Weather & Climate Chapters 3-5.1

urban heat island

An urbanized region that is significantly warmer than surrounding rural areas.

heat

Energy transferred between materials or systems due to their temperature differences.

temperature

The average kinetic movement of atoms and molecules of a substance.

advection

The horizontal movement of a property of the atmosphere, such as heat, humidity, or pollution.

conduction

The process by which energy is transferred through a substance or between objects that are in direct contact.

convection

The transfer of heat through vertical movement of mass within a fluid (liquid or gas).

radiation

The process by which wave energy travels through the vacuum of space or through a physical medium such as air or water.

sensible heat

A type of heat energy that is detectable as a change in temperature and can be measured with a thermometer.

absorption

The ability of an object or a material to assimilate electromagnetic energy and convert it to another form of energy, usually thermal infrared energy.

albedo

The reflectivity of a surface, given as the percentage of incoming radiation that the surface reflects.

reflection

The process of returning a portion of the radiation striking a surface back in the general direction from which the radiation came.

scattering

The process by which solar radiation is redirected in random directions as it strikes physical matter.

transmission

The unimpeded movement of electromagnetic energy through a medium such as air, water, or glass.

radiative equilibrium temperature

The temperature of an object resulting from the balance between incoming and outgoing energy.

greenhouse effect

The process by which a planet's atmosphere is warmed as greenhouse gases (such as water vapor, carbon dioxide, and methane) and clouds absorb and counterradiate heat.

global heat engine

A system that involves movement of heat from low to high latitudes and low to high altitudes as a result of heating differences.

plane of the ecliptic

The flat plane traced by the orbital paths of the planets in the solar system.

circle of illumination

The line separating night from day, where sunrise and sunset are occurring.

subsolar point

The single point at which the Sun's rays are perpendicular to Earth's surface at or near noon; restricted to between 23.5 degrees north and south latitude.

Tropic of Cancer

The 23.5-degree north parallel; the maximum latitude of the subsolar point in the Northern Hemisphere.

Tropic of Capricorn

The 23.5-degree south parallel; the maximum latitude of the subsolar point in the Southern Hemisphere.

December solstice

A seasonal marker that occurs when the subsolar point is at 23.5 degrees south, on about December 21.

solar altitude

The altitude of the Sun above the horizon, in degrees.

June solstice

The seasonal marker that occurs when the subsolar point is 23.5 degrees north latitude, about June 21.

March equinox

The seasonal marker that occurs when the subsolar point is over the equator about March 20.

September equinox

The seasonal marker that occurs when the subsolar point is over the equator about September 22.

Antarctic Circle

The 66.5-degree south parallel.

Arctic Circle

The 66.5-degree north parallel.

continental effect

The increase in seasonality with distance from the oceans.

specific heat (or specific heat capacity)

The heat required to raise the temperature of 1 gram of any material by 1 °C.

photovoltaic cell (PV cell)

A semiconductor that converts sunlight directly into electricity, using silicon cells.

renewable energy

Energy that comes from sources that are not depleted when used, such as sunlight or wind.

cloud

An aggregation of microscopic water droplets and ice crystals suspended in the air.

condensation

A change in the state of water from gas to liquid.

evaporation

The change in the state of water from liquid to gas.

hydrologic cycle

The circulation of water within the atmosphere, biosphere, lithosphere, and hydrosphere.

precipitation

Solid or liquid water that falls from the atmosphere to the ground.

transpiration

The loss of water to the atmosphere by plants.

evapotranspiration

The combined processes of evaporation and transpiration.

hydrogen bond

A bond between water molecules that results from the attraction between one water molecule's positive end and another's negative end.

latent heat

Energy that is absorbed or released during a change in the state of a substance, such as during evaporation or condensation of water.

apparent temperature

The temperature perceived by people as a result of low air temperatures coupled with wind or high air temperatures coupled with atmospheric humidity; apparent temperatures can be found using the wind chill and heat index charts (see Appendix III: Wind Chill and Heat-Index Charts).

humidity

The amount of water vapor in the atmosphere.

hygrometer

An instrument used to measure humidity.

saturation

The point at which an air parcel's water vapor content is equal to its water vapor capacity.

millibar (mb)

A measure of atmospheric pressure; average sea level, pressure is 1013.25 mb.

saturation vapor pressure

The vapor pressure at which saturation occurs.

specific humidity

The water vapor content of the atmosphere, expressed in grams of water per kilogram of air (g/kg).

vapor pressure

The portion of air pressure exerted exclusively by molecules of water vapor.

air parcel

A body of air of uniform humidity and temperature.

relative humidity (RH)

