acid rain: rain having a pH lower than 5.6 (the pH of natural rainwater), usually caused by sulfuric acid and/or nitric acid from air pollution.
advection: Horizontal transport of temperature, pressure, moisture or some other atmospheric attribute solely by the wind.
aerosol: particulate matter put into the atmosphere. Examples are smoke and dust.
air mass: a body of air that has the same temperature, moisture, and stability properties.
altimeter: an instrument that measures the altitude of something with respect to a set level.
altitude: the vertical distance above a certain level, usually sea level or ground level.
altocumulus: a middle cloud type that is white and/or gray in color and occurs as a layer or patch with a waved aspect. Generally have shadowed parts.
altostratus: a middle cloud type that appears as a gray or bluish sheet of stretched out, fibrous, or uniform appearance. Overcast clouds.
anemometer: an instrument that measures wind speed.
anticyclone: a body of air that circulates clockwise and outward in the Northern Hemisphere and counterclockwise and outward in the Southern Hemisphere, and is associated with a high atmospheric pressure.
anvil: the top part of a cumulonimbus cloud that is flat and spread out, with a shape that looks like a blacksmith's anvil.
aphelion: the point where the earth is farthest (152 million kilometers) from the sun while in its orbit. Currently, this occurs on July 5.
aurora: appears as greenish-white and sometimes pink light in the night sky over the middle and high latitudes. Called the aurora borealis in the Northern Hemisphere and the aurora australis in the Southern Hemisphere. Caused by electrons interacting with each other in the upper atmosphere.
barometer: instrument that measures atmospheric pressure.
Bermuda High: a subtropical anticyclone that occurs over the North Atlantic Ocean.
biosphere: the zone between the earth and the atmosphere in which most life forms reside.
black frost: when vegetation is frozen on the inside during a frost in which there is no moisture. A black frost will almost always kill the plant.
boiling point: The temperature at which a substance will change from a liquid state to a gas state. The boiling point of water is 100°C and 212°F.
calm: when the air conditions have no movement or wind.
calorie: the amount of heat required to raise the temperature of one gram of water one degreeCelsius.
ceilometer: an instrument used to measure cloud height.
cirriform: describes high clouds formed with small, mostly ice particles that result in white, transparent clouds that often produce a halo when the sun shines through them.
cirrus: a high cloud made up of ice crystals that appear fibrous and silky.
climatology: scientific study of the climate, the causes of a climate, and the use of data to solve climate related problems.
cold front: when a colder, higher pressure air mass moves so that cold air replaces warmer air. Storm activity is often associated with the moving in of a cold front.
condensation: when a vapor becomes a liquid due to some change. Occurs often when the temperature becomes cooler, the humidity becomes greater, or the air pressure changes.
conduction: transfer of energy through a body by particles or molecules within that body. Energy flows from the hotter parts to the cooler parts.
contrail: a trail of condensed water vapor formed by jet aircraft flying at high altitudes. This trail is formed by the water vapor added to the air by engine exhaust, as well as particles in the exhaust that act as cloud nuclei.
convection: vertical energy transfer in the atmosphere.
cooling degree day: a measurement used to determine how much fuel will be needed to cool indoor environments. There is one cooling degree-day unit for each degree that the mean daily temperature departs from 65 °F or 18 °C.
cumulonimbus: a dense cloud with a lot of vertical development that looks like a large tower. Often the top is flattened, and spreads out above the cloud in an anvil shape. This is a storm cloud, and is often associated with precipitation and severe weather.
cumulus: a low cloud that develops from the bottom up. Appear white and puffy, with a cauliflower-like top. Small ones are associated with fair weather(cumulus humilis), but as they grow vertically, they can produce rain showers (cumulus congestus).
degree: 1) a unit of temperature, describing an interval on a scale of Fahrenheit or Celsius. 2) a unit of angular distance, used to describe distance of latitude and longitude.
degree-day unit: a measure of how much the mean daily temperature differs from a base temperature in Fahrenheit and Celsius. Used to determine how much fuel will be needed to heat or cool indoor environments.
desertification: when an area begins to develop desert-like conditions due to lack of water, deforestation, overgrazing and over cropping.
