AIR MASS
a large body of air that has nearly the same temperature and humidity at the same altitude throughout

AIR PRESSURE
the force that air exerts

ATMOSPHERE
the layer of gases that surrounds the earth - The atmosphere is often classified into five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere.

BAROMETER
a device that measures air pressure and predicts changes in the weather

CLIMATE
the pattern of weather in an area over a long period of time

CLOUD
a mass of very tiny water droplets, called cloud droplets, in the sky

COALESCENCE
the growth of a tiny cloud droplet, by collision with other droplets, into a raindrop, heavy enough to fall from a cloud

CONDENSATION
the change from a gas to a liquid - Water vapor condenses into liquid water.

CONDENSE
to change from a vapor to a liquid

DEW POINT
the temperature at which air can't hold any more moisture (i.e., relative humidity is 100%) and the water vapor it condenses - When air reaches its dew point, clouds, fog, or dew can form.

ECOSYSTEM
the coexistence and interaction of living and non living things in an environment

EVAPORATE
to change from a liquid to a gas

EVAPORATION
the change from a liquid to a gas - Liquid water evaporates into water vapor.

EXOSPHERE
the outermost layer of the atmosphere

FOG
a cloud in contact with the ground

FREEZING POINT
temperature at which a liquid changes to a solid

FRONT
the boundary between air masses of different temperatures and humidities - A cold front forms when a cold air mass pushes into a warm air mass, and a warm front forms when a warm air mass replaces a retreating cold air mass.

GREENHOUSE EFFECT
the warming of the earth that occurs when the atmosphere traps heat waves radiated from the earth in much the same way that heat in a greenhouse is trapped by the glass walls and roof

HAIL
ice crystals surrounded by layers of ice

HIGH PRESSURE SYSTEM
an area of above normal pressure (also known as a High or anti-cyclone.) - In the Northern Hemisphere, winds of a high-pressure system spiral clockwise and outward; in the Southern Hemisphere they spiral counterclockwise and outward.  A high-pressure system usually brings fair weather. 

HUMIDITY
the amount of water vapor in the air - The relative humidity is the amount of water vapor in the air compared to the maximum the air could hold at that temperature, expressed as a percentage.

HURRICANE
a severe storm of spiraling winds that starts over warm ocean water

IONOSPHERE
the layer of the atmosphere named for the electrically charged atoms or molecules (ions) it contains.  The ionosphere can overlap some other atmospheric layers.

JET STREAM
a ribbon of high speed winds that blows high above the earth in the upper 
troposphere

LIGHTNING
an electric charge that travels between a cloud and the ground or between clouds

LOW PRESSURE SYSTEM
an area of below normal pressure (also known as a Low or cyclone)- In the Northern Hemisphere, winds of a low-pressure system spiral counterclockwise and inward; in the Southern Hemisphere they spiral clockwise and inward.  A low-pressure system usually brings stormy weather. 

MELTING POINT
temperature at which a solid changes to a liquid

MESOSPHERE
the layer of the atmosphere that lies above the stratosphere

METEOROLOGY
the study of the state of the atmosphere and of weather forecasting - A scientist who studies meteorology is called a meteorologist.

PRECIPITATION
rain, snow, and other forms of water or ice that fall from clouds

PREVAILING WINDS
winds that blow more or less steadily from a specific direction - The three main prevailing winds are the trade winds, the polar easterlies, and the prevailing westerlies.

REVOLUTION
orbital motion around a point - The earth revolves around the sun.

ROTATION
the act of spinning on an axis - The earth rotates on its axis.

SNOW
solid precipitation that falls in the form of white ice crystals

STRATOSPHERE
the layer of the atmosphere that lies above the troposphere

SUPERCOOLED
the condition in which water remains in a liquid form at below-freezing temperatures

THERMOSPHERE
the layer of the atmosphere that lies above the mesosphere

THUNDER
the shock waves from rapidly expanding air caused by heating as lightning passes through it - The rapidly expanding air causes a bang or rumble.

TROPOSPHERE
the layer of the atmosphere that lies closest to the earth - Most of our weather occurs in the troposphere.

WATER CYCLE
the movement of water as it evaporates from lakes, rivers, soil, and oceans, rises into the atmosphere, condenses, and falls as precipitation

WEATHER
condition of the earth's atmosphere as it affects its inhabitants

WEATHERING
process by which rocks and materials are broken down by wind, water, and ice.