Unifying Concept:

One of the most influential aspects of weather is air pressure. You learned in the previous lesson the various gases that make up the structure of the atmosphere. Like all matter, these gases have volume and mass. This being the case, the molecules and atoms of these gases exert pressure. The closer one is to sea level, the greater the pressure. This is because the atoms and molecules are more closely packed together and there are more of them than those higher in the atmosphere (remember the charts from the upper air soundings).

Recall from your upper air soundings for the three different stations, that the air pressure at the same altitude for each station was slightly different. These pressure differences were caused by differences in air density, and air density is related to temperature. You may recall from other courses that temperature is directly related to the average kinetic energy (energy of motion) of the molecules and atoms that compose the substance. Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air,therefore they exert less pressure than cooler air.

It is the temperature and altitude which determines the air pressure at any point on the earth's surface. Knowing the air pressure is useful because one can use air pressure readings to help determine weather and wind changes as well as altitude. The standard air pressure is 14.7 pounds per square inch or 1013.25 mb (29.92 inches of mercury). World wide, the pressure ranges from 970 to1040 mb (28.64 to 30.71 inches of mercury). Air pressure readings are determined by using an instrument known as a barometer.

Another factor that affects air pressure is humidity. As the temperature of a column of air increases, so does its capacity to hold more water vapor. The more water vapor that is held in a column of air, the less pressure it will exert when compared to the same volume of air at a cooler temperature.

Because air pressure is so important in the prediction of weather changes, meteorologist will take air pressure readings at elevations above sea level, and correct them to sea level pressure. This is done so that air pressure recorded at Denver (elevation 5,330 ft) can be compared to air pressure reading recorded in San Diego (elevation 15 ft). Without the mathematical correction, one could not compare accurately, the air pressures from different areas around the globe.

Monday's lesson discussed the structure of the Earth's atmosphere. This structure leads directly to what is known as the high and low pressure systems associated with weather. These weather systems can stretch for hundreds, even thousands of miles and generally travel from west to east. These systems sit for several days over land and/or water and take on the characteristics of the land masses they form over.

Air masses with a center of high pressure, is associated with fair weather. When your weather is influenced by a high pressure weather system, one can expect the weather to be cold or warm, humid or dry, depending upon where the mass of air formed. And even though the air in high pressure systems sink and is warmed by compression causing the cloudless and fair weather, there are times when air in a high pressure system can be forced up, resulting in clouds and precipitation. As a high pressure system approaches your area from the west, expect the barometric pressure to increase and then decrease as it passes.

Expect temperatures to increase as it approaches and then decrease as it passes to the east.The wind direction will also shift, but the direction of shift will be dependent upon whether the system passes to the south or north of you. If the system passes to the north expect the wind shift to be from the north to northeast to southeast to south. If the system passes to the south, expect the wind shift to occur from the north to northwest to west to southwest, and south.

Air masses with low pressure at their center is associated with inclement weather. As with a high pressure system, the type of weather one experiences with a low pressure system is dependent upon whether the system passes to the north or south of you.

Expect the barometric pressure to decrease and then increase as the system passes from west to east. If the center of the low passes to the north, expect the temperature to decrease and then icrease; increasing cloudiness leading to precipitation and then clearing; winds will shift from the southeast to south, and then to the southwest. If the center of the low passes to the south, expect increasing cloudiness, leading to precipitation and then clearing, temperatures slowly decrease; winds shifting from the southeast to northeast to north, to northwest. As you have learned, both of these types of weather systems influence the everyday weather we experience.