The ratio of water vapor content to water vapor capacity, expressed as a percentage.

dew point (or dew-point temperature)

The temperature at which air becomes saturated.

dew-point depression

The difference between the air temperature and the dew point.

adiabatic cooling

The cooling of an air parcel through expansion.

adiabatic warming

The warming of an air parcel through compression.

dry adiabatic rate

The rate of temperature change in an unsaturated parcel of air; 10 °C/1,000 m (5.5 °F/1,000 ft).

lifting condensation level (LCL)

The altitude at which an air parcel becomes saturated.

moist adiabatic rate

The rate of cooling in a saturated air parcel; usually about 6 °C/1,000 m (3.3 °F/1,000 ft).

rain shadow

The dry, leeward side of a mountain range.

stable atmosphere

A condition in which air parcels are cooler and denser than the surrounding air and will not rise unless forced to do so.

unstable atmosphere

A condition in which air parcels rise on their own because they are warmer and less dense than the surrounding air.

convective uplift

The rising of an air parcel that is warmer and less dense than the surrounding air.

orographic uplift

The rising of air over mountains.

convergent uplift

The rising of air as a result of converging airflow.

frontal uplift

The rising of warm air masses where they meet relatively cold air masses.

cirrus

A high cloud with a feathery appearance that is composed of ice crystals.

cumulonimbus

A cloud that extends high into the atmosphere and is capable of strong vertical development and of producing severe weather.

cumulus

A dome-shaped, bunched cloud, with a flat base and billowy upper portions.

nimbostratus

Rain-producing low-level sheets of clouds.

stratus

A cloud type characterized by low, flat sheets of clouds.

advection fog

Fog that results from moist air moving over a cold surface, such as a lake or a cold ocean current, which lowers air temperature to the dew point.

fog

A cloud at or near ground level that reduces visibility to less than 1 km (0.62 mi).

radiation fog (or valley fog)

Fog that results when the ground radiates its heat away at night, cooling the air above it to the dew point.

collision and coalescence

The process by which cloud droplets merge to form raindrops.

condensation nucleus

A small particle in the atmosphere, about 0.2 μm in diameter, on which water vapor condenses.

ice-crystal process (or Bergeron process)

The process by which ice crystals grow within a cloud to form snow.

hail

Hard, rounded pellets of ice that precipitate from cumulonimbus clouds with strong vertical airflow.

cloud seeding

The introduction of artificial substances, such as silver iodide, to modify or enhance precipitation from clouds.

geoengineering

The deliberate, global-scale modification of Earth's environments to improve living conditions for humans.

monsoon

A seasonal reversal of winds, characterized by moist summer onshore airflow and dry winter offshore airflow.

barometer

An instrument used to measure air pressure.

sea-level pressure

Air pressure that has been adjusted to sea level.

thermal air pressure

Air pressure caused by heating or cooling of air. Warm air is associated with low pressure, and cold air is associated with high pressure.

dynamic air pressure

Air pressure caused by air movement.

pressure-gradient force

The force resulting from changes in barometric pressure across Earth's surface.

Coriolis effect (or Coriolis force)

The perceived deflection of moving objects in relationship to Earth's surface.

friction layer

The layer of the atmosphere where wind is slowed by friction with Earth's surface; extends about 1 km (3,280 ft) above the surface.

isobar

A line drawn on a map connecting points of equal pressure. Isobars are quantitative representations of the changing molecular density of the air over a geographic region.

aerovane

A combination of an anemometer and a wind vane that measures wind speed and direction.

anemometer

(pronounced an-eh-MOM-eter) An instrument used to measure wind speed.

geostrophic wind

A high-altitude wind that experiences strong Coriolis force deflection and moves along a path parallel to the pressure gradient rather than across it.

wind vane (or weather vane)

An instrument used to measure wind direction.

anticyclone

A meteorological system in which air flows away from a high-pressure region, creating clockwise circulation in the Northern Hemisphere and counterclockwise circulation in the Southern Hemisphere.

cyclone

A meteorological system in which air flows toward a low-pressure region, creating counterclockwise circulation in the Northern Hemisphere and clockwise circulation in the Southern Hemisphere.

offshore wind

A coastal wind that flows from land to sea.

onshore wind

A coastal wind that flows from sea to land.

prevailing wind

A wind that blows from the direction that is most common during a specified window of time.

doldrums

A low-wind region near the equator, associated with the ITCZ.

horse latitudes

The low-wind regions centered on 30 degrees north and south.

ITCZ

(short for intertropical convergence zone; or equatorial trough) The discontinuous band of thermal low pressure and thunderstorms that encircles Earth in the tropics.