Air pressure: Air has weight. Air pressure is the weight of the air, or atmosphere, pushing down on Earth. The closer you are to sea level, the higher the air pressure because there?s that much air above you. The higher you are, there is less air pressure. Barometers measure air pressure.
Blizzard: A major snowstorm with strong winds of 35 miles per hour or more. The most notorious blizzard was the ?Great White Hurricane? that paralyized the East Coast of the United States in 1888. (See also Major Storms timeline.)
Clouds: Little drops of water hanging in the atmosphere. Clouds come in a variety of shapes. The major types of clouds are cirrus (thin, feathery), cirrocumulus (small patches of white), cirrostratus (thin, white sheets), stratus (a low, gray blanket), cumulous (flat-bottomed, white, putty), and cumulonimbus (mountains of dark, heavy clouds). (See also What Kind of Cloudy Is It?)
Drought: A long period of no rainfall in a region. Droughts can destroy crops, dry up water supplies, and sometimes lead to widespread hunger or famine. The lack of moisture in the soil can also cause dust storms. (See also Droughts and Heat Waves.)
Global Warming: The theory that the Earth's temperature is growing warmer. In the future, global warming is expected to have damaging consequences to human life and the Earth's ecosystems. Many climatologists believe that increasing concentrations of carbon dioxide and other ?greenhouse gasses? in the atmosphere are the cause of global warming.
Hail: Pellets of ice and snow created within clouds, that then fall to Earth. Hailstones can sometimes be quite large and can cause significant damage. The largest hailstone ever recorded in the U.S. was found in Aurora, Neb., on June 22, 2003. It measured 7 in. in diameter and 18.7 in. in circumference. The costliest U.S. hailstorm took place on July 11, 1990, in Denver, Colorado. The total damage was $625 million.
Humidity: The amount of water vapor in the air. Relative humidity is the amount of water in the air compared to the amount of water the air can hold at that temperature. When the relative humidity reaches 100%, the air has reached its dew point. Once the air reaches this point, the water vapor turns back into water in the form of rain, snow, clouds or fog.
Hurricane: Major storms with strong winds ranging from 40 m.p.h. to 150 m.p.h. Violent storms occurring in the region of the Atlantic Ocean are known as hurricanes. When they appear in the Pacific they are called typhoons. (See also Hurricane Season.)
Lightning: Flashes of electrical discharges moving through the atmosphere during thunderstorms. There are different types of lightning: forked lightning (a jagged streak), sheet or streak lighting (a broad flash across the sky), and very rarely as a ball lightning (an illuminated ball). (See also Lightning Advice.)
Precipitation: Condensed moisture that falls to the earth as rain, sleet, snow, frost, or dew.
Snow: When clouds become too heavy with humidity, water falls from them. In colder clouds, this water forms ice crystals that fall from the sky as snow.
Rainbow: The refraction (bending) of sunlight passing through raindrops or fog sometimes causes a beautiful arc of colors to appear in the sky for a brief period. The sun, the arc, and the person observing it must be aligned just so in order for the rainbow to be visible.
Sleet: A mixture of falling rain and snow, or rain and ice pellets.
Thunder: The loud noise that follows lightning during a thunderstorm. You can estimate how many miles away a storm is by counting the number of seconds between the flash of lightning and the clap of thunder. Divide the number of seconds by five to get the distance in miles. The lightning is seen before the thunder is heard because light travels faster than sound.
Tornado: A tornado is a dark funnel-shaped cloud made up of violently rotating winds that can reach speeds of up to 300 m.p.h. The diameter of a tornado can vary between a few feet and a mile, and its track can extend from less than a mile to several hundred miles. (See also Tornadoes.)
Typhoon: A hurricane taking place in the Pacific Ocean.
Water cycle: The process of water changing from one state to another and its movement from one place to another. For example, when it rains, water drops fall to Earth. This water evaporates from the surface of Earth and enters the atmosphere as water vapor. The water vapor then condenses into droplets that form clouds.
Wind Chill: The wind chill temperature indicates how cold people feel when outside. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. The wind therefore makes it feel much colder. If, for example, the temperature is 0F and the wind is blowing at 15 mph, the wind chill is ?19F. At this wind chill temperature, exposed skin can freeze in 30 minutes. (See also Revised Wind Chill Factor.)