Montreal Weather

A

Absolute humidity
Absolute zero
Absorptivity
Acclimatization
Accretion
Actual evapotranspiraton
Actual vapor pressure
Adiabatic process
Advection
Advection fog
Aerovane
Air density
Air mass
Air pressure
Albedo
Altimeter
Altocumulus
Altocumulus castellanus
Altocumulus lenticularis
Altostratus
Ambient air
Ambient temperature
Anemometer
Aneroid barometer
Annual range of temperature
Anomalies
Arctic air
Atmospheric window
Autumnal equinox

B

Barograph
Barometer
Beaufort scale
Black body
Blizzard
Bora
Bowen ratio
Boyle's law

C

Ceilometer
Centrifugal force
Centripetal force
Charles's law
Chinook
Cirrocumulus
Cirrostratus
Cirrus
Climate
Cloud base
Cloudburst
Cloud cover
Cloud deck
Cloud seeding
Coalescence
Cold fog
Cold front
Condensation
Condensation nuclei
Conduction
Continental air mass
Continental Climate
Continental polar air
Continental tropical air
Contrail (condensation trail)
Convection
Convective condensation level
Convergence
Cooling degree-day
Coriolis effect
Crepuscular rays
Cumulonimbus
Cumulus
Cumulus Congestus
Cup anemometer
Cutoff high
Cutoff low

D

Daily range of temperature
Degree days
Density
Deposition
Deposition nuclei
Desert
Dew
Dew point (dew-point temperature)
Diffraction
Diffuse insolation
Direct insolation
Divergence
Downbursts
Downdraft
Drainage basin
Drizzle
Drought
Dry adiabatic rate
Dry climate
Dust devil (or whirlwind)

E

Eddy
Effective emissivity
Emissivity
Emittance
Entrainment
Environmental lapse rate
Equilibrium vapor pressure
Equinox
Evaporation
Evapotranspiration
Evaporation fog
Exosphere

F

Fall Freeze date
Fall streaks
Foehn
Fog
Forced convection
Free convection
Freeze
Freeze free season
Freezing rain and freezing drizzle
Front
Frontal fog
Frost (also called hoarfrost)
Frost point
Frozen dew
Funnel cloud

G

Geostrophic wind
Glaciation
Glaciated cloud
Glaze
Glory
Graupel
Green flash
Ground fog
Growing degree-day
Growing season

H

Haboob
Hail
Hailstones
Hair hygrometer
Halos
Haze
Heat
Heat capacity
Heat index (HI)
Heat of fusion
Heat of melting
Heating degree-day
Heat lightning
Heiligenschein
Heterosphere
High inversion fog
Highland climate
Hoarfrost
Homosphere
Humid continental climate
Humid Subtropical Climate
Hurricane
Hydrograph
Hygrometer
Hypothermia

I

Ice Cap Climate
Ice fog
Ice nuclei
Ice pellets
Indian summer
Infrared radiation
Insolation
Intertropical convergence zone (ITCZ)
Inversion
Ion
Ionosphere
Iridescence
Isobar
Isotach
Isotherm

J

January thaw
Jet stream

K

Katabatic wind

L

Lake breeze
Lake-effect snows
Land breeze
Lapse rate
Latent heat
Lenticular cloud
Lightning
Longwave radiation

M

Magnetosphere
Mammatus clouds
Marine climate
Maritime air mass
Maritime polar air
Maritime tropical air
Mean annual temperature
Mesoscale
Mesosphere
Meteorology
Microburst
Microclimate
Microscale
Millibar (mb)
Mirage
Mist
Mistral
Mixing ratio
Moist adiabatic rate
Molecular viscosity
Mountain and valley breeze

