Weather refers to the state of the atmosphere and includes temperature, precipitation, humidity, cloudiness, visibility, pressure, and winds. Weather, as opposed to climate, includes the short-term variations of the atmosphere, ranging from minutes to months. Climate is typically considered the weather that characterizes a particular region over time.
The weather must be measured and records kept to gain an understanding of the forces at work and to yield the information on the averages and extremes. By studying weather records, atmospheric scientists may be able to predict the weather ahead on scales of weeks to months with greater accuracy and modify more successfully the weather to increase precipitation or ameliorate severe storms.
Causes of Weather
The five factors that determine the weather of any land area are: the amount of solar energy received because of latitude; the area's elevation or proximity to mountains; nearness to large bodies of water and relative temperatures of land and water; the number of such storm systems as cyclones, hurricanes, and thunderstorms resulting from air-mass differences; and the distribution of air pressure over the land and nearest oceans, which produces varying wind and air mass patterns.
How these five factors interact over the North American continent is an excellent example of how the weather of the United States is produced. Because the landmass of North America encompasses a greater range of latitude than longitude (10° to 80° north latitude), a great amount of differential heating occurs. This in turn creates air-mass differences. The presence of a large water area, the Gulf of Mexico, below the southern states affects the character of air masses and placement of pressure centers and storm systems over the eastern half of the continent. Warm, moist air from the south often meets cold, dry air from the north over the central United States. These contrasting air masses include: continental Arctic and polar, from cold land sources; maritime polar, from cold ocean regions; and the warmer and moist Gulf or Atlantic oceanic sources. Air from the Pacific Ocean affects the weather in the western mountains of the United States, which in turn affects the interaction of cold and warm air in the eastern United States.
The air movements resulting from these five weather-producing factors of North America provide an exceptional variety of weather among regions, including droughts, floods, and every known form of severe storm, including hail, ice storms, and tornadoes. These extremes alternate with calm periods of clouds or sunshine. Thunderstorms yield about half of the total precipitation in most of the United States — 80 percent in drier mountain climates, 65 percent in the Great Plains, 50 percent in the Midwest, and 40 percent in the East.
The weather in most places is sensitive to a few key factors. For example, severe drought in the sub-Saharan region of Africa is thought to occur when onshore winds from the Atlantic Ocean change direction by at least 60° in a relatively small area. A seasonal shift of this type is presumably related to slight differences in ocean temperatures. Such differences in turn may have resulted from changes in cloudiness related to a slight shift in hemispheric pressure patterns.
In most locales the weather changes as a result of the diurnal (night/day) cycle and the annual cycle. The latter encompasses daily, monthly, and seasonal variations. These two cycles reveal the Sun as the major factor influencing the weather. At a given time the weather differs greatly with distance. During heavy rains, for example, the differences in the amount of rainfall between regions in proximity are often great. The diurnal cycle exists everywhere but varies by climate type. Within a climatic zone it varies by season. Honolulu, with an equable oceanic weather regime dominated by the trade winds, has a night-to-day range of 6° C (11° F) in July, whereas St. Louis, with a continental climate, has an average diurnal range of 12.8° C (23° F) in July. In January, however, the St. Louis diurnal range is 9° C (16° F).
All weather information for a given area is derived by a local weather station, and daily high and low temperature values for a month are averaged. These two values are averaged to yield the "mean," or "normal," monthly temperature. The highest average values of precipitation occur in locales with strong maritime or orographic influences. These values occur in warm or cold climates. Extremely low precipitation averages occur in interior zones far from moist air or at locales where cold oceanic currents stabilize the air.
In a similar fashion, the daily wind and humidity values are averaged to yield their monthly means, and the daily rainfall and snowfall values are totaled — along with the number of days with rain, thunderstorms, freezing temperatures, and clear or cloudy skies. Certain monthly extremes are also identified on a monthly basis, including the highest and the lowest temperature, the heaviest one-day precipitation (rain or snow), and the fastest wind speed. These summaries are often combined and become the basis for describing the weather of a region. Atmospheric scientists also study weather-producing conditions, such as fronts and high- and low-pressure areas, to describe how the region's weather is produced.
Weather variations that are excessive during a given time span are often called extremes. Records of weather are examined to define extremes. Absolute extremes are the highest and lowest values of a weather element observed over the entire period of record. The highest temperature ever recorded is 57.8° C (136° F), in the Libyan Desert. The lowest is –89.2° C ( –128.6° F), in Antarctica. The world's highest one-year precipitation for a given area was 26,461 mm (1,042 in) at Cherrapunji, India, and the lowest was 0.8 mm (0.03 in), in the Atacama Desert.
Great temperature extremes have been recorded at a number of points within the United States. For example, the record highest and lowest temperatures at Fairbanks, Alaska, are 37.2° C and –54.5° C (99° F and –66° F), respectively, a difference of 91.6° C (165° F ). Those at Boise, Idaho, are 45.6° C and –42.8° C (114° F and –45° F), a range of 88.4 °C/159° F. The greatest temperature change during a 24-hour period in the United States occurred in Billings, Mont., when the temperature fell 56.5° C (101°F), from 7.8° C to –48.3° C (46° F to –55° F).
by Stanley A. Changnon, Jr.
Bibliography: Batten, Louis J., Weather, 2d ed. (1985); Bender, Lionel, Weather: Science Facts (1992); Eagleman, Joe R., Weather Concepts and Terminology (1989); Holford, Ingrid, Guinness Book of Weather Facts and Feats, 2d ed. (1984); Mason, John, Weather and Climate (1991); Ruffner, J. A., and Bair, F. E., Weather Almanac, 7th ed. (1994); Upgren, Arthur, and Stock, Jurgen, Weather: How It Works and Why It Matters (2000).