This page last changed on Jan 25, 2011 by stepheneb.

This style guide uses styled text, to be certain the information is understandable make sure that the next word: ITALICS, appears in italics.

As a style guide on units this document is oriented toward experimental data.

Graphing: Axis Labeling

Whenever possible SI units should be used as the default representation. Axes labels normally have just two components, quantities and units. Quantity refers to the type of phenomena being measured, example: velocity. Units refer to the specification for how the quantity is reported, example: meters per second. Units also have standard symbolic abbreviations such as "m/s".

There are other items that are part of the dependent or independent dataset associated with an axis but are usually not reported on an axis. There may be a label which is a textual description of what is being measured, example: "speed of the car". There may also be a symbolic variable which can be used to specify the dataset in a formula, for example multiplying the velocity of a car by its mass to get momentum.


  • Value: the numeric quantity of a measurement
  • Quantity: what phenomena is being measured, ex: mass
  • Units
    • Name: the full SI name, ex: kilogram
    • Unit-symbol: the abreviation, ex: kg
    • ASCII unit-symbol: derived units often need additional ASCII representations, square meters = m3
    • Prefixes: m: milli, k: kilo, etc
  • Symbolic variables: temperature Kelvin: T, temperature degrees Celsius: t, heat: Q, etc.

There are only 7 base units in SI,

Base quantity Unit Name Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

All the other SI units are derived from these for example

velocity meters per second m/s

However some units while derived from the base units have special names in which the derivation is not obvious. For example:

force newton N

The actual derivation for N from base SI units is: m - kg - s2

When the raised dot can't be used to indicate multiplication use a single space instead like this: m kg s2. If the typographical system supports it the space should be a thin non-breaking space.

The last element in the expression above represents seconds squared. This form is used when the display mechanism doesn't support superscripts.

Other Units

There are a few other units that are not SI but are acceptable to use with SI.

Name Symbol Value in SI units
minute (time) min 1 min = 60 s
hour (time) h 1 h = 60 min = 3600 s
day (time) d 1 d = 24 h = 86 400 s
degree (plane angle) ° 1° = ( /180) rad
minute (plane angle) ' 1' = (1/60)° = (/10 800) rad
second (plane angle) '' 1'' = (1/60) = (/648 000) rad

from NIST Rules and Style Conventions for Expressing Values of Quantities

In the expression for the value of a quantity, the unit symbol is placed after the numerical value and a space is left between the numerical value and the unit symbol.

The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle: °,','' and , respectively (see Table 6), in which case no space is left between the numerical value and the unit symbol

Here's a simple example:

  • I measured the air temperature in the room and labeled it T.
  • The standard base unit name for thermodynamic temperature is Kelvin, the standard symbolic variable for Kelvin is T.
  • T = 295.55 Kelvin
  • The standard base SI unit-symbol for thermodynamic temperature is K:
  • T = 295.55 K
  • Degree Celsius is a derived unit with a special name:
  • t = 22.4 degrees Celcius
  • The unit-symbol for degree Celsius is °C.
  • t = 22.4 °C

There are also standard symbolic variables which are often used with certain units. For example T is used for representing a temperature value measured in Kelvin while t is used to represent a temperature value measured in Celsius.

To specify that the variable t is used to represent room temperature (as opposed to outside temperature) a subscripted non-italicized suffix is used however both italicizing and subsripting are impractical with certain computer display environments such as the plain text of an email message. In this case I suggest using an underscore: t_room.

  • NIST: Guide for the Use of the International System of Units (SI)
  • SI Guide by James R. Frysinger
  • The Unified Code for Units of Measure, Gunther Schadow and Clement J. McDonald, Regenstrief Institute for Health Care and Indiana University School of Medicine
  • Rowlett's dictionary of Units of Measurements

From an email to Carolyn about the Jason Albedo unit:

You should probably be consistent throughout the activities in your use of units and variables. The standard convention for variables is to italicize them. I recommend describing and using units instead of just constants. See my examples below.

I am using the raised dot "- " between units to indicate that they are multiplicative. An alternative is to use a single space character.


NIST: Guide for the Use of the International System of Units (SI)

Albedo comments:

The datacollection should be based on collecting data from each cup for 30 minutes instead of waiting for the same temperature rise. If you wait for the same temperature rise then you have created equivalent amounts of absorbed heat however the clear water will take much longer because of it's higher albedo. By doing a constant amount of time you more closely simulate for example the amount of energy absorbed by areas of the earth with different albedos during a single day.

IN the Exploring Data section have the students record the temperature rise (t_rise = t_final - t_initial) for the two equal time intervals for the two solutions.

In the Albedo activity the Drawing Conclusions section is actually Data Analysis and Drawing Conclusions.

Suggested text for the Drawing Conclusions section in the Albedo activity (it still needs a wrap up paragraph but I have to drive home now):

You have already measured the increase in temperature for the two solutions. In order to calculate the increase in heat energy you'll need one more piece of information, the specific heat capacity of water. The specific heat capacity of water refers to the energy required to raise one gram of water one degree Celcius. At 20 °C it takes 4.18 Joules of heat energy to raise one gram of water one degree Celcius. Conveniently one gram of water is almost exactly equal to one milliliter of water.

Let T represent the change in temperature of the 50 mL solution. The change in heat energy Q due to the change in temperature can be calculated as follows:

Q = 4.18 J/mL- °C x 50 mL x T °C

So if this was the data collected from the experiment:

coffee water
initial 8 8 degrees Celcius
final 32 17 degrees Celcius
change 24 9 degrees Celcius

The heat energy absorbed by the two solutions would be calculated as follows:

Q coffee = 4.18 x 50 x 24 = 5016 Joules
Q water = 4.18 x 50 x 9 = 1881 Joules

Since the coffee is quite black we can make a reasonable estimate that it's albedo is 0 – almost all of the light energy striking the surface is absorbed and not reflected. That then means that we can say that 5016 Joules of light energy fell on the surface of the cup during the 30 minute experiment. If only 1881 Joules were absorbed by the clear water that means that 3135 Joules of energy were reflected. Remember the equation for albedo:

A = Q reflected / Q total

Therefore with the data above we can calculate the albedo of the water and white foam cup as follows:

A water = 3135 / 5016 = 0.625

An albedo value of 0.625 is very close the the 0.7 value atmospheric scientists measure for the earths polar ice caps. What is similar between your cup with water in it and the polar ice caps?


Marily has corrected my units in the past. She is pretty insistent on mL (not italicized. I will check with her tomorrow if we can italicize and not capitalize the base unit.

Marily is correct, you misunderstood me (or I wasn't clear enough), units are never italicized. Italicize variables such as T to represent the temperature change or Q to represent the change in heat energy.

See the following section in the NIST manual on SI units I referenced earlier:

More on Printing and Using Symbols and Numbers in Scientific and Technical Documents


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