N

Nacreous clouds
Nimbostratus
Noctilucent clouds
Nocturnal inversion

O

Offshore breeze
Onshore breeze
Orographic uplift
Orographic precipitation

P

Permafrost
Photodissociation
Photon
Pileus cloud
Polar air mass
Polar climates
Potential energy
Potential evapotranspiration (PE)
Potential temperature
Precipitable water vapor
Precipitation
Prevailing wind
Probability forecast
Psychrometer
Pyranometer

R

Radar
Radiant energy (radiation)
Radiation fog
Radiation inversion
Radiosonde
Rain
Rain gage
Rain Shadow
Rawinsonde
Reflection
Refraction
Refractive index
Relative humidity
Rime ice

S

Santa Ana
Saturation vapor pressure
Scattering
Scintillation
Sea breeze
Sea level pressure
Semiarid
Sensible heat transfer
Sensible temperature
Shear
Sheet lightning
Shortwave radiation
Shower
Sleet
Smog
Snow
Snowflake
Snow flurries
Snow grains
Snow pellets
Snow rollers
Snow squall (shower)
Solstice
Southern oscillation
Specific heat
Specific humidity
Spontaneous nucleation (freezing)
Spring freeze date
Squall line
Station pressure
Steam fog
Steppe
Storm surge
Stratocumulus
Stratopause
Stratosphere
Stratus
Subarctic climate
Sublimation
Subsidence
Subsidence inversion
Summer solstice
Sundog
Sun pillar
Supersaturated air
Surface inversion
Synoptic scale

T

Taiga
Temperature
Temperature inversion
Terminal velocity
Thermal
Thermograph
Thermometer
Thermosphere
Thunder
Tipping bucket rain gage
Tornado
Trade winds
Transpiration
Tropical air mass
Tropical depression
Tropical disturbance
Tropical storm
Tropopause
Troposphere
Tundra Climate
Turbulence
Twilight
Typhoon

U

Ultraviolet radiation
Upslope fog
Upslope precipitation
Upwelling
Urban heat island

V

Valley breeze
Valley fog
Vapor pressure
Vernal equinox
Virga
Virtual temperature
Viscosity
Visibility
Visible light
Vorticity

W

Warm front
Water balance
Water budget
Water equivalent
Weather
Weighing bucket rain gage
Wet-bulb depression
Wet-bulb temperature
White frost
Wind chill equivalent temperature
Wind-chill factor
Wind shear
Wind Vane
Windsock
Winter solstice

Absolute humidity

The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.

Absolute zero

A temperature of -273ºC, -460ºF, or 0ºK. Theoretically, there is no molecular motion at this temperature.

Absorptivity

The efficiency of radiation absorption.

Acclimatization

The gradual adjustment of the body to new climatic or other environmental conditions, for example, the adjustment to low levels of oxygen at high altitudes.

Accretion

The growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.

Actual evapotranspiraton

The rate of water lost from vegetation and soil, ordinarily at a slower rate than the potential rate.

Actual vapor pressure

See vapor pressure.

Adiabatic process

A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. In an adiabatic process compression always results in warming, and expansion results in cooling.

Advection

The horizontal transfer of any atmospheric property by the wind.

Advection fog

Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.

Aerovane

A device that resembles a wind vane with a propeller at one end. Used to indicate wind speed and direction.

Air density

Mass per unit volume of air; about 1.275 km per cubic meter at 0ºC and 1000 millibars.

Air mass

A large expanse of air having similar temperature and humidity at any given height.

Air pressure

The cumulative force exerted on any surface by the molecules composing air.

Albedo

The percent of radiation returning from a surface compared to that which strikes it.

Altimeter

An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.

Altocumulus

A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.

Altocumulus castellanus

An altocumulus showing vertical development, individual cloud elements have towerlike tops, often in the shape of tiny castles.

Altocumulus lenticularis

A lens-shaped altocumulus cloud; a mountain-wave cloud generated by the disturbance of horizontal airflow caused by a prominent mountain range.

Altostratus

A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.

Ambient air

The air surrounding a cloud, or the air surrounding rising or sinking air parcels.

