Polar climates(6)

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

Positive Feedback(5)

A process that results in an amplification of the response of a system to an external influence. For example. increased atmospheric water vapor in response to global warming would be a positive feedback on warming. because water vapor is a GHG.

Potential energy(6)

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

Potential evapotranspiration ((PE))(6)

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

Potential temperature(6)

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.


Abbreviation for precipitation.


The tendency of the Earth's axis to wobble in space over a period of 23.000 years. The Earth's precession is one of the factors that results in the planet receiving different amounts of solar energy over extended periods of time.

Precipitable water vapor(6)

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


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

Prevailing wind(6)

The wind direction most frequently observed during a given period.

Probability forecast(6)

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


A chance. or likelihood. that a certain event might happen.


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.


An instrument that measures the amount of radiation.


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)(6)

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 budget(1)

A measure of all the inputs and outputs of radiative energy relative to a system. such as Earth. See Earth Radiation Budget Experiment.

Radiation fog(6)

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(6)

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


Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object. See ultraviolet radiation. infrared radiation. solar radiation. longwave radiation. terrestrial radiation.

Radiative cooling(1)

Cooling process of the Earth's surface and adjacent air. which occurs when infrared (heat) energy radiates from the surface of the Earth upward through the atmosphere into space. Air near the surface transfers its thermal energy to the nearby ground through conduction. so that radiative cooling lowers the temperature of both the surface and the lowest part of the atmosphere.

Radiative Forcing(1)

A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing. solar radiation coming to the Earth would continue to be approximately equal to the infrared radiation emitted from the Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation. radiating it back toward the surface and creating a warming influence (i.e.. positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).


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

Rain gage(6)

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

Rain Shadow(6)

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


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


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


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


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

Refractive index(6)

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

Relative humidity(4)

An estimate of the amount of moisture in the air relative to the amount of moisture that the air can hold at a specific temperature. For example. if it's 70�F near dawn on a foggy summer morning. the relative humidity is near 100%. During the afternoon the temperature soars to 95�F and the fog disappears. The moisture in the atmosphere has not changed appreciably. but the relative humidity drops to 44% because the air has the capacity to hold much more moisture at a temperature of 95�F than it does at 70�F. But even when the relative humidity is 'low' at 44%. it's a very humid day when the temperature is 95�F. For this reason. a better measure of comfort is dew point.

Retrogression or retrograde motion(3)

Motion that is backwards from the usual way things move in the Northern Hemisphere extratropics which is from west to east. In meteorology. the term is used in relation to atmospheric waves or pressure systems. When meteorologists say that a pattern will retrograde. they mean that the troughs and ridges will end up further west than they were previously. Normal motion (over the United States) is progressive. or prograde. which means (weather systems move) from west to east.

Rime ice(6)

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


The degree of salt in water. The rise in sea level due to global warming would result in increased salinity of rivers. bays and aquifers.

Santa Ana(6)

The local name given a foehn wind in southern California.

Saturation vapor pressure(6)

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.)


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


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

Sea breeze(6)

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(6)

The atmospheric pressure at mean sea level.

Sea level(1)

The datum against which land elevation and sea depth are measured.

Sea surface temperature (SST)(6)

The temperature of the layer of seawater (approximately 0.5 m deep) nearest the atmosphere.

Sea surface temperature anomaly(6)

Temperature of emitted energy from the sea surface. SST anomaly = (SST-SST mean). where SST = sea surface temperature. Snow water content and snow water equivalent are different terms for the same parameter.


See Steppe.

Sensible heat transfer(6)

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

Sensible temperature(6)

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


Opportunities to remove atmospheric CO2. either through biological processes (e.g. plants and trees). or geological processes through storage of CO2 in underground reservoirs.


See wind shear.

Sheet lightning(6)

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

Shortwave radiation(6)

A term most often used to describe the radiant energy emitted from the sun. in the visible and near ultraviolet wavelengths.
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Americans’ health, security and economic wellbeing are tied to climate and weather. Every day, we see communities grappling with environmental challenges due to unusual or extreme events related to climate and weather. In 2011, the United States experienced a record high number (14) of climate- and weather-related disasters where overall costs reached or exceeded $1 billion. Combined, these events claimed 670 lives, caused more than 6,000 injuries, and cost $55 billion in damages. Businesses, policy leaders, resource managers and citizens are increasingly asking for information to help them address such challenges.