Learn about the terms used when studying precipitation.
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 radi­ation to detect an object or area for observation and receives the reflected or transmitted radiation. 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.

atmosphere search for term

The envelope of gases that surrounds Earth.


biosphere search for term

All life on Earth; the parts of the geosphere, hydrosphere, and atmosphere in which living organisms can be found.


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 search for term

The process by which molecules of water vapor in the air become liquid water.

convection search for term

The transfer of thermal energy by movement of a fluid. 


drought search for term

Long periods of low precipitation.


evaporation search for term

The process by which water molecules in liquid water escape into the air as water vapor.

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.


geologic time search for term

All life on Earth; the parts of the solid Earth (geosphere) , hydrosphere, and atmosphere in which living organisms can be found.

geosphere search for term

The layer of Earth under both atmosphere and oceans; It is comprised of the core, the mantle, and the crust.

global warming search for term

A gradual increase in the average temperature of Earth’s surface.

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.

greenhouse effect search for term

The process by which heat is trapped in the atmosphere by gases that form a “blanket” around the Earth.

greenhouse gasses search for term

Gases in the atmosphere that trap energy. The primary greenhouse gases in the Earth's atmosphere are water vapour, carbon dioxide, methane, nitrous oxide, and ozone. 

groundwater search for term

Water that collects or flows beneath Earth’s surface, filling in porous spaces in soil, sediment, and rocks.


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.

heat search for term

The transfer of thermal energy  from one object to another because of a difference in temperature.

hurricane search for term

A tropical storm that has winds of about 119 kilometers per hour or higher; the scientific term is “tropical cyclone”.

hydrology search for term

The scientific study of the properties, distribution, and effects of water on the earth's surface, in the soil and underlying rocks, and in the atmosphere.

hydrosphere search for term

All of Earth's water, including surface water (water in oceans, lakes, and rivers), groundwater (water in soil and beneath the Earth's surface), snowcover, ice, and water in the atmosphere, including water vapor.


La Nina search for term

A recurring climate event in the Pacific Ocean in which surface waters are colder than normal.

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.


meteorologists search for term

Scientist who study the causes of weather and try to predict it.

microwaves search for term

A type of electromagnetic energy; the portion of electromagnetic energy between radio waves and infrared radiation, which is often measured by scientific instruments to determine atmospheric conditions from space.

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.


Non-Sun-Synchronous Orbit search for term

When an object, such as a satellite, is circling the Earth at such a speed that it crosses over the same location at different times of the day. This allows the satellite to collect measurements during different solar conditions (i.e. morning, evening, night). 


passive sensor search for term

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

photosynthesis search for term

The process by which plants use light energy to convert water and carbon dioxide into energy.

porous search for term

Material that is able to easily absorb fluids or allow liquid to pass through.

precipitation search for term

In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena) 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.


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.

rain gauge search for term

An instrument that is used to measure liquid precipitation.

reflectivity search for term

A measure of the fraction of incident radiation falling on a surface 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.


satellite search for term

Anything that orbits or circles something else: man-made satellites are artificial machines that are put in space in order to collect information or for communication.

swath search for term

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


temperature search for term

 A measure of how hot or cold an object is compared to a reference point.

thermal energy search for term

The total energy of motion in the particles of a substance.

thermal radiation search for term

The direct transfer of heat energy

thunderstorm search for term

A localized storm often accompanied by heavy precipitation and frequent thunder and lightning.

tornado search for term

A rapidly whirling, funnel-shaped cloud that reaches down to touch Earth's surface and often causes great damage to structures and people.

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..


water cycle search for term

The continual movement of water among Earth’s atmosphere, oceans, and land surface through evaporation, condensation, and precipitation.

water vapor search for term

Water in the form of a gas. 

weather search for term

The condition of Earth’s atmosphere at a particular place and time.