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cohesion
the attraction of molecules for other molecules of the same kind, such as occurs between water molecules as a result of hydrogen bonding. it causes beading of water on hydrophobic surfaces and surface tension that allows a steel needle to float on a water surface
adhesion
the attraction of molecules of one kind for molecules of another kind; in water, it causes capillary action in which water moves upward against gravity as water molecules stick to the molecules of other substances, such as paper, glass, or soil
phase change
the change in state among the solid, liquid, and gas phases of water (ice, water, and water vapor). each phase change involves the absorption or release of latent heat
freezing and melting
describes the familiar phase changes of water between solid and liquid
condensation
the process through which water vapor in the air becomes liquid water. this is the basis of the process that forms clouds
evaporation
the process through which liquid water becomes water vapor. this phase change is is called vaporization when water is at boiling temperature
deposition
the process through which water vapor changes directly to a solid, as when it attaches to an ice crystal in the formation of frost. you may have seen this on your car windshield on a cold morning or inside your freezer
sublimation
the phase change of ice, a solid, directly to water vapor, a gas. the opposite process is deposition, in which water vapor changes directly to ice
latent heat of vaporization
the heat energy absorbed from the environment in a phase change from liquid to water vapor at boiling point; under normal sea-level pressure, 540 calories must be added to each gram of boiling water to achieve a phase change to water vapor
latent heat of condensation
the heat energy released to environment in a phase change from water vapor to liquid; under normal sea-level pressure, 540 calories are released from each gram of water vapor that changes phase to water at boiling, and 585 calories are released from each gram of water vapor that condenses at 20°C (68°F)
latent heat of sublimation
the heat energy absorbed (680 calories for one gram of ice) in the phase change from ice to water vapor (no liquid phase). the change from water vapor to ice is deposition, which releases a comparable amount of heat
saturation
the point at which air is holding the maximum amount of water vapor is possible at that temperature, and beyond which point any addition of water vapor will cause net condensation; also known as saturation equilibrium, the point at which evaporation and condensation are in balance
humidity
water vapor content of the air. the capacity of the air for water vapor is mostly a function of the temperature of the air and the water vapor
vapor pressure
that portion of total air pressure that results from water vapor molecules, expressed in millibars (mb). at a given dew-point temperature, the maximum capacity of the air is termed its saturation vapor pressure
saturation vapor pressure
the vapor pressure, expressed in millibars, of saturated air (air that is carrying the maximum possible amount of water vapor for the current temperature)
specific humidity
the mass of water vapor (in grams) per unit mass of air (in kilograms) at any specified temperature. the maximum mass of water vapor that a kilogram of air can hold at any specified temperature is termed its maximum specific humidity
atmospheric stability
the tendency of the atmosphere to either encourage or discourage vertical air movement; stable air parcels resist upward displacement, while unstable air parcels rise until they reach an altitude where the surrounding air has a similar temperature and density
adiabatic
pertaining to the change in temperature of a vertically moving parcel of air, cooling by expansion as it rises or heating by compression as it sinks, occurring without any exchange of heat between the air parcel and the surrounding environment
dry adiabatic rate (DAR)
the rate at which an unsaturated parcel of air cools (if ascending) or heats (if descending); a rate of 10°C per 1000 m (5.5 F° per 1000 ft)
moist adiabatic rate (MAR)
the rate at which a saturated parcel of air cools in ascent; a rate of 6 C° per 1000 m (3.3 F° per 1000 ft). this rate may vary, with moisture content and temperature, from 4 C° to 10 C° per 1000 m (2 F° to 6 F° per 1000 ft)
cloud
an aggregate of tiny water droplets and ice crystals suspended in the air and great enough in volume and concentration to be visible
warm clouds
form in the tropics and have temperatures above 0°C (32°F) from top to bottom,, they are great masses of tiny liquid droplets, each invisible without magnification
