Working through this chapter of the study guide will enable you to:

1. Show how precipitation can form, and identify the most common kinds.
2. Name the various types of air masses and describe their source regions, then show how frontal regions move and how they affect our weather.
3. Describe the various types of storms that can occur in our atmosphere, and tell the conditions under which these storms can form.
4. Identify the sources of the major pollutants that threaten our environment, and explain how pollution can affect our weather and climate.

Discussion

Two of the most obvious manifestations of weather are wind and precipitation. Wind not only serves as a distribution mechanism to moderate the temperature extremes that would exist on Earth without such large-scale air circulation, it also provides localized cooling and protection from the concentration of airborne pollutants. On the negative side, wind can sometimes produce great amounts of damage and even cause loss of life when storm conditions become severe.

Precipitation is also the result of atmospheric processes involving air movement. In this case, the amount of moisture in the air, and whether the particular air mass under consideration is stable or unstable, determines the type and amount of precipitation that occurs. A close interdependence among temperature, dew point, and moisture content plays a crucial role in the weather picture and in the overall climate of any given geographic region on Earth.


Section  20.1Condensation and Precipitation

When water vapor in the atmosphere is cooled to the dew point, it does not always condense out into clouds. If air is cooled below the dew point and condensation has not occurred, the air is said to be supersaturated or supercooled. Condensation is greatly enhanced and often quite rapid when the proper hygroscopic nuclei, such as dust particles or ice crystals, are available. These nuclei initiate the condensation of droplets that eventually fall as precipitation. This process not only causes rain (or snow) but also serves as a cleaning mechanism by removing the dust particles from the air.

Two processes are responsible for the formation of raindrops that are large enough to fall as precipitation. One is the Bergeron process, in which ice crystals in the upper portions of a cloud become the condensation nuclei for large droplets that then fall as precipitation. The other is the coalescence process, which involves the collection of many small droplets into large ones because of collisions between the smaller droplets. The result can take the form of rain, snow, sleet, or hail. Moisture can also condense directly out of the air on the surface of the ground as dew or frost.


Section  20.2Air Masses

An air mass is a localized region of the atmosphere that has taken on physical characteristics that differ from those of the surrounding stable air. The physical properties of air masses are related to the source regions over which they form. These regions are grouped into general classes by the surface itself, either maritime (over water) or continental (over land), and the general latitude (arctic, polar, tropical, or equatorial) of the region in which they form. A very important characteristic of an air mass is its temperature in relation to the temperature of the surface over which it is moving. Air masses are therefore classified as warm air masses or cold air masses.

The boundaries of air masses are referred to as fronts or frontal zones. These zones mark the interaction of large warm and cold air masses where they meet. When these frontal zones produce unstable weather conditions, large-scale storms may result. The severity of the weather along such a front is determined by the difference in temperature between the two air masses and the vertical slope of the interface itself. Warm air in an advancing warm front often rises and climbs over the denser, cooler air in a cold front. This can result in severe turbulence and heavy precipitation as the rising warm air cools below its dew point.

Cold fronts generally travel more quickly than warm fronts, and when a cold front moves into an area, the temperature often drops rapidly and dramatically. This temperature drop is often accompanied by severe rainfall or snowfall, and because of its rapid onset, is commonly referred to as a cold snap. Advancing warm fronts move more slowly and generally produce a more gradual change in weather conditions with less violent storm activity. Precipitation is often prevalent along the leading edges of warm fronts as well as cold fronts.


Section  20.3Storms

Severe atmospheric disturbances are called storms. The extent and severity of storms depend on many factors that are covered in detail in the textbook. Some common types of localized storms are rainstorms, thunderstorms, ice and snowstorms, and tornadoes.

One of the most violent and widespread global storm systems is called a tropical storm. Tropical storms go by different names in various parts of the world, but they are all characterized by high-speed rotating winds that cover wide areas. Other names often used for such storms are hurricane, typhoon, and cyclone. These storms build up over water, which provides them with plenty of moisture and energy from latent heat. They often have winds associated with them in excess of 150 to 200 mi/h. Such winds can be awesomely destructive when they reach shore (make landfall). Over land, the storms lose their energy, but vast amounts of damage and even loss of life can be caused by flooding and wind. One of the primary duties of the National Weather Service is to locate and track storms and provide warnings about when, where, and how severe hurricanes will be when they strike land. Many lives can be saved when people take such warnings seriously and prepare for these events or evacuate areas that severe weather is known to be approaching.


Section  20.4Atmospheric Pollution

Pollution can affect our health, living conditions, and weather. Some people even feel that our global climate may already be adversely influenced by this problem. There are several natural phenomena that affect our environment over which we have little or no control, such as volcanoes, heavy rains and floods, and fires caused by lightning. We will therefore limit our definition of pollution to mean any undesirable contributions to the environment resulting from the activities of human beings.

As the population of Earth (currently over 6 billion) continues to increase, the discharge of waste products into our environment is becoming an increasingly complex and critical problem. When the quantity of some pollutants is small, nature can accept this abuse and recover; that is, the overall quality of air or water is not permanently degraded. This is no longer the case with many of the polluting by-products of modern living, and it appears that severe and perhaps irreversible damage may soon be done to our environment if we do not learn to be more responsible.

Many different kinds of pollutants affect our environment. The most serious of these seem to be the ones that degrade the purity of the air. The products of combustion and industrial activity are the primary sources of such pollution. The complete burning of fossil fuels such as coal, gas, and oil uses up oxygen, forms water and carbon dioxide, and can produce nitrogen oxides when combustion temperatures are high. Incomplete combustion of carbon-based materials can produce carbon monoxide and various other products that result in smog, acid rain, and perhaps even long-term climatic changes on a global scale. Some of the hydrocarbons produced by incomplete combustion further complicate this problem by reacting with oxygen in the air in the presence of sunlight to form photochemical smog. Airborne pollutants cause severe eye and lung irritation, form carcinogens (substances that can cause cancer), and increase the ozone (O₃) levels near large cities where hydrocarbons combine with nitrogen oxides from vehicle exhaust.


Section  20.5Pollution and Climate

Air pollution involves the activities of human beings, and its major concentration is around heavily populated and industrialized areas. Thermal pollution gives rise to the urban heating effects that can cause the temperature and retained heat near cities to be higher than in adjacent rural areas. This can cause changes in convection patterns and lead to localized, self-contained, thermal circulation systems that trap pollutants in these urban areas. The same process can result in the formation of a dust dome over a city, which prevents incoming solar radiation (insolation) from correcting the thermal-inversion effects that sometimes occur.

The overall climate of the planet may change because of atmospheric pollution. Although there is no conclusive proof, it appears that air pollution may affect wind patterns, climate zones, and local temperature extremes. Such processes could lead to expansion of desert areas and elevated or lowered overall atmospheric temperature, which in turn could result in the melting the polar ice caps or cause the onset of a new ice age. These effects could be triggered by changes in Earth's albedo if the formation of clouds in the atmosphere is affected by pollutants. Detrimental changes may occur in the large-scale wind patterns and ocean currents that distribute heat, moisture, and nutrients around the world. A good example of this is El Niño. Excess amounts of sulfur dioxide and nitrogen dioxide in the atmosphere can give rise to sulfuric and nitric aerosols that cause acid rain. The release of CFCs into the air can also produce a thinning of the ozone layer that protects the surface of Earth from harmful ultraviolet radiation.

Many unanswered questions remain about the short- and long-term effects of pollution on our environment. It is, however, becoming increasingly evident that we must all pay more attention to, and take better care of, the environment, or future generations could find themselves in serious trouble.

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