Ambient temperature

Temperature of the surrounding (ambient) air.

Anemometer

An instrument designed to measure wind speed.

Aneroid barometer

An instrument designed to measure atmospheric pressure. It contains no liquid.

Annual range of temperature

The difference between the warmest and coldest months at any given location.

Anomalies

Departures of temperature, precipitation, or other weather elements from long-term averages.

Arctic air

A very cold and dry air mass that forms primarily in winter and the northern interior of North America.

Atmospheric window

A region of the electromagnetic spectrum from 8 to 12 µm where the atmosphere is transparent to radiation.

Autumnal equinox

The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.

Barograph

A recording instrument that provides a continuous trace of air pressure variation with time.

Barometer

An instrument that measures atmospheric pressure. The two most common barometers are the mercury barometer and the aneroid barometer.

Beaufort scale

A scale of wind strength based on visual assessment of the effects of wind on seas and vegetation.

Black body

A hypothetical object that absorbs all of the radiation that strikes it. It also emits radiation at a maximum rate for its given temperature.

Blizzard

A severe weather condition characterized by low temperatures and strong winds (greater than 32 mi/hr) bearing a great amount of snow. When these conditions continue after the falling snow has ended, it is termed a ground blizzard.

Bora

A cold katabatic wind that originates in Yugoslavia and flows onto the coastal plain of the Adriatic Sea.

Bowen ratio

The ratio of energy available for sensible heating to energy available for latent heating.

Boyle's law

When the temperature is held constant, the pressure and density of an ideal gas are directly proportional.

Ceilometer

An instrument that automatically records cloud height.

Centrifugal force

A force directed outward, away from the center of a rotating object; equal in magnitude to the centripetal force but in the opposite direction.

Centripetal force

An inward-directed force that confines an object to a circular path; equal in magnitude to the centrifugal force but in the opposite direction.

Charles's law

With constant pressure, the temperature of an ideal gas is inversely proportional to the density of the gas.

Chinook

A warm, dry wind on the eastern side of the Rocky Mountains. In the Alps, the wind is called a Foehn.

Cirrocumulus

A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.

Cirrostratus

A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.

Cirrus

A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

Climate

The accumulation of daily and seasonal weather events over a long period of time. A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Cloud base

The lowest portion of a cloud.

Cloudburst

Any sudden and heavy rain shower.

Cloud cover

The amount of the sky obscured by clouds when observed at a particular location.

Cloud deck

The top of a cloud layer, usually viewed from an aircraft.

Cloud seeding

The introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation.

Coalescence

The merging of cloud droplets into a single larger droplet.

Cold fog

See Supercooled cloud.

Cold front

The leading edge of a cold air mass.

Condensation

Process by which water changes phase from a vapor to a liquid.

Condensation nuclei

Small particles in the atmosphere that serve as the core of tiny condensing cloud droplets. These may be dust, salt, or other material.

Conduction

The transfer of heat by molecular activity from one substance to another, or through a substance. Transfer is always from warmer to colder regions.

Continental air mass

An air mass that forms over land; it is normally relatively dry.

Continental Climate

A climate lacking marine influence and characterized by more extreme temperatures than in marine climates: therefore, it has a relatively high annual temperature range for its latitude.

Continental polar air

Relatively dry air mass that develops over the northern interior of North America; very cold in winter and mild in summer.

Continental tropical air

Warm, dry air mass that forms over the subtropical deserts of the south-western United States.

Contrail (condensation trail)

A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

Convection

Motions in a fluid that result in the transport and mixing of the fluid's properties. In meteorology, convection usually refers to atmospheric motions that are predominantly vertical, such as rising air currents due to surface heating. The rising of heated surface air and the sinking of cooler air aloft is often called free convection. (Compare with forced convection.)

Convective condensation level (CCL)

The level above the surface marking the base of a cumiliform cloud that is forming due to surface heating and rising thermals.