cold clouds
form in the midlatitudes and have temperatures below -40°C (-40°F). they are masses of ice crystals, typically occurring above 6000 m (20,000 ft) in altitude
mixed-phase clouds
cumulonimbus or thunderhead clouds, containing both liquid cloud droplets and solid ice crystals
cloud droplet
a tiny water droplet, typically 0.02 mm in diameter, that constitutes the initial composition of clouds. they form as rising air cools to the dew-point temperature, becomes saturated, and condenses on cloud-condensation nuclei in the lower atmosphere
cloud-condensation nuclei
microscopic particles (such as dust, soot, or ash) necessary as matter on which water vapor condenses to form cloud droplets; giant cloud condensation nuclei, such as sea salt, have a diameter greater than 0.01 mm
stratiform
flat and layered clouds with horizontal development are classed as this
cumuliform
puffy and globular clouds with vertical development are classed as this
cirroform
wispy clouds, usually high in altitude and made of ice crystals are classed as this
stratus
a stratiform (flat, horizontal) cloud generally below 2000 m (6500 ft)
cumulus
bright and puffy cumuliform clouds up to 2000 m (6500 ft) in altitude
altocumulus
middle-altitude clouds composed of ice and water that occur in several forms: patchy rows, wave patterns, a “mackerel sky,” or lens-shaped “lenticular” clouds
cumulonimbus
a towering, precipitation-producing cumulus cloud that is vertically developed across altitudes associated with other clouds; frequently associated with lightning and thunder and thus sometimes called a thunderhead
fog
a cloud, generally stratiform, in contact with the ground, with visibility usually reduced to less than 1 km (3300 ft)
radiation fog
formed by radiative cooling of a land surface, especially on clear nights in areas of moist ground; occurs when the air layer directly above the surface is chilled to the dew-point temperature, thereby producing saturated conditions
advection fog
active condensation formed when warm, moist air moves laterally over cooler eater or land surfaces, causing the lower layers of the air to be chilled to the dew-point temperature
upslope fog
forms when moist air is forced to higher elevations along a hill or mountain and is thus cooled
valley fog
the settling of cooler, more dense air in low-lying areas; produces saturated conditions and fog
evaporation fog
a fog formed when cold air flows over the warm surface of a lake, ocean, or other body of water; forms as the water molecules evaporate from the water surface into the cold, overlying air; also known as steam fog or sea smoke
precipitation
rain, snow, sleet, and hail, occurring when condensed water droplets or ice crystals in a cloud become large enough to fall with gravity. in the water balance, the moisture supply (P)
collision-coalescence process
the process by which raindrops form in clouds with temperatures above freezing. updrafts of rising air force the condensation of water vapor onto cloud-condensation nuclei, which then move aloft, mixing and colliding to form raindrops that eventually fall under their own weight
rain
precipitation in the form of liquid water droplets that forms by condensation and precipitation in air with a temperature above freezing or by melting of ice crystals as they pass through a war layer of the atmosphere
snow
precipitation in the form of ice crystals, pellets, or aggregates of ice crystals known as snowflakes
freezing rain
sometimes known as glaze. occurs when raindrops fall through a thin layer of subfreezing temperatures, they will become supercooled and, upon reaching the ground, will instantly freeze onto cold surfaces
sleet
ice pellets that form as raindrops fall through a layer of subfreezing air near the ground and freeze
ice storms
winter storms with freezing rain and sleet that can form ice coatings on roads, power lines, and crops
hail
ice pellets formed when graupel (small snow pellets) circulates within a cloud, moving repeatedly above and below the freezing level, adding layers of ice. eventually the graupel forms hail that falls to the ground when it becomes too heavy to stay aloft
air mass
a distinctive, homogeneous body of air that has taken on the moisture and temperature characteristics of its source region
continental polar (cP)
air masses that form only in the northern hemisphere and are most developed in winter and cold-weather conditions. an area covered by this in winter experiences cold, stable, air; clear skies; high pressure; and anticyclonic wind flow. the southern hemisphere lacks the necessary continental landmasses at high altitude
