Understanding Scientific Units

Earth science data parameters have very specific definitions and units. The links below will access some definitions for science parameter units by the topic of interest.
Temperature | Pressure | Radiation | Aerosols | Air Quality | Clouds | Surface
All Unit Definitions | Full Science Glossary | The Metric System

C
centimeter:
A unit measuring distance or height. It is used as a unit for precipitable water in the LAS. This is the height of the layer of water that would exist if all the water vapor in the atmospheric column were condensed to liquid at the Earth’s surface.  
C
column number density:
A unit often used in remote sensing. The number of something (particles, atoms, molecules, …) in an atmospheric column over an area. The area could be a square centimeter, square meter, square kilometer, etc… This provides information about the total amount of that substance in the column, but does not provide any information about how it is distributed: uniformly mixed in the atmosphere (i.e., CO2), vs concentrated near the surface (i.e. pollution), vs concentrated at some other level (i.e., the ozone layer). Because the number of atoms or molecules is usually large, scientific notation is used. For example: 1.e15 NO2 molecules in a column over one square centimeter of the surface (1,000,000,000,000,000 NO2 molecules in an atmospheric column)..  
  Related MY NASA DATA Activity:  Parts Per Billion Activity (PowerPoint)
Illustration of molecules within an atmospheric column
Image courtesy MY NASA DATA Team
C
concentration:
the strength of a solution or mixture, or the amount of a substance in a liquid or gas. Units of measurement will vary depending upon the substance and medium (e.g. milligrams per meter cubed, parts per billion volume, etc.).  
D
degree:
a standard unit of measure for several purposes to include temperature on a scale such as Celsius, Fahrenheit or Kelvin, latitude or longitude to locate or designate places on Earth, and planar angles equal to a 360th of a circle.   Fahrenheit, Celsius and Kelvin temperature scales
Image courtesy NASA explores
D
dobson unit:
a standard unit measure for ozone in a column of air.  
Graphical explanation
J
joule:
a standard unit of energy (radiation) or work (mechanics). For energy, one joule is equal to one watt second.   unit relationships
Image courtesy NASA
K
kilometer:
A metric unit of distance equivalent to 1,000 meters. One kilometer is a little more than one half of a mile.  
The Metric System
L
latitude:
a measure which identifies the northsouth location of a point on the Earth. It is the angle between the line connecting a point on the Earth and the Earth’s center, and the equatorial plane of the Earth. There are three ways to express latitude. You may be most familiar with 0-90 North and 0-90 South. In the computer era this became -90 to +90, where -45 equivalent to 45 South. The third method is less familiar and is called the colatitude. Colatitude is 0 at the north pole, 90 at the equator, and 180 at the south pole. So, 45 South is equivalent to a colatitude of 135.  
  Related MY NASA DATA Activity:  Latitude and longitude tool.
diagram showing latitude
L
longitude:
a measure which identifies the eastwest location of a point on the Earth. It is the angular distance along a line of latitude from the Greenwich Meridian – a reference longitude set to be zero degrees. There are three equivalent ways to express longitude, and scientists tend to use them interchangeably. You may be most familiar with longitude as 0-180 East, and 0-180 West. It can also be expressed as 0-360 East, or just 0-360. In that case, 270 East is equivalent to 90 West. The third system arose in the computer era, when carrying both a number (0-180) and a character (East or West) was inconvenient. The new convention of -180 to +180 was then developed. In this case,-90 is equivalent to 90 West.  
  Related MY NASA DATA Activity:  Latitude and longitude tool.
diagram showing longitude
M
micrometer or micron:
one-millionth of a meter (10.e-6 or 0.000001 m). Human hair is approximately 100 microns in diameter.  
The Metric System
M
millibar:
a unit of measure used for atmospheric pressure. Standard atmospheric pressure is 1013.25 millibars, or mb, so this offers a very sensitive scale: 1 millibar represents a one tenth of one percent change in atmospheric pressure.  
More about surface pressure units
M
mm per day:
this unit provides a measure of the amount of precipitation in one day. Precipitation is measured in units of height as if a layer of water or condensed snow exists at the surface. It is NOT measured in units of volume. Within the LAS, Monthly Precipitation is the average amount of rain or snowfall per day that fell over one month’s time. To obtain the total monthly precipitation estimate, multiply this measurement by the number of days in that month.  
  Related MY NASA DATA Activity:  Science Project: Measuring Local Precipitation
M
mole:
a standard unit of measure for the amount of a substance. One mole (mol) of a substance is numerically equal to the molecular weight of that substance. For gases, one mole is equal to Avogadro’s number of molecules (6.022e23). In atmospheric chemistry, moles are sometimes used to quantify atmospheric constituents or pollutants.  
N
nanometer:
One billionth of a meter (10.e-9 or 0.000000001 m). There are a billion (1,000,000,000) nanometers in one meter.  
The Metric System
N
number density:
A unit often used in chemistry and physics. The number of something (particles, atoms, molecules, …) in a given volume. The volume could be a cubic centimeter, cubic meter, or some other volume. This can be used in atmospheric science to describe the concentration of aerosols or trace gases. Because the number of atoms or molecules is usually large, scientific notation is used. For example: 3.e12 ozone molecules in a cubic centimeter inside the ozone layer (3,000,000,000,000 ozone molecules in a cubic centimeter).  
  Related MY NASA DATA Activity:  Parts Per Billion Activity (PowerPoint)
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
percentage:
proportion in regard to the whole, or a fraction or ratio measured as part of 100.  
P
psu:
practical salinity unit or psu is one gram of salt per 1000 grams of water.  
More information PODAAC Sea Surface Salinity
S
steradian:
a standard unit of solid angle. One steradian is the solid angle made at the center of a sphere by an area on the surface of the sphere equal to the square of the sphere’s radius.  
Illustration of a steradian
T
TEST DAN:
TEST DAN.  
SSE Website
POWER/SSE Logo
Image courtesy NASA
W
watt:
a standard unit of power. In radiation studies, watts are used to quantify the flux of shortwave or longwave energy across a unit area such as Earth’s surface (watts per meter squared).   unit relationships
Image courtesy NASA