Convergence

An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.

Cooling degree-day

A form of degree-day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. A cooling degree-day is a day on which the average temperature is one degree above a desired base temperature.

Coriolis effect

A deflective force arising from the rotation of the earth on its axis; affects principally synoptic-scale and global-scale winds. Winds are deflected to the right of the initial direction in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Crepuscular rays

Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.

Cumulonimbus

An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud.

Cumulus

A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are called cumulus humilis. Those that exhibit much vertical growth are called cumulus congestur or towering cumulus.

Cumulus Congestus

An upward building convective cloud with vertical development between that of a cumulus cloud and a cumulonimbus.

Cup anemometer

An instrument used to monitor wind-speed. Wind rotation of cups generates and electric current calibrated in wind speed.

Cutoff high

Anticyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Cutoff low

Cyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Daily range of temperature

The difference between the maximum and minimum temperatures for any given day.

Degree days

Computed from each day's mean temperature (max+min/2). For each degree that a day's mean temperature is below or above a reference temperature is counted as one degree day.

Density

The ratio of the mass of a substance to the volume occupied by it.

Deposition

A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. (Also called sublimation in meteorology.)

Deposition nuclei

Tiny particles in the atmosphere that serve as the core of tiny ice crystals as water vapor changes to the solid form. These are also called ice nuclei.

Desert

One of two types of dry climate-the driest of the dry climates.

Dew

Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Dew point (dew-point temperature)

The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. When the dew point falls below freezing it is called the frost point.

Diffraction

The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Diffuse insolation

Solar radiation that is scattered or reflected by atmospheric components (clouds, for example) to the earth's surface.

Direct insolation

Solar radiation that is transmitted directly through the atmosphere to the earth's surface without interacting with atmospheric components.

Divergence

An atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region.

Downbursts

A severe localized downdraft that can be experienced beneath a severe thunderstorm. (Compare Microburst)

Downdraft

Downward moving air, usually within a thunderstorm cell.

Drainage basin

A fixed geographical region from which a river and its tributaries drain water.

Drizzle

Small drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.

Drought

A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry adiabatic rate

The rate of change of temperature in a rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 10ºC per 1000 m (5.5ºF per 1000 ft).

Dry climate

A climate in which yearly precipitation is not as great as the potential loss of water by evaporation.

Dust devil (or whirlwind)

A small but rapidly rotating wind made visible by the dust, sand, and debris it picks up from the surface. It develops best on clear, dry, hot afternoons.

Eddy

A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Effective emissivity

A correction factor, dependent on the radiational characteristics of the earth -atmosphere system, that permits application of black body radiation laws to the earth-atmosphere system

Emissivity

The fractional amount of radiation emitted by a given object or substance in comparison to the amount emitted by a perfect emitter.

Emittance

The rate at which a black body radiates energy across all wave-lengths.

Entrainment

The mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.

Environmental lapse rate

The rate of decrease of temperature with elevation. It is most often measured with a radiosonde.

Equilibrium vapor pressure

The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called saturation vapor pressure.

Equinox

The time when the sun crosses the plane of the earth's equator occurring about March 21 and September 22.

Evaporation

The process by which a liquid changes into a gas.

Evapotranspiration

Vaporization of water through direct evaporation from wet surfaces and the release of water vapor by vegetation.

Evaporation fog

Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

Exosphere

The outermost portion of the atmosphere.

Fall Freeze date

The date of occurrence in the fall of the first minimum at or below a temperature threshold.

Fall streaks

Falling ice crystals that evaporate before reaching the ground.

Foehn

See Chinook.

Fog

A cloud with its base at the earth's surface. It reduces visibility to below 1 km.

Forced convection

On a small scale, a form of mechanical stirring taking place when twisting eddies of air are able to mix.

Free convection

Convection triggered by intense solar heating of the earth's surface.

Freeze

A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost.

