Glossary

Term	Description
Absorption	In terms of energy, the interception of radiant energy or sound waves.
Active sensor	Active sensors provide their own source of energy to illuminate the objects they observe. An active sensor emits radiation in the direction of a target and then detects and measures the radiation that is reflected or scattered back from the target. Compare with Passive Sensor.
Aerosols	Aerosols are tiny particles (e.g., dust, smoke, sea salt) suspended in the atmosphere that have great importance for the quality of the air we breathe, as well as playing a role in cloud formation and evolution, affecting the warming and cooling of our planet.
Algorithm	A self-contained step-by-step set of operations to be performed. For example, scientists use mathematical algorithms on computers to process the raw data from sensors and turn them into measurements of geophysical variables.
Atmospheric column	A vertical pillar defined by a unit area on Earth’s surface and bounded by the top of the atmosphere that is used to quantify an atmospheric parameter such as pressure, ozone, or precipitable water.
Backscatter lidar	Backscatter is the deflection of radiation (e.g., from a laser beam) in a direction opposite to that from which it originated (i.e., through an angle of 180°). Lidar is a detection system that works on the principle of radar, but uses light from a laser. A lidar instrument transmits a laser pulse through the atmosphere. Backscattered light resulting from a laser pulse hitting air molecules, aerosols, and clouds in the atmosphere is collected using a telescope, providing the altitude and optical properties of aerosol and cloud features in the atmosphere.
Bright band	A narrow, intense radar echo due to water-covered ice particles below the level where falling ice particles enter air with temperatures above 0°C. In many situations, the radar reflectivity is greatest in the bright band compared to other altitudes.
Climate	Weather refers to short-term atmospheric conditions. Climate on the other hand, describes what an area's typical weather conditions are like over a long period of time — 30 years or more. Climate encompasses averages and variability (including extremes) of meteorological variables such as precipitation, temperature, humidity, winds and other measures of the weather.
Cloud	A cloud is a visible mass of condensed water vapor in the form of small droplets or ice particles (or a mixture thereof) floating in the atmosphere. Clouds form when tiny suspended aerosol particles and water vapor get caught up in rising air motions.
Cloud feedback	Feedback is an event that occurs when the output of a system is used as input back into the system as part of a chain of cause and effect. Cloud feedback is the coupling between cloudiness and the surface air temperature where a change in surface air temperature leads to a change in clouds, which, in turn, can influence surface air temperature.
Coalescence	In cloud physics, the merging of two water drops into a single larger drop after collision.
Condensation nuclei	A particle upon which water vapor condenses. It may be either in a solid or liquid state.
Conically scanning	A scanning pattern for sensors in which the antenna traces a cone pattern around its central axis.
Convection	Convection refers to atmospheric motions in the vertical direction. As molecules of air close to Earth's surface warm, they spread apart, and the parcel of air containing them becomes less dense and therefore lighter than its surroundings. This less-dense air rises and transfers its thermal energy or heat to higher altitudes, while cooler, more dense air sinks towards the ground. That cooler air can then be warmed by the Earth's surface and rise, continuing the convection pattern. Convection plays a key role in the formation of clouds and even thunderstorms.
Convective clouds	Convective clouds are vertically oriented clouds with significant rising and sinking air motions that can quickly contribute to precipitation. These clouds can produce a range of rain events from weak spring showers to intense local thunderstorms and organized systems like tropical cyclones.
Cumulonimbus	Cumulonimbus is a dense, towering vertical cloud, forming from water vapor carried by powerful upward air currents. These clouds may also be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along squall lines. These clouds are capable of producing lightning and other dangerous severe weather such as strong winds, tornadoes and hailstones.
Cyclone	A system of winds rotating around an area of low atmospheric pressure, with a counterclockwise (northern hemisphere) or clockwise (southern hemisphere) circulation.
Detrainment	The transfer of air from an organized air current to the surrounding atmosphere; the opposite of entrainment .
Dew point	The temperature to which air must be cooled for saturation to occur, exclusive of air pressure or moisture content change. At that temperature, dew begins to form and water vapor condenses into a liquid.
