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Temperature, Dew Point, and Relative Humidity
Measuring
Temperature
You probably don’t need to be told that a thermometer is an instrument
for measuring temperature. Generally speaking, it is a glass tube of
small bore in which either alcohol or mercury expands and contracts
with the rise and fall of the temperature of the surrounding medium.
Most Navy thermometers are mercury-filled and practically all of them
use the Fahrenheit (F) scale, where the freezing point of water is 32°
and its boiling point is 212°. Temperature in meteorology, however, is
sometimes expressed according to the Celsius (C) (formerly Centigrade)
scale, where the freezing point of water is 0° and its boiling point is
100°.
You may be required to convert a Fahrenheit reading to Celsius, or vice
versa. Knowing that 32°F = 0°C, to change a Fahrenheit reading to
Celsius, you first subtract 32° and then multiply the remainder by 5/9.
Example: Say you want to change 41°F to Celsius. Subtracting 32°
from 41° gives 9°. Multiply 9° by 5/9, and you get 45/9, or 5°C.
To change from Celsius to Fahrenheit, simply reverse the procedure.
First multiply the Celsius temperature by 9/5, then add 32°. In the
previous example, to change 5°C back to Fahrenheit, first multiply it by
9/5, which gives you 45/5, or 9°. Adding 32” gives you 41°F.
You may also use the following formulas:
F = 1.8
C + 32 (Multiply Celsius temp by 1.8, then add 32 to find
Fahrenheit)
C = F -32 divided by 1.8 (Fahrenheit minus 32, then divide by 1.8 to
find Celsius)
A thermometer must be read properly to obtain an accurate result. First,
if you must handle it, be sure that you do not touch the lower part of the
glass containing the alcohol or mercury, because the heat from your
body can affect the height of the mercury or alcohol column. Make
certain that the top of the column is level with your eyes; otherwise you
will be reading a higher or lower graduation than the one actually
indicated. The top of the column is in the shape of a curve called a
meniscus. It is the bottom of this curve that indicates the reading for an
alcohol thermometer; the top, for a Mercury thermometer.
10-26
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