Freeze free season

The number of days between the last spring freeze date and the first fall freeze date.

Freezing rain and freezing drizzle

Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze.

Front

The transition zone between two distinct air masses.

Frontal fog

See Evaporation fog.

Frost (also called hoarfrost)

A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point is below freezing).

Frost point

See Dew point.

Frozen dew

The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.

Funnel cloud

A rotating conelike cloud that extends down-ward from the base of a thunderstorm. When it reaches the surface it is called a tornado.

Geostrophic wind

A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.

Glaciation

The conversion of all the supercooled liquid water in a cloud into ice crystals, thus reducing the growth rate of ice crystals and hail.

Glaciated cloud

A cloud or portion of a cloud where only ice crystals exist.

Glaze

A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is called an icing storm.

Glory

Colored rings that appear around the shadow of an object.

Graupel

See Snow pellets

Green flash

A small, green color that occasionally appears on the upper part of the sun as it rises or sets.

Ground fog

See Radiation fog.

Growing degree-day

A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates.

Growing season

The number of days between the last spring freeze date and the first fall freeze date.

Haboob

A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Hail

Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Hailstones

Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair hygrometer

An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Halos

Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Haze

Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat

A form of energy transferred between systems by virtue of their temperature differences.

Heat capacity

The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat index (HI)

An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat of fusion

Heat released when water changes phase from liquid to solid; 80 calories per gram

Heat of melting

Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating degree-day

A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69 ºF (18 ºC); computed by subtracting the day's average temperature from 65 ºF.

Heat lightning

Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

Heiligenschein

A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

Heterosphere

The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High inversion fog

A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland climate

Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Hoarfrost

Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

Homosphere

The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid continental climate

A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40 and 50º north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate

A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Hurricane

A severe tropical cyclone having winds in excess of 64 knots (74 mi/hr).

Hydrograph

An instrument that provides a continuous trace of relative humidity with time.

Hygrometer

An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Hypothermia

The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.

Ice Cap Climate

A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice fog

A type of fog composed of tiny suspended ice particles that forms at very low temperatures.

Ice nuclei

Particles that act as nuclei for the formation of ice crystals in the atmosphere.

Ice pellets

See Sleet

Indian summer

An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Infrared radiation

Electromagnetic radiation with wavelengths between about 0.7 and 1000 µm. This radiation is longer than visible radiation but shorter than microwave radiation.

Insolation

The incoming solar radiation that reaches the earth and the atmosphere.

Intertropical convergence zone (ITCZ)

The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.

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.

Iridescence

Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30º from the sun.

Isobar

A line connecting points of equal pressure

Isotach

A line connecting points of equal wind speed.

Isotherm

A line connecting points of equal wind temperature.

January thaw

A period of relatively mild weather around January 20 to 23 that occurs primarily in New England; an example of a singularity in the climatic record.

Jet stream

Relatively strong winds concentrated within a narrow band in the atmosphere.

Katabatic wind

Any wind blowing downslope. Usually cold.

Lake breeze

A wind blowing onshore from the surface of a lake.

Lake-effect snows

Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water.

Land breeze

A coastal breeze that blows from land to sea, usually at night.

Lapse rate

The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.)

Latent heat

The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.

Lenticular cloud

A cloud in the shape of a lens.

Lightning

A visible electrical discharge produced by thunderstorms.

Longwave radiation

A term most often used to describe the infrared energy emitted by the earth and the atmosphere.

Magnetosphere

The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.

Mammatus clouds

Clouds that look like pouches hanging from the underside of a cloud.

Marine climate

A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime air mass

An air mass that originates over the ocean. These air masses are relatively humid.

Maritime polar air

Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Maritime tropical air

Warm, humid air mass that forms over tropical and subtropical oceans.

Mean annual temperature

The average temperature at any given location for the entire year.

Mesoscale

The scale of meteorological phenomena that ranges in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.

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.