Disdrometer	An instrument that measures the size of precipitation particles and counts the frequency of occurrence in individual size ranges (i.e., distribution of sizes). Some disdrometers provide only a measure of size, while others also provide a measure of the shape and fall speed of individual precipitation particles such as raindrops and snowflakes
Diurnal cycle	Sub-daily variability of the atmosphere in response to the variation of solar insolation over the course of the day. Examples include the variation of temperature between an afternoon maximum and nighttime minimum and the afternoon development of thundershowers over land and their nighttime or early morning dissipation.
Doppler radar	A radar system that measures the motion of cloud and/or precipitation particles either toward or away from the radar by detecting the change in frequency of the reflected wave energy caused by the Doppler shift. (An example of the Doppler shift is the change in pitch of an ambulance siren to a listener as it passes by.)
Downscaling	In numerical modeling, downscaling refers to techniques used with global models with low spatial resolution to produce regional and local outputs at higher spatial resolution. Downscaling applies observation-based relationships between big picture weather events (e.g., atmospheric pressure for a wide area) and local variables (e.g., local storm systems and rainfall over a few square miles) to achieve a higher resolution model result.
Electromagnetic	Relating to the interrelation of electric currents or fields and magnetic fields. The electromagnetic spectrum is the range of wavelengths or frequencies over which electromagnetic radiation extends.
emission	The generation and sending out (or discharge) of something, such as gases, particles, or radiant energy. It is considered separately from the processes of absorption or scattering.
entrainment	To draw in and transport something (such as solid particles or gas) by the flow of a fluid.
eyewall	In a tropical cyclone, the arc of clouds that partially or completely surrounds the eye — the area about which the rest of the storm rotates and where the lowest surface pressures are found. The fastest surface winds are usually under this arc of clouds. Embedded in these clouds are vigorous convective cells that transport upward the majority of air and moisture within a tropical cyclone.
flux	The rate of flow of a fluid (e.g., air), radiant energy, or particles across a given area.
Forecasting	Forecasting is the process of predicting or estimating future events based on past and present data through analysis of trends or application of mathematical or computer models
frequency	In physics, the rate at which a vibration occurs that constitutes a wave, either in a material (as in sound waves) or in an electromagnetic field (as in radio waves and light). In remote sensing, frequency is used to quantify electromagnetic radiation by denoting the number of oscillations of the perpendicular electric and magnetic fields per second. This measurement is expressed in hertz (Hz), or number of cycles per second. One megahertz (MHz) is equal to one million cycles per second while one gigahertz (GHz) is equal to one billion cycles per second.
global atmospheric circulation	The movement of major air masses around the globe that serves as the primary means by which atmospheric heat energy is redistributed from the equator toward the poles.
Hail	Precipitation composed of balls or irregular lumps of ice. Hail is produced in cumulonimbus clouds when large frozen raindrops or ice particles act as embryos that grow by accumulating supercooled liquid droplets. Violent updrafts in the cloud carry and suspend the particles in below-freezing air, allowing the frozen core to accumulate more ice. When the hail stone becomes too heavy to be suspended by rising air currents, it falls to the ground.
High Spectral Resolution Lidar (HSRL)	HSRL is used to characterize clouds and, more importantly, small particles in the atmosphere called aerosols. HSRL is similar to radar, but with radio waves replaced with laser light. Lidar allows researchers to see the vertical dimension of the atmosphere and the advanced HSRL makes measurements that can even distinguish among different aerosol types and their sources. The HSRL technique takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals. HSRL enables studies of aerosol size, composition, distribution and movement.
hot tower	A tall cumulonimbus cloud that reaches or overshoots the tropopause. In the 1960s, Malkus and Riehl proposed that most of the upward leg of the atmosphere‘s global circulation occurs near the Equator inside of hot towers. They also proposed that hot towers were responsible for most of the latent heat release inside of the eyewall of tropical cyclones.
humidification	The act of increasing the moisture content of a gas, usually air.
hydrometeor	A hydrometeor is liquid or solid water suspended in the atmosphere. Categories of hydrometeors include small cloud droplets, larger rain drops, and many types and shapes of ice crystals, including snow, graupel, and hail.
inclined orbit	Satellites with "inclined" orbits travel in an orbit plane between an equatorial orbit (e.g., geosynchronous satellites) and a polar orbit (near 90° inclination with respect to the equatorial plane). Inclined orbits are typically used to focus observations within a latitudinal band centered on the equator and to measure sub-daily variability such as the diurnal cycle.