dew: when liquid water from the atmosphere condenses on plants and objects near the ground.
dewpoint: the temperature that air must reach before condensation can occur. The air is saturated at this point.
diffuse sky radiation: the energy from the sun that is scattered by molecules, clouds and aerosols in the atmosphere.
direct radiation: the energy from the sun that hits the earth without being obstructed by clouds, molecules, or aerosols. It is the sunlight that creates a shadow at the surface.
diurnal: referring to daily phenomena.
drought: a long period of time where there is abnormally dry weather due to little or no precipitation.
dry adiabatic lapse rate: the rate at which an unsaturated parcel of air will decrease in temperature as it moves through the atmosphere. The rate is a decrease of about 9.767°C per kilometer upwards.
dry season: in certain climates, it is a period of one month or more where precipitation is at a minimum.
dryline: a boundary that separates warm, dry air from warm, moist air. This boundary usually represents a zone of instability where thunderstorms often develop due to the mixing of the different types of air.
dust devil: a small rotating column of wind that can be seen due to the dust and debris that it picks up.
duststorm: when strong winds blow over a dusty area, blowing the dust into the air and reducing visibility significantly.
easterlies: winds that come from the East. Examples are equatorial easterlies, tropical easterlies (trade winds), and polar easterlies.
eclipse: when one celestial body, such as the sun or the moon, hides another from view.
El Niño : warming of ocean waters in the eastern tropical pacific, resulting in warmer waters off the coasts of Ecuador and northern Peru. Associated with heavy rainfall in those regions, and also affects weather patterns in North America as well. Usually begins to occur around Christmas, and is so named for El Niño, the Spanish name for the Christ Child.
electromagnetic radiation: form of energy emitted through space and reaches the earth. Solar electromagnetic radiation comes from the sun and drives all energy processes on earth.
electromagnetic radiation spectrum: all the types of radiation put in order from smallest wavelength to largest wavelength. Please see table* below.
elevation: vertical distance from sea level to a point affixed to the ground. The elevation of a mountain, for example, may be 10,000 feet.
ENSO Cycle: El Niño Southern Oscillation Cycle. The ocean-atmosphere interactions in the tropical and equatorial Pacific region involving abnormal sea surfae temperatures and atmospheric pressures. These abnormalities often result in changes in the trade winds. Occurs at 2-7 year intervals and may last up to two years.
equator: imaginary circle having 0° latitude that runs horizontally around the center of the earth. Is equally far from both the North and South poles.
equinox: either of two points where the sun's rays will shine directly over the equator at noon, and all locations on earth will theoretically experience twelve hours of daylight. The vernal equinox occurs on March 21 and the autumnal equinox occurs on September 22 in the Northern Hemisphere. The dates are reversed for the Southern Hemisphere.
erosion: when rock and soil are moved from one place to another by running water, precipitation, ice, or wind.
evaporation: the change of state from a liquid to a vapor.
evapotranspiration: the combined water vapor put into the air through evaporation from water on earth's surface and plants giving off water to the atmosphere.
exosphere: the outermost portion of the atmosphere.
Fahrenheit: temperature scale where water freezes at 32 °F and water boils at 212°F at sea level air pressure.
flash flood: a flood that rises and falls rapidly with little or no advance warning, usually due to intense rainfall over a small area.
flood: when water overflows the confines of a body of water and gathers over low-lying areas.
fog: a visible gathering of water droplets suspended in the air near the earth's surface.
freezing point: the temperature at which a substance changes from a liquid to a solid. For water, it is 0°C or 32°F.
front: boundary where two air masses with different temperatures and moisture content meet.
frost: 1) a cover of ice crystals produced when atmospheric water vapor is deposited directly on a surface when the temperature is below freezing, 2) the condition that exists when the temperature of the earth's surface and earth-bound objects falls below freezing. An example is a killing frost, where a frost will end the growing season in agriculture because it freezes crops and they die.
frost point: temperature where air is cooled enough to become saturated, but forms ice instead of dew.