Microburst

A strong localized downdraft less than 4 km wide that occurs beneath severe thunderstorms. A strong downdraft greater than 4 km across is called a downburst.

Microclimate

The climate structure of the air space near the surface of the earth.

Microscale

The smallest scale of atmospheric motions.

Millibar(mb)

A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb.

Mirage

A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is, it is called a superior image. When an object appears lower than it actually is, it is an inferior mirage.

Mist

Very thin fog in which visibility is greater than 1.0 km (0.62 mi).

Mistral

A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing ratio

The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Moist adiabatic rate

The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6ºC per 1000 m (3.3ºF per 1000 ft) is used.

Molecular viscosity

The small-scale internal fluid friction that is due to the random motion of the molecules within a smooth-flowing fluid, such as air.

Mountain and valley breeze

A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Nacreous clouds

Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.

Nimbostratus

A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail.

Noctilucent clouds

Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

Nocturnal inversion

See Radiation inversion.

Offshore breeze

A breeze that blows from the land out over the water. Opposite of an onshore breeze.

Onshore breeze

A breeze that blows from the water onto the land. Opposite of an offshore breeze.

Orographic uplift

The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.

Orographic precipitation

Rainfall or snowfall from clouds, induced by topographic uplift.

Permafrost

A layer of soil beneath the earth's surface that remains frozen throughout the year.

Photodissociation

The splitting of a molecule by a photon.

Photon

A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.

Pileus cloud

A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud.

Polar air mass

A cold air mass that forms in a high-latitude source region.

Polar climates

Climates in which the mean temperature of the warmest month is below 10ºC; climates that are too cold to support the growth of trees.

Potential energy

The energy that a body possesses by virtue of its position with respect to other bodies in the field of gravity.

Potential evapotranspiration (PE)

The amount of moisture that, if it were available, would be removed from a given land area by evaporation and transpiration.

Potential temperature

The temperature that a parcel of dry air would have if it were brought dry adiabatically from its original position to a pressure of 1000 mb.

Precipitable water vapor

The depth of water that would result if all the vapor in the atmosphere above a location were condensed into liquid water.

Precipitation

Any form of water particles-liquid or solid-that falls from the atmosphere and reaches the ground.

Prevailing wind

The wind direction most frequently observed during a given period.

Probability forecast

A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions.

Psychrometer

An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Pyranometer

An instrument that measures the amount of radiation.

Radar

An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.

Radiant energy (radiation)

Energy propagated in the form of electromagnetic waves. These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 300,000 km per sec.

Radiation fog

Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as ground fog and valley fog.

Radiation inversion

An increase in temperature with height due to radiational cooling of the earth's surface. Also called a nocturnal inversion.

Radiosonde

A balloon-borne instrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station.

Rain

Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.

Rain gage

A device-usually a cylindrical container-for measuring rain-fall.

Rain Shadow

The region on the leeside of a mountain where the precipitation is noticeable less than on the windward side.

Rawinsonde

An instrument carried by weather balloons to measure the temperature, humidity, pressure, and winds of the atmosphere.

Reflection

The process whereby a surface turns back a portion of the radiation that strikes it.

Refraction

The bending of light as it passes from one medium to another

Refractive index

The ratio of the speed of light in a vacuum to its speed in a transparent medium.

Relative humidity

The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at the particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.

Rime ice

A white, granular deposit of ice formed by the freezing of water drops when they come in contact with an object.

Santa Ana

The local name given a foehn wind in southern California.

Saturation vapor pressure

The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. (See Equilibrium vapor pressure.)

Scattering

The process by which small particles in the atmosphere deflect radiation from its path into different directions.

Scintillation

The apparent twinkling of a star due to its light passing through regions of differing air densities in the atmosphere.

Sea breeze

A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a sea breeze front.

Sea level pressure

The atmospheric pressure at mean sea level.

Semiarid

See Steppe.