Ka-band	A microwave band in which the wavelengths vary from 1.11 cm – 7.5 mm or a frequency of 27 – 40 GHz. For radars, this band is used to measure light to moderate rainfall; at higher rainfall rates, the signal from the radar tends to get fully extinguished, i.e., no signal is obtained from the region of heavy rainfall. AOS will include a Ka-band Doppler radar in a polar orbit. https://aos.gsfc.nasa.gov/spaceborne.htm.
Ku-band	A microwave band in which the wavelengths vary from 1.67-2.4 cm or a frequency of 12-18 GHz. For radars, this band is used to measure moderate to heavy rainfall and has less sensitivity to lighter precipitation. AOS will include a Ku-band Doppler radar in an inclined orbit. https://aos.gsfc.nasa.gov/spaceborne.htm
latent heat	The amount of heat given up or absorbed when a substance changes from one state to another, such as from a liquid to a solid.
Lidar	Light Detection and Ranging (Lidar) is similar to radar, but instead of bouncing radio waves off its target, lidar uses short pulses of laser light. Some of that light reflects off of tiny particles in the atmosphere and back to a telescope aligned with the laser.
mesoscale convective system	A group of thunderstorms that is typically several hundred kilometers across and lasting 6-12 hours or sometimes longer. The rain that falls from these systems can lead to dangerous flooding with tremendous socioeconomic impacts.
microphysics	Cloud microphysics is the study of hydrometeors in various categories and the processes that change hydrometeors from one category into another (e.g., cloud ice, cloud water, rain, and several kinds of frozen precipitation).
microwave radiometer	A microwave radiometer is a sensitive receiver designed to measure energy emitted at millimeter-to-centimeter wavelengths (frequencies of 1—1000 GHz) known as "microwaves." In atmospheric science, microwave radiometers are used to measure the column total liquid and/or ice water content, precipitation, and profiles of temperature and humidity, depending on the microwave frequencies measured.
model	A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. This representation typically involves certain simplifications and assumptions.
multi-angle polarimeter	Light travels as waves, which oscillate perpendicularly to the direction of travel. Before it reaches the Earth, sunlight is unpolarized, meaning that the direction of the oscillations are random. However, as that light interacts with the Earth's atmosphere or surface, the light that is scattered may have a preferred oscillation direction - it is polarized. An example of this is the polarized glare reflecting off the surface of a lake or other body of water. Polarized sunglasses help reduce that glare by blocking specific polarization directions. A polarimeter measures how much and what type of polarization exists in scattered light, which is used to understand the nature of clouds or particles (aerosols) in the atmosphere. It does so at multiple viewing angles to gather as much information as possible and better understand clouds and aerosols.
nadir	In terms of satellites, the downward-looking direction along the shortest distance to the planetary surface.
nowcasting	Nowcasting is a form of very short-range weather forecasting, covering only a very specific geographic area. A nowcast is loosely defined as a forecast for the coming 12-hour period based on very detailed observational data.
nucleation	The process by which droplets of liquid can condense from a vapor or ice crystals can form from supercooled droplets.
orography	In atmospheric science, orography is roughly synonymous with 'topography', i.e., the location and altitude of mountains, hills, and other major features on the earth's surface.
passive sensor	Passive sensors detect natural energy (radiation) that is emitted or reflected by the object or scene being observed. Reflected sunlight and microwave energy are common sources of radiation measured by passive sensors.
phase	The physical state of matter of a substance: gas, liquid or solid. For water, phases include water vapor, liquid water or ice.
planetary boundary layer	In meteorology, the planetary boundary layer, also known as the atmospheric boundary layer, is the lowest part of the atmosphere. Its behavior is directly influenced by its contact with Earth's surface.
polar orbit	Satellite orbits with inclinations near 90 degrees with respect to the equator are called "polar orbits". This is because satellites in this orbit cross over (or nearly over) the north and south poles.
precipitation	In meteorology, precipitation (the collective name for falling hydrometeors) is the result of condensation of atmospheric water vapor that produces water drops or ice particles of sufficient size or mass to fall back to earth. This is in contrast to cloud particles, either liquid or ice, in which the droplets or particles are sufficiently small to remain suspended in the cloud. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail.
