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GLOSSARY

Our comprehensive earth science glossary is available to assist in your understanding of the MY NASA DATA microsets, lesson plans and computer tools. Please select the first letter of the word you wish to check for a definition and further information.

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All definitions
P
PAR:
acronym for Photosynthetically Active Radiation, energy in the visible light region of the electromagnetic spectrum (400 - 700 nanometer wavelength) that plants use for photosynthesis, measured in watts per meter squared. PAR is an important parameter of interest for ecological and hydrological climate studies (carbon, oxygen and water cycles).   More Information about Photosynthesis tree canopy
Image courtesy MY NASA DATA
P
parameter:
a measureable or derived earth science quantity within a data set, such as temperature, that usually varies by location and in time.  
P
parts per billion volume:
one method of expressing chemical concentration, usually of a gas. For example, 20 ppbv of ozone means that, if a sample of gas contained one billion units, then 20 of the units would be ozone.     Related MY NASA DATA Activity:  Parts Per Billion Activity (PowerPoint)
P
Pascal:
The SI unit for pressure measurement, named after Blaise Pascal, a French mathematician and physicist. 1 Pa = 1000 millibars (mb). Atmospheric pressure at Earth's surface averages 101325 Pa.  
P
percentage:
proportion in regard to the whole, or a fraction or ratio measured as part of 100.  
P
phase:
the physical state of matter of a substance: gas, liquid or solid; for water, phases include water vapor, liquid water or ice.  
P
photometer:
An instrument for measuring the amount of light. Depending on the purpose, a variety of filters can be used to measure light of specific wavelengths. A photometer can also be used to measure ambient light, or only a narrow beam that comes directly from a source like the Sun.   An AERONET sun photometer installed on an ocean platform
Image courtesy The NASA Langley COVE Surface Validation Group
P
photosynthesis:
the chemical process by which plants and some bacteria capture the energy of the Sun, and convert it to food. This chemical reaction combining carbon dioxide and water is catalyzed by chlorophyll, a green pigment in plant cells. The main product of photosynthesis is a carbohydrate, such as the sugar glucose, and oxygen which is released to the atmosphere.  
P
phytoplankton:
small plant-like organisms, such as the algae and bacteria found in marine ecosystems, that perform photosynthesis.   More information about phytoplankton   Related MY NASA DATA Activity:  Lesson: Phytoplankton in the Gulf of Maine
P
pixel:
a picture element, the smallest element that can be displayed on an image or computer monitor, often used as a unit of measurement for image size and resolution.   More information about pixels
P
pollution:
a general term refering to increased concentrations of undesired matter (solids, liquids or gases) in Earth's water, atmosphere or land that is manmade.  
P
population density:
number of people per unit area.  
P
precipitable water:
the amount of water in the form of vapor (gas) that is in a column of atmosphere. Under the right conditions, this water vapor can condense into liquid water and turn into clouds or precipitation. Precipitable water has units of centimeters, so one must imagine condensing all the moisture in the column of air into a small layer of water and then measuring its height.  
P
precipitation:
Water that falls from clouds to the ground. This can be in the form of rain, snow, sleet, hail, etc.   More info from the Global Precipitation Measurement mission   Related MY NASA DATA Activity:  Science Project: Measuring Local Precipitation
P
pressure:
see definition for atmospheric pressure.  

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