Stores all PMM-related glossary terms.
Click one of the letters above to advance the page to terms beginning with that letter.


active sensor search for term

A remote-sensing sys­tem (e.g., an instrument) that transmits its own radiant energy to detect an object or area for observation and receives the reflected or transmitted energy. Radar is an example of an active system. Compare with passive sensor.

algorithm search for term

A self-contained step-by-step set of operations to be performed. Scientists use mathematical algorithms on computers to process the raw data from the GPM satellites into precipitation rates.

asynoptic search for term

As meteorology has developed, certain hours have been designated 'synoptic' hours, and observation times standardised around these points; the MAIN synoptic hours currently being 00, 06, 12 and 18 UTC (formerly GMT), with intermediate hours at 03, 09, 15 and 21 UTC. Increasingly however, observing systems (e.g. satellite, radar-networks, drifting buoys etc.) provide data at times other than these 'fixed hours' - these are designated non, or 'asynoptic' observations. NWP models can assimilate these observations during the initialisation process.


atmospheric column search for term

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.

Go to top


bright band search for term

A narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest.

Go to top


climate search for term

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elements in a given region over a long period of time. Climate can be contrasted to weather, which is the present condition of these same elements and their variations over shorter time periods.

condensation nuclei search for term

A particle upon which water vapor condenses. It may be either in a solid or liquid state.

conically-scanning search for term

A scanning pattern for sensors in which the antenna traces a cone pattern around its central axis.

Go to top


dBZ search for term

Decibels of Z. It is a meteorological measure of equivalent reflectivity (Z) of a radar signal reflected off a remote object. The reference level for Z is 1 mm6 m−3, which is equal to 1 μm3. It is related to the number of drops per unit volume and the sixth power of drop diameter.

dew point search for term

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 search for term

An instrument that measures the distribution of precipitation drop sizes in the atmosphere i.e. the number of small droplets, number of large droplets, and where they occur.

doppler radar search for term

A radar system which differentiates between fixed and moving targets by detecting the change in frequency of the reflected wave caused by the doppler effects. The system can also measure target velocity with high accuracy.

downscaling search for term

In numerical modeling, downscaling refers to the techniques used on global models with low spatial resolution to produce regional and local outputs. Downscaling applies observation-based relationships between big picture weather events (for example, atmospheric pressure for a wide area) and local variables (for example, local storm systems and rainfall over a few square miles) to achieve a higher resolution model result.

drainage basin search for term

A drainage basin or watershed is an area of land that collects water from rain, snowmelt and groundwater at one location, often a stream that then flows into another body of water like a river, or ultimately the ocean. The boundaries of a drainage basin are defined by the topography at a given scale.

Go to top


evapotranspiration search for term

The sum of evaporation and plant transpiration. Potential evapotranspiration is the amount of water that could be evaporated or transpired at a given temperature and humidity, if there was plenty of water available. Actual evapotranspiration can not be any greater than precipitation, and will usually be less because some water will run off in rivers and flow to the oceans. If potential evapotranspiration is greater than actual precipitation, then soils are extremely dry during at least a major part of the year.

eyewall search for term

The arc of clouds that partially or completely surrounds the eye of a major tropical cyclone. The fastest surface winds are usually under this arc of clouds. Embedded in these clouds are vigorous convective cells with rapid updrafts that transport the majority of the upward flux of air and moisture in the eyewall.

Go to top


flux search for term

The amount that flows through a unit area per unit time.

frequency search for term

Number of oscillations per unit time or number of wavelengths that pass a point per unit time. In other words, the rate of oscillation of a wave. 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.

Go to top


global atmospheric circulation search for term

The movement of major air masses around the globe, which is the primary means by which heat energy is redistributed from the equator toward the poles.

graupel search for term

Graupel, also called soft hail or snow pellets, refers to precipitation that forms when supercooled droplets of water condense on a snowflake, forming a 2–5 mm ball of rime; the snowflake acts as a nucleus of condensation in this process. The term graupel is the German word for this meteorological phenomenon. Graupel is sometimes referred to as small hail, although the World Meteorological Organization defines small hail as snow pellets encapsulated by ice, a precipitation halfway between graupel and hail.

groundwater recharge search for term

Refers to the process of water moving from the surface downward to the water table where it replenishes the groundwater supply.

Go to top


hail search for term

Precipitation composed of balls or irregular lumps of ice. Hail is produced when large frozen raindrops, or almost any particles, in cumulonimbus clouds act as embryos that grow by accumulating supercooled liquid droplets. Violent updrafts in the cloud carry the particles in freezing air, allowing the frozen core to accumulate more ice. When the piece of hail becomes too heavy to be carried by upsurg­ing air currents it falls to the ground.

hot tower search for term

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. Around the same time, the same scientists proposed that hot towers were responsible for most of the latent-heat release inside of the eyewall of tropical cyclones (Meteorological Monographs, 2003, Chapters 10 and 4). Our understanding of hot towers was improved by the 3D observations of the TRMM satellite, which could be used to create a global catalog of hot towers.

hydrometeor search for term

A hydrometeor is liquid or solid water suspended in the atmosphere, from the Greek 'hydro' meaning 'water' and 'meteoros' for 'high in the air' (American Heritage College Dictionary). Categories of hydrometeors include cloud water, precipitation, sea spray, and wind-blown snow (AMS Glossary of Meteorology).

