Chapter 1
Acid rain
Cloud droplets or raindrops combining with gaseous pollutants, such as oxides of sulfur and
nitrogen, to make falling rain (or snow) acidic—pH less than 5.0. If fog droplets combine with
such pollutants it becomes acid fog.
Aerosols
Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere
from either natural or human (anthropogenic) sources, such as the burning of fossil fuels.
Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols.
Air density
See Density.
Air pressure
(atmospheric
pressure)
The pressure exerted by the mass of air above a given point, usually expressed in millibars
(mb), inches of mercury (Hg) or in hectopascals (hPa).
Atmosphere
The envelope of gases that surround a planet and are held to it by the planet’s gravitational
attraction. The earth’s atmosphere is mainly nitrogen and oxygen.
Carbon dioxide
(CO2)
A colorless, odorless gas whose concentration is about 0.038 percent (385 ppm) in a volume
of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is
important in the earth’s atmospheric greenhouse effect. Solid CO2 is called dry ice.
Chlorofluorocarbons Compounds consisting of methane (CH4) or ethane (C2H6) with some or all of the hydrogen
(CFCs)
replaced by chlorine or fluorine. Used in fire extinguishers, as refrigerants, as solvents for
cleaning electronic microcircuits, and as propellants. CFCs contribute to the atmospheric
greenhouse effect and destroy ozone in the stratosphere.
Climate
The accumulation of daily and seasonal weather events over a long period of time.
Cloud
A visible aggregate of tiny water droplets and/or ice crystals in the atmosphere above the
earth’s surface.
Cloudburst
Any sudden and heavy rain shower.
Cyclone
An area of low pressure around which the winds blow counterclockwise in the Northern
Hemisphere and clockwise in the Southern Hemisphere.
Density
The ratio of the mass of a substance to the volume occupied by it. Air density is usually
expressed as g/cm3 or kg/m3.
Drought
A period of abnormally dry weather sufficiently long enough to cause serious effects on
agriculture and other activities in the affected area.
Exosphere
The outermost portion of the atmosphere.
Extratropical
cyclone
A cyclonic storm that most often forms along a front in middle and high latitudes. Also called
a middle-latitude cyclonic storm, a depression, and a low. It is not a tropical storm or
hurricane.
Front
The transition zone between two distinct air masses.
Galaxy
A huge assembly of stars (between millions and hundreds of millions) held together by
gravity.
Heterosphere
The region of the atmosphere above about 85 km where the composition of the air varies with
height.
Homosphere
The region of the atmosphere below about 85 km where the composition of the air remains
fairly constant.
Hurricane
A tropical cyclone having winds in excess of 64 knots (74 mi/hr).
Hypoxia
A condition experienced by humans when the brain does not receive sufficient oxygen.
Inversion
An increase in air temperature with height.
Ion
An electrically charged atom, molecule, or particle.
Ionosphere
An electrified region of the upper atmosphere where fairly large concentrations of ions and
free electrons exist.
Isothermal layer
A layer where the air temperature is constant with increasing altitude. In an isothermal layer,
the air temperature lapse rate is zero.
Knot
A unit of speed equal to 1 nautical mile per hour. One knot equals 1.15 mi/hr.
Lapse rate
The rate at which an atmospheric variable (usually temperature) decreases with height. (See
Environmental lapse rate.)
Low
See Extratropical cyclone.
Mesopause
The top of the mesosphere. The boundary between the mesosphere and the thermosphere,
usually near 85 km.
Mesosphere
The atmospheric layer between the stratosphere and the thermosphere. Located at an
average elevation between 50 and 80 km above the earth’s surface.
Meteorology
The study of the atmosphere and atmospheric phenomena as well as the atmosphere’s
interaction with the earth’s surface, oceans, and life in general.
Middle latitudes
The region of the world typically described as being between 30° and 50° latitude.
Middle-latitude
cyclone
See Extratropical cyclone.
Molecule
A collection of atoms held together by chemical forces.
Nitrogen (N2)
A colorless and odorless gas that occupies about 78 percent of dry air in the lower
atmosphere.
Outgassing
The release of gases dissolved in hot, molten rock.
Oxygen (O2)
A colorless and odorless gas that occupies about 21 percent of dry air in the lower
atmosphere.
Ozone (O3)
An almost colorless gaseous form of oxygen with an odor similar to weak chlorine. The
highest natural concentration is found in the stratosphere where it is known as stratospheric
ozone. It also forms in polluted air near the surface where it is the main ingredient of
photochemical smog. Here, it is called tropospheric ozone.
Ozone hole
A sharp drop in stratospheric ozone concentration observed over the Antarctic during the
spring.
Photodissociation
The splitting of a molecule by a photon.
Plate tectonics
The theory that the earth’s surface down to about 100 km is divided into a number of plates
that move relative to one another across the surface of the earth. Once referred to as
continental drift.
Pollutants
Any gaseous, chemical, or organic matter that contaminates the atmosphere, soil, or water.
Pressure
The force per unit area. See also Air pressure.
Radiosonde
A balloon-borne instrument that measures and transmits pressure, temperature, and humidity
to a ground-based receiving station.
Sounding
An upper-air observation, such as a radiosonde observation. A vertical profile of an
atmospheric variable such as temperature or winds.
Stratosphere
The layer of the atmosphere above the troposphere and below the mesosphere (between 10
km and 50 km), generally characterized by an increase in temperature with height.
Temperature
inversion
An increase in air temperature with height, often simply called an inversion.
Thermometer
An instrument for measuring temperature. The most common is liquid-in-glass, which has a
sealed glass tube attached to a glass bulb filled with liquid.
Thermosphere
The atmospheric layer above the mesosphere (above about 85 km) where the temperature
increases rapidly with height.
Thunderstorm
A convective storm (cumulonimbus cloud) with lightning and thunder. Thunderstorms can be
composed of an ordinary cell, multicells, or a rapidly rotating supercell.
Tornado
An intense, rotating column of air that often protrudes from a cumuliform cloud in the shape of
a funnel or a rope whose circulation is present on the ground. (See Funnel cloud.)
Tropopause
The boundary between the troposphere and the stratosphere.
Troposphere
The layer of the atmosphere extending from the earth’s surface up to the tropopause (about
10 km above the ground).
Visibility
The greatest distance at which an observer can see and identify prominent objects.
Water vapor
Water in a vapor (gaseous) form. Also called moisture.
Weather
The condition of the atmosphere at any particular time and place.
Weather elements
The elements of air temperature, air pressure, humidity, clouds, precipitation, visibility, and
wind that determine the present state of the atmosphere, the weather.
Wind
Air in motion relative to the earth’s surface.
Wind direction
The direction from which the wind is blowing.
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