Fujita Tornado Scale: classifies tornadoes from F0 (wind speeds of 40-72 mph) to F5 (261-318 mph) on the basis of rotational speeds derived from the damage they cause. Developed by T. Theodore Fujita.
funnel cloud: a quickly rotating column of air coming down from a cloud base but not reaching the ground. When it touches the ground it becomes a tornado.
greenhouse effect: a natural occurrence caused by certain greenhouse gases in the Earth's atmosphere. Because greenhouse gases trap longwave energy and reemit it back to the Earth's surface, the global temperature is changed 33 degrees Celsius, or 59 degrees Fahrenheit warmer than it would otherwise be. Some scientists think that the increase in greenhouse gases in the atmosphere (caused by pollution- mainly the burning of fossil fuels and deforestation) may be causing a rise in global temperature.
Gulf Stream: a warm ocean current that flows from the Gulf of Mexico and the Carribean Sea north along the North American coast, and northeast across the Atlantic Ocean.
halo: a ring of light that appears to encircle the sun or moon. This occurs when the light from the sun or moon shines through cirrus clouds.
Heat Index: temperature that describes how hot it feels taking into account high air temperatures and high humidity levels.
heating degree day: a measurement used to determine how much fuel will be needed to heat indoor environments. There is one heating degree-day unit for each degree that the mean daily temperature departs from 65 °F or 18 °C.
high clouds: clouds found between 3-8 kilometers (10000-25000 feet) in polar regions and 6-18 kilometers (20000-60000 feet) in tropical regions. Contains cirriform clouds (cirrus, cirrocumulus, and cirrostratus).
high pressure system: area of maximum air pressure and anticyclonic air movement.
humidity: the measure of how much water there is in the air.
hurricane: a severe tropical cyclone with maximum 1-minute sustained surface wind speeds greater than 74 mph.
hydrologic cycle: the exchange of water between the earth, the oceans, and the atmosphere. Also called the water cycle.
hydrosphere: the part of planet earth that is covered with water and ice.
Indian summer: occurring during mid to late autumn after the first frost, a period of unusually warm weather with clear skies and cool nights.
infrared radiation: the portion of energy that has a wavelength between 0.8 micrometers (just above visible radiation) to 0.1 millimeters (microwave radiation).
instability: a state of the atmosphere when a parcel of air will keep moving either up or down from its starting position when moved. An unstable atmosphere is often associated with severe weather.
Intertropical Convergence Zone (ITCZ): belt of clouds and thunderstorms that runs parallel to the equator and marks where the trade winds of both hemispheres come together. Shifts seasonally, and is associated with monsoons and dry seasons in some regions.
isobar: a line of equal pressure resembling a contour line on a map.
isotherm: a line passing through points of equal temperature on a map.
jet stream: narrow river of very strong horizontal winds usually located in the upper troposphere along fronts.
Kelvin: the SI (Standard International) unit of temperature. A degree symbol is not used. One Kelvin equals one degree Celsius, but the base temperature of Kelvin is absolute zero (-273° C), where there is absolutely no movement of molecules at all.
Knot: a measure of speed equal to the velocity of one nautical mile traveled in one hour. Usually used in marine studies. One knot equals 1.151 miles per hour or 1.852 kilometers per hour.
La Niña: a period of strong trade winds and abnormally low sea surface temperatures in the central and eastern tropical Pacific. It is the opposite of an El Niño. Spanish for little girl child.
latent heat: heat released or absorbed per unit of mass when a substance changes phase. When water evaporates from liquid to steam, it absorbs heat. When liquid water freezes, it releases latent heat.
latitude: the distance in degrees on the earth measured from the equator, running horizontally. Lines of latitude are described as North or South of the equator, and are parallel to each other.
lee: the side of an object that is shielded from the wind.
lithosphere: the outer solid portion of the planet earth, made up of the earth's crust and upper mantle. All the rock on earth.
longitude: the distance in degrees on the earth measured East and West from the Prime Meridian (in Greenwich England). Lines of a certain degree of longitude are called meridians, and because they run vertically over the earth, they are not parallel. Meridians converge at the North and South Poles.
low clouds: clouds found between the earth's surface and 2 kilometers (0-6500 feet) in all geographic regions. Contains stratocumulus and stratus clouds, and sometimes nimbostratus, cirrus and cumulonimbus clouds.
