is often associated with cloud development and precipitation. Therefore, lower pressure associated with moist air can suggest inclement damp weather.
On the other hand, dry air which is heavier is often associated with blue skies and fair weahter. The dry air will be at higher pressure than a corresponding equal volume of moist air at the same temperature.
Air masses tend to take on characteristics of the land over which they form. So for instance, air masses forming over northern Canada which is very cold and dry, often descend into the United States as high pressure systems and will bring cool, calm weather. On the other hand, the Gulf of Mexico is moist and warm. Often low pressure systems will migrate northward out of the Gulf of Mexico and will spread precipitation across the east coast of the United States.
Informal Weahter Discussion
After our discussion on atmospheric pressure, I wish to engage in a interactive student discussion about weather experiences students have had that they now understand better or hope to learn later in the semester.