radar	Radar uses radio waves to detect the presence, direction, distance, and speed of objects by sending out pulses of high-frequency electromagnetic waves that are reflected off the object back to the source.
radiative heating (cooling)	The process by which temperature increases (decreases) due to an excess (deficit) of absorbed radiation over emitted radiation.
radiometer	An instrument that measures the intensity of radiation emitted at specific wavelengths. Radiometers are often used to derive meteorological parameters such as precipitation, temperature, and water vapor depending on the frequencies measured. More on radar and lidar.
reflectivity	In meteorology, a measure of the fraction of incident radiation falling on a surface in (or volume of) the atmosphere that is turned back without being absorbed. Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.
rime	An opaque, granular deposit of ice formed by the rapid freezing of supercooled water drops as they fall upon an exposed object.
runoff	Runoff occurs when falling rain cannot be adequately absorbed by soil; thus, instead of going into the ground, the water flows over the surface. It occurs in nature when the soil is saturated with water or the surface is impervious to water, such as asphalt in urban environments.
scattering	The process in which electromagnetic radiation or particles are deflected or diffused.
snow	Precipitation that forms due to the growth of ice crystals from water vapor, often in complex forms or through aggregation of ice crystals into larger snowflakes.
spectrometer	A spectrometer is used to separate and measure wavelengths of light as it interacts with materials, including trace gases and airborne particles. A spectrometer covers several bandwidths in the Visible (VIS), Near Infrared (NIR) and Short-Wave Infrared (SWIR), Long Wave Infrared (LWIR), and Thermal Infrared (TIR) ranges of the electromagnetic spectrum. For meteorology, spectrometers can provide ozone profiles, be used to monitor various trace gases and air quality, measure cloud properties, and measure reflected and emitted radiation.
suborbital	Occurring below the altitudes of orbiting satellites, typically referring to measurements of the atmosphere from aircraft or surface platforms.
sun-synchronous orbit	Describes a satellite orbit in which the satellite passes over the same place on Earth at the same time each day. For example, a satellite in a sun-synchronous orbit will cross the equator several times per day and each crossing will occur at 3:00 p.m. local time.
supercooled water	Liquid water below 0 degrees Celsius (32 Fahrenheit). Smaller droplets can remain supercooled longer and at cooler temperatures than larger droplets. Similarly, freezing is delayed when the water droplets are less contaminated (i.e., purer).
swath	The portion of Earth's surface or atmosphere measured by an instrument during a single satellite overpass.
tropical depression	A tropical depression is an area of low pressure that has clouds and thunderstorms with winds less than 38 mph (61 kph). It does not necessarily have an eye or the typical cyclone structure.
tropopause	Located right above the planetary boundary layer, the troposphere is the layer in which most weather phenomena take place on Earth. The tropopause is the boundary between the troposphere and the stratosphere above it. The tropopause lies at approximately 17 kilometers (11 mi) altitude above equatorial regions and approximately 9 kilometers (5.6 mi) above polar regions. It is the atmospheric level where the air ceases to cool with increasing altitude.
W-band	A microwave band in which the wavelengths vary from 2.7 – 4 mm or a frequency of 75 – 110 GHz. For radars, this band is used to profile clouds and light rainfall; at moderate and higher rainfall rates, the signal from the radar tends to get fully extinguished, i.e., no signal is obtained from the region of heavier precipitation. AOS will include a W-band Doppler radar in a polar orbit (https://aos.gsfc.nasa.gov/spaceborne.htm).
water vapor	Water vapor is water present in the atmosphere in gaseous form. Water vapor is a greenhouse gas that plays a key role in climate feedbacks because of its heat-trapping ability. Warmer air holds more moisture than cooler air. Therefore, as greenhouse gas concentrations increase and global temperatures rise, the total amount of water vapor in the atmosphere also increases, further amplifying the warming effect. In addition, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.
weather	Weather refers to atmospheric conditions such as the combination of temperature, humidity, precipitation, cloudiness, visibility, and wind over a short period of time, while climate is the weather of a specific region averaged over a long period of time. We talk about weather in terms of the near future: "What will it be like today?" "What is the temperature right now?" and "Will we get rain this week?"
wet removal	Taking away (for example, of an aerosol) via precipitation. Aerosol typically form the nucleus of water drops and are removed from the atmosphere when those drops grow to precipitation size and fall to the surface.