Sensible heat transfer

Movement of heat from one place to another as a consequence of conduction or convection or both.

Sensible temperature

The sensation of temperature that the human body feels in contrast to the actual temperature of the environment as measured with a thermometer.

Shear

See wind shear.

Sheet lightning

A fairly bright lightning flash from distant thunderstorms that illuminates a portion of the cloud.

Shortwave radiation

A term most often used to describe the radiant energy emitted from the sun, in the visible and near ultraviolet wavelengths.

Shower

Intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy.

Sleet

A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets.

Smog

Originally smog meant a mixture of smoke and fog. Today, smog means air that has restricted visibility due to pollution, or pollution formed in the presence of sunlight-photochemical smog.

Snow

Solid precipitation in the form of minute ice flakes that occur below 0ºC.

Snowflake

An aggregate of ice crystals that falls from a cloud

Snow flurries

Light showers of snow that fall intermittently.

Snow grains

Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.

Snow pellets

White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow rollers

A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.

Snow squall (shower)

An intermittent heavy shower of snow that greatly reduces visibility.

Solstice

Either of the two times of the year when the sun is the greatest distance from the celestial equator, occurring about June 22 and December 22. See winter solstice and summer solstice.

Southern oscillation

The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during El Nino events.

Specific heat

The ratio of the heat absorbed (or released) by the unit mass of the system to the corresponding temperature rise (or fall).

Specific humidity

The ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

Spontaneous nucleation (freezing)

The freezing of pure water without the benefit of any nuclei.

Spring freeze date

The date of occurrence in the spring of the last minimum at or below a temperature threshold.

Squall line

Any nonfrontal line or band of active thunderstorms.

Station pressure

The actual air pressure computed at the observing station.

Steam fog

See Evaporation fog.

Steppe

One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Steppe also refers to the short-grass vegetation associated with this semiarid climate.

Storm surge

An abnormal rise of the sea along a shore. Primarily due to the winds of a storm, especially a hurricane.

Stratocumulus

A low cloud, predominantly stratiform with low, lumpy, rounded masses, often with blue sky between them.

Stratopause

The boundary between the stratosphere and the mesosphere.

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.

Stratus

A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subarctic climate

A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers. Places within this climatic realm experience the highest annual temperature ranges on earth.

Sublimation

The process whereby ice changes directly into water vapor without melting. In meteorology, sublimation can also mean the transformation of water vapor into ice. (See Deposition.)

Subsidence

The slow sinking of air, usually associated wit high-pressure areas.

Subsidence inversion

A temperature inversion produced by the adiabatic warming of a layer of sinking air.

Summer solstice

Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23.5º N, the Tropic of Cancer.

Sundog

A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called parhelion.

Sun pillar

A vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight of ice crystals.

Supersaturated air

A condition that occurs in the atmosphere when the relative humidity is greater that 100 percent.

Surface inversion

See Radiation inversion

Synoptic scale

The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent. Also called the cyclonic scale.

Taiga

The northern coniferous forest; also a name applied to the subarctic climate.

Temperature

The degree of hotness or coldness of a substance as measured by a thermometer. It is also a measure of the average speed or kinetic energy of the atoms and molecules in a substance.

Temperature inversion

An extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the troposphere.

Terminal velocity

The constant speed obtained by a falling object when the upward drag on the object balances the downward force of gravity.

Thermal

A small, rising parcel of warm air produced when the earth's surface is heated unevenly.

Thermograph

A recording instrument that gives a continuous trace of temperature with time.

Thermometer

An instrument used to measure temperature.

Thermosphere

The atmospheric layer above the mesosphere. It extends from 90 km to outer space.

Thunder

The sound due to rapidly expanding gases along the channel of a lightning discharge.

Tipping bucket rain gage

A device that accumulates rainfall in increments of 0.01 in. by containers that alternately fill and empty (tip).