low pressure system: area of minimum air pressure, and cyclonic air movement.
lunar eclipse: when the earth is in a direct line between the sun and the moon. Because the moon reflects the sun's light and it cannot do this when the earth is in the way, it may appear dim or even orange in color.
meridian: If a circle is drawn vertically around Earth at its largest point and ran through both poles, one half of that circle running from the North pole to the South pole would be a meridian. Describes a line of longitude. mesosphere: part of the atmosphere that has an altitude of between about 50 kilometers and about 70-80 kilometers.
meteorology: study of the phenomena of the atmosphere. Includes physics, chemistry and dynamics of the atmosphere as well as the direct effects upon the earth's surface, oceans and life in general. Oriented towards developing a complete understanding, accurate prediction, and artificial control of atmospheric occurrences.
middle clouds: clouds found between 2-4 kilometers (6500-13000 feet) in polar regions and 2-8 kilometers (6500-25000 feet) in tropical regions. Contains altocumulus and altostratus clouds, and sometimes nimbostratus.
moist adiabatic lapse rate: the decrease in temperature as a saturated parcel of air moves up through the atmosphere. This rate is 0.55°C per 100 meters.
monsoon: when the principal wind direction changes noticeably with the season. The main cause is the heating and cooling of a nearby ocean depending on summer and winter, such as the Indian Ocean and the India monsoon for example.
nimbostratus: usually considered a middle cloud, is gray and often dark. The edges are not easy to see because of continuously falling rain, snow, or sleet. This cloud does not have lightning, thunder or hail.
nimbus: a rain cloud.
Northern Hemisphere: the half of the earth located above the equator, from 0° to the North Pole.
occluded front: when a cold front overtakes a warm front, and three air masses with different temperatures conflict. The resulting frontal boundary type will depend on how they meet.
orographic: weather phenomena caused by mountains.
orographic cloud: a cloud whose form is changed as high terrain, like a mountain, changes the airflow over the area. These clouds are often irregularly shaped, like a lens or a banner.
ozone layer: a place in the stratosphere (about 10-50 kilometers up) where the high ozone (O3 molecules) concentration shields the Earth from harmful ultraviolet radiation.
Pascal: The SI (Standard International)unit of pressure equal to the pressure resulting from a force of one Newton acting over an area of one square meter.
perihelion: opposite of aphelion. The point where the earth is nearest to the sun (147 million kilometers) while in its orbit. Currently, this occurs on January 3, but varies when there is a leap year.
permafrost: layer of soil or bedrock beneath earth's surface that is permanently frozen and has been for thousands of years.
pH scale: a measure of the range of acidity and alkalinity of a substance, on a scale from 0-14, with acids being on the lower end and bases on the higher end. A value of 7 is neutral.
precipitation: any form of water particles that fall from the clouds and reach the ground. Examples are rain, snow, sleet, and hail. Fog, dew, frost, and clouds are not forms of precipitation.
psychrometer: instrument used to measure the water vapor content of the air.
radar: an electronic instrument that broadcasts and receives microwave signals back from precipitation areas, and determines their location, height, movement, and intensity.
rainbow: an arc displaying all colors in the visible light spectrum. Formed when light from the sun is reflected and refracted through water droplets. Always appears on the side of the sky opposite of the sun. reflectivity: a measure of how well a surface turns the part of the radiation it receives back in the direction from which the radiation came.
refraction: bending of light as it passes through something with different properties, such as a light beam bending as it travels through glass.
relative humidity: ratio of the actual vapor pressure of the air to the saturation vapor pressure for the surrounding air temperature. In other words, how much water the air is holding divided by how much water the air is capable of holding.
Saffir-Simpson Scale: a hurricane intensity scale created to quantify the amount of damage they will cause upon landfall. The scale is derived using wind speeds and the central atmospheric pressure of the storm, from 1 (minimal damage) to 5 (intense).
saturation: when the air is holding the maximum amount of water that it can hold. If any more water is added to the air, the water will condense and leave the air in the form of liquid.
sea level: the height or level of the sea, used as a reference for elevation. "Above" or "below" sea level.
season: a way of dividing up the calendar year according to regular occurrences such as the equinoxes and solstices. For example, in the Northern Hemisphere, from the vernal equinox to the summer solstice is Spring (March 21 to June21). From that solstice to the autumnal equinox is Summer (June21-September 22). From that equinox to the winter solstice is Autumn (September 22-December 22). From that solstice to the vernal equinox is Winter (December 22-March 21).