Go to top


infiltration search for term

Refers to the process of surface water moving into soil. Infiltrating water does not necessarily reach groundwater, but it can.

Go to top


Ka-band search for term

Radar and microwave band in which the wavelengths vary from 1.11 cm-7.5 mm, a frequency of 27-40 GHz. The dual-frequency radar on the GPM satellite will include a Ka-band 35 GHz frequency for measuring ice precipitation and light rain and a Ku-band 14 GHz frequency for measuring heavy precipitation.

Ku-band search for term

Radar and microwave band in which the wavelengths vary from 1.67-2.4 cm, a frequency of 12-18 GHz. The 13 Ghz TRMM radar is a Ku-band radar.

Go to top


latent heat search for term

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.

Go to top


mesoscale convective system search for term

A group of thunderstorms that is typically several hundred kilometers across and lasting 6-12 hours or sometimes longer (Cotton and Anthes, 1989, Chapter 12; Meteorological Monographs, 2001, Chapter 9). The rain that falls from these systems can lead to dangerous flooding with tremendous socioeconomic impacts. Predicting and monitoring these high-impact natural hazard events depends on accurate and timely knowledge of precipitation at local and regional scales.

microphysics search for term

Cloud microphysics is the study of the population of hydrometeors in various categories and the processes that change hydrometeors from one category into another. For example, a simple microphysics scheme in a cloud model might include the equations that govern the creation and destruction of cloud ice, cloud water, rain, and several kinds of precipitation ice. Source: Chapter 6 of Wallace and Hobbs (2006)

model search for term

A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.

Go to top


NCEI search for term

Formerlly the National Climatic Data Center (NOAA), the name has changed to National Centers for Environmental Information (NCEI), although the web domain is still ncdc.  Learn more at

nowcasting search for term

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. A more restrictive definition of a nowcast is a detailed description of current weather conditions, from which one can extrapolate (project) the weather conditions for the following two hours. Source: McGraw-Hill Encyclopedia of Science and Technology, 7th ed., vol. 12, p. 116.

Go to top


orography search for term

When modeling the atmosphere, orography is roughly synonymous with 'topography', i.e., the location and altitude of mountains, hills, and other major features on the earth's surface. In precipitation science, this typically refers to adding a realistic altitude grid to an atmospheric model.

Go to top


passive sensor search for term

A system using only radiant energy emitted by the object being viewed, or reflected by the object, or from a source other than the system. Compare with active sensor.

phase search for term

The physical state of matter of a substance: gas, liquid or solid. For water, phases include water vapor, liquid water or ice.

polar orbit search for term

Satellite orbits with inclinations near 90 degrees are called polar orbits because the satellite crosses over (or nearly over) the north and south poles.

precipitation search for term

In meteorology, precipitation (the collective name for falling hydrometeors) is any product of the condensation of atmospheric water vapor that falls quickly out of a cloud. This is in contrast to cloud water, which is ice or liquid water that falls slowly enough that it can remain in the air for hours. The main forms of precipitation include drizzle, rain, sleet, snow, graupel and hail. It occurs when a local portion of the atmosphere becomes saturated with water vapour and the water condenses.

Go to top


radar search for term

Radar uses radio waves to detect objects. The antenna sends out pulses of radio waves that are reflected back by the object they touch, which are then picked up by the dish.

radiometer search for term

An instrument for quantitatively measuring the intensity of electromagnetic radiation in some band of wavelengths in any part of the electromagnetic spectrum. Usually used with a modifier, such as an infrared radiometer or a microwave radiometer.

reflectivity search for term

A measure of the fraction of incident radiation falling on a impinging on a surface or volume of the atmosphere that is turned back from it by reflection. Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.

runoff search for term

Runoff occurs when falling rain cannot be absorbed by soil, and 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 in urban environments when there is asphalt instead of soil on the ground.

Go to top


subsurface flow search for term

Refers to the flow of water through the ground. Subsurface flow includes water in unsaturated soil and groundwater aquifers.

sun synchronous search for term

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’s Sun synchronous orbit might cross the equator 12 times a day, each time at 3:00 p.m. local time.

supercooled water search for term

Liquid water below 0 degrees Celcius (32 Fahrenheit). Smaller droplets can remain supercooled longer and at cooler temperatures than can larger droplets. Similarly, freezing is delayed when the water droplets are more pure

swath search for term

The portion of Earth's surface or atmosphere measured by an instrument during a single satellite overpass.

Go to top


transpiration search for term

The release of water to the atmosphere from plants. Plants release water vapor from small pores (stomata) in their leaves as a part of photosynthesis..

tropical depression search for term

A tropical depression is an area of low pressure that has clouds and thunderstorms with winds less than 38 mph (61 kmh). It does not necessarily have an eye or the typical cyclonic structure.

Go to top


water vapor search for term

The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, 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.

Go to top