Tornado

An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope and touches the ground. (See Funnel cloud.)

Trade winds

The winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low.

Transpiration

The release of water vapor to the atmosphere by plants.

Tropical air mass

A warm-to-hot air mass that forms in the subtropics.

Tropical depression

A mass of thunderstorms and clouds generally with a cyclonic wind circulation of between 20 and 34 knots

Tropical disturbance

An organized mass of thunderstorms with a slight cyclonic wind circulation of less than 20 knots.

Tropical storm

Organized thunderstorms with a cyclonic wind circulation between 35 and 64 knots.

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).

Tundra Climate

Found almost exclusively in the northern hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a long, bitterly cold winter.

Turbulence

Any irregular or disturbed flow in the atmosphere that produces gusts and eddies.

Twilight

The time immediately before sunrise and after sunset when the sky remain illuminated.

Typhoon

A hurricane that forms in the western Pacific Ocean.

Ultraviolet radiation

Electromagnetic radiation with wave-lengths longer than X-rays but shorter than visible light.

Upslope fog

Fog formed as moist, stable air flows upward over a topographic barrier.

Upslope precipitation

Precipitation that forms due to moist, stable air gradually rising along an elevated plain. Upslope precipitation is common over the western Great Plains, especially east of the Rock Mountains.

Upwelling

The rising of water (usually cold) toward the surface from the deeper regions of a body of water.

Urban heat island

The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas.

Valley breeze

See Mountain breeze.

Valley fog

See Radiation fog.

Vapor pressure

The pressure exerted by the water vapor molecules in a given volume of air.

Vernal equinox

The equinox at which the sun approaches the Northern Hemisphere and passes directly over the equator. Occurs around March 20.

Virga

Precipitation that falls from a cloud but evaporates before reaching the ground. (See Fall streaks.)

Virtual temperature

An adjustment applied to the real air temperature to account for a reduction in air density due to the presence of water vapor.

Viscosity

The resistance of fluid flow.

Visibility

The greatest distance an observer can see and identify prominent objects.

Visible light

That portion of the electromagnetic spectrum from 0.4 to 0.7 µm wavelengths that is visible.

Vorticity

A measure of the spin of a fluid, usually small air parcels. Absolute vorticity is the combined vorticity due to the earth's rotation and the vorticity due to the air's circulation relative to the earth. Relative vorticity is due to the curving of the air flow and wind shear.

Warm front

The leading edge of a warm air mass.

Water balance

The comparison of actual and potential evapotranspiration with the amount of precipitation, usually on a monthly basis.

Water budget

Balance sheet for the inputs and outputs of water to and from the various global water reservoirs.

Water equivalent

The depth of water that would result from the melting of a snow sample. Typically about 10 inches of snow will melt to 1 inch of water, producing a water equivalent of 10 to 1.

Weather

The state of the atmosphere in terms of such variables as temperature, cloudiness, precipitation, and radiation.

Weighing bucket rain gage

A device that is calibrated so that the weight of rainfall is recorded directly in terms of rainfall in millimeters or in inches.

Wet-bulb depression

The difference in degrees between the air temperature (dry-bulb temperature) and the wet-bulb temperature.

Wet-bulb temperature

The lowest temperature that can be obtained by evaporating water into the air.

White frost

Ice crystals that form on surfaces instead of dew when the dew point is below freezing.

Wind chill equivalent temperature

A theoretical air temperature at which the heat loss from exposed skin under calm conditions is equivalent to the heat loss at the actual air temperature and under the actual wind speeds.

Wind-chill factor

The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.

Wind shear

A difference in wind speed or direction between two wind currents in the atmosphere.

Wind Vane

An instrument used to determine wind direction.

Windsock

A large, conical, open bag designed to indicate wind direction and relative speed; usually used at small airports.

Winter solstice

Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23.5ºS, the Tropic of Capricorn.

Montreal Weather

Current Weather for Montreal, Quebec

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