maritime polar (mP)
air masses in the northern hemisphere that sit over the northern oceans. within them, cool, moist, unstable conditions prevail throughout the year
maritime tropical (mT)
there are two types of these air masses that influence northern America; the gulf/atlantic and the pacific. the humidity experienced in the north American east and midwest is created by the gulf/atlantic air mass, which is particularly unstable and active from late spring to early fall. in contrast, the pacific air mass is stable to conditionally unstable and generally lower in moisture content and available energy. as a result, the western US receives lower average precipitation than the rest of the country
convergent lifting
results when air flows toward an area of low pressure. air flowing from different directions forces lifting and displacement of air upward, initiating adiabatic processes
convectional lifting
happens when air is stimulated by local surface heating. air passing over warm surfaces gains buoyancy and lifts, initiating adiabatic processes
orographic lifting
occurs when air is forced over a barrier such as a mountain range. the uplift of a migrating air mass as it is forced to move upward over a mountain range, a topographic barrier. the lifted air cools adiabatically as it moves upslope; clouds may form and produce increased precipitation
frontal lifting
occurs as air is displaced upward along the leading edges of contrasting air masses
rain shadow
the area of the leeward slope of a mountain range where precipitation receipt is greatly reduced compared to the windward slope on the other side
chinook winds
North American term for a warm, dry, downslope airflow characteristic of the rain-shadow region on the leeward side of mountains
cold front
the leading edge of an advancing cold air mass; identified on a weather map as a line marked with triangular spikes pointing in the direction of frontal movement
warm front
the leading edge of an advancing warm air mass, which is unable to push cooler, passive air out of the way; tends to push the cooler, underlying air into a wedge shape; identified on a weather map as a line marked with semicircles pointing in the direction of frontal movement
squall line
a zone slightly ahead of a fast-advancing cold front where wind patterns are rapidly changing and blustery and precipitation is strong
stationary front
a frontal area of contact between contrasting air masses that shows little horizontal movement; winds in opposite directions on either side of the front flow parallel along the front
midlatitude cyclone
an organized area of low pressure, with converging and ascending airflow producing an interaction of air masses; migrates along storm tracks. such lows or depressions form the dominant weather pattern in the middle and higher latitudes of both hemispheres
blizzard
snowstorms with frequent gusts or sustained winds greater than 56 kmph (35 mph) for a period of time longer than 3 hours and blowing snow that reduces visibility to 400 m (0.5 mi) or less
thunderstorm
a type of turbulent weather accompanied by lightning and thunder. characterized by a buildup of giant cumulonimbus clouds that can be associated with squall lines of heavy rain, including sleet, blustery winds, hail, and tornadoes
single-cell thunderstorm
an isolated, short-lived thunderstorm fueled by the rapid upward movement of warm, moist air
lightning
flashes of light caused by tens of millions of volts of electrical charge heating the air to temperatures of 15,000°C to 30,000°C
thunder
the violent expansion of suddenly heated air, created by lightning discharges, which sends out shock waves as an audible sonic bang
supercell thunderstorm
a sever thunderstorm with a deep, persistently rotating updraft that can produce heavy rain, large hail, and tornadoes
derecho
a destructive windstorm characterized by strong linear winds in excess of 93 kmph (58 mph) and a damage path at least 386 km (240 mi) long; associated with thunderstorms and bands of showers crossing a region
tornado
an intense, destructive cyclonic rotation, developed in response to extremely low pressure; generally associated with mesocyclone formation
mesocyclone
a large, rotating atmospheric circulation, initiated within a parent cumulonimbus cloud at midtroposphere elevation; generally produces heavy rain, large hail, blustery winds, and lightning; may lead to tornado activity
funnel cloud
the visible swirl extending from the bottom side of a cloud, which may or may not develop into a tornado. a tornado is one of these that has extended all the way to the ground
waterspout
an elongated, funnel-shaped circulation formed when a tornado exists over water
tropical cyclone
a rotating low-pressure storm system originating in the tropics, with strong winds, thunderstorms, heavy rain, and storm surge; characterized by closed isobars and circular organization
hurricane
a tropical cyclone that is fully organized and intensified in inward spiraling rainbands; ranges from 160 to 960 km (100 to 600 mi) in diameter, with wind speeds in excess of 119 kmph (65 knots, or 74 mph); a name used specifically in the Atlantic and eastern Pacific
typhoon
a tropical cyclone that occurs in the western Pacific; same as a hurricane except for location
storm surge
a large quantity of seawater pushed inland by the strong winds associated with a tropical cyclone
meteorology
the scientific study of the atmosphere, including its physical characteristics and motions; related chemical, physical, and geological processes; the complex linkages of atmospheric systems; and weather forecasting
weather
the short-term condition of the atmosphere, as compared to climate, which reflects long-term atmospheric conditions and extremes. temperature, air pressure, relative humidity, wind speed and direction, daylength, and sun angle are important measurable elements that contribute to it
outgassing
the release of trapped gases from rocks, forced out through cracks, fissures, and volcanoes from within Earth; the terrestrial source of Earth’s water
eustasy
refers to worldwide changes in sea level that are related not to movements of land, but rather to changes in the volume of water in the oceans
hydrologic cycle
a simplified model of the flow of water, ice, and water vapor from place to place. water flows through the atmosphere and across the land, where it is stored as ice and as groundwater. solar energy empowers the cycle
evaporation
the movement of free water molecules away from a wet surface into air that is less than saturated; the phase change of water to water vapor
transpiration
the movement of water vapor out through the pores in leaves; the water is drawn by the plant roots from soil-moisture storage
evapotranspiration
the merging of evaporation and transpiration water loss into one term
interception
a delay in the fall of precipitation toward Earth’s surface caused by vegetation or other ground cover
infiltration
water access to subsurface regions of soil moisture storage through penetration of the soil surface
overland flow
surplus water that flows across the land surface toward stream channels. together with precipitation and subsurface flows, it constitutes the total runoff from an area
surface runoff
surplus water that flows across the ground surface toward stream channels when soils are saturated or when the ground is impermeable; also called overland flow
percolation
the process by which water permeates the soil or porous rock into the subsurface environment
soil-moisture zone
the area of water stored in soil between the ground surface and the water table. water in this zone may be available or unavailable to plant roots, depending on soil texture characteristics
base flow
the portion of streamflow that consists of groundwater
actual evapotraspiration (AE)
the actual amount of evaporation and transpiration occurring at any given location; derived in the water balance by subtracting the deficit from potential evapotranspiration (PE)
potential evapotranspiration (PE)
the amount of moisture that would evaporate and transpire under optimum moisture conditions where adequate precipitation and soil moisture are present. it is the total water demand, which if not satisfied results in a deficit, or moisture shortage
soil-moisture storage
the retention of moisture within the soil. in the water balance, this is the savings account that can accept deposits (soil-moisture recharge) or allow withdrawals (soil-moisture utilization) as conditions change
gravitational water
that portion of surplus water that percolates downward from the capillary zone, pulled by gravity to the groundwater zone
capillary water
soil moisture, most of which is accessible to plant roots; held in the soil by cohesion of water molecules and adhesion between water molecules and soil
field capacity
water held in the soil by hydrogen bonding against the pull of gravity, remaining after water drains from the larger pore spaces; the available water for plants
porosity
the spaces, or voids, between particles in a material such as soil or rock
hygroscopic water
the portion of soil moisture that is so tightly bound to each soil particle that it is unavailable to plant roots; the water, along with some bound capillary water, that is left in the soil after the wilting point is reached