SI Unit: short for International System of Units, and denotes the standard unit used internationally for different measures of variables.
solar eclipse: when the moon is in a direct line between the earth and the sun, and the sun appears obscured. The moon's shadow will be cast on the earth, making some parts of the earth dark for a short length of time.
solstice: point in the earth's orbit of the sun where the sun's rays are shining directly as far north or as far south as they possibly can shine, due to the tilt of the earth on its axis. In the Northern Hemisphere, the summer solstice is June 21, and the winter solstice is December 22. The dates are reversed for the Southern Hemisphere.
Southern Hemisphere: the half of the earth located below the equator, extending from 0° to the South pole.
stability: a state of the atmosphere when a parcel of air will return to its starting position when moved up or down. This is because the parcel of air is denser than its surrounding air. Stability is often associated with fair weather.
stratosphere: the layer between the troposphere and the mesosphere in the earth's atmosphere. It is located at an altitude between 10 and 50 kilometers, and is the area where most beneficial ozone formation occurs.
stratus clouds: a low cloud type that forms a gray layer with a uniform base. Does not usually produce precipitation. Overcast skies.
temperature inversion: when a layer of air has a temperature increase instead of decrease with altitude.
thermal pollution: an increase in air or water temperature caused by heat from man-made sources.
thermocline: A temperature gradient within layer of a body of water that is notably greater than the temperature gradients above and below it.
thermometer: an instrument used for measuring temperature.
thermosphere: the atmospheric shell that goes from the top of the mesosphere to outer space. The temperature increases with altitude, and its height is about 80 kilometers.
tidal wave: 1) the periodic variations of sea level caused by the moon's pull, and also caused by earthquakes 2) in everyday language, referring to any unusually high and destructive water level along a shore.
tornado: small mass of air that spins rapidly about an almost vertical axis and forms a funnel cloud that contacts the ground. Comes down from a cumulonimbus cloud and is considered probably the most destructive of all weather systems.
troposphere: the layer of the atmosphere that goes from the earth's surface to the tropopause. The lowest 6-16 kilometers (4-10 miles), where all the earth's weather takes place.
typhoon: a severe tropical cyclone with sustained surface wind speeds greater than 74 mph in the western North Pacific. Same as a hurricane, but given a different name depending on the location of the storm.
ultraviolet radiation:electromagnetic radiation with wavelengths ranging from 1-300 nanometers. Can be divided into three types-UVA (responsible for tanning the skin and some types of skin cancers), UVB (responsible for sunburn and skin cancers), and UVC (potentially lethal radiation, but does not reach the earth's surface because of protection from the ozone layer).
unsaturated: is able to hold more water than it currently is holding.
warm front: any front that moves so that warm air replaces colder air.
warning: for a tornado, thunderstorm, flood, or hurricane. Indicates that the event is definitely going to happen or is in the process of occurring. Issued by the U.S. National Weather Service.
watch: for a tornado, thunderstorm, flood, or hurricane. Indicates that the conditions are ideal for one of the aforementioned events to occur. Issued by the U.S. National Weather Service so that the public can take the necessary precautions or even leave the area before the event occurs.
westerlies: the dominant west-to-east motion of the atmosphere centered over the middle latitudes (35°-65°) of both hemispheres.
wind chill equivalent temperature: air temperature index that attempts to quantify how cold the air feels when skin is exposed to low air temperature and wind. The faster the wind blows, the colder the air will feel.
wind shear: the rate of change of wind speed or wind direction over a short period of time and a short distance. Can occur vertically or horizontally.
wind vane: instrument used to measure wind direction.
zephyr any soft, gentle breeze.
Geer, Ira W.(Ed.). (1996). Glossary of Weather and Climate. Boston